Data Journalism

---
name: data-journalism
description: Data journalism workflows for analysis, visualization, and storytelling. Use when analyzing datasets, creating charts and maps, cleaning messy data, calculating statistics or building data-driven stories. Essential for reporters, newsrooms and researchers working with quantitative information.
---

# Data journalism methodology

Systematic approaches for finding, analyzing and presenting data in journalism.

## Story structure for data journalism

### Data journalism framework

The framework for data journalism was established by Philip Meyer, a journalist for Knight-Ridder, Harvard Nieman Fellow and professor at UNC-Chapel Hill. In his book *The New Precision Journalism*, Meyer encourages journalists to treat journalism "as if it were a science" by adopting the scientific method:

- Make observations / formulate a question
- Research the question / collect, store, and retrieve data
- Formulate a hypothesis
- Test the hypothesis, using both qualitative (interviews, documents) and quantitative (data analysis) methods
- Analyze the results and reduce them to the most important findings
- Present them to the audience

The process is iterative, not sequential.

### The data story arc

**1. The hook (nut graf)**
- What's the key finding?
- Why should readers care?
- What's the human impact?

**2. The evidence**
- Show the data
- Explain the methodology
- Acknowledge limitations

**3. The context**
- How does this compare to the past?
- How does this compare to elsewhere?
- What's the trend?

**4. The human element**
- Individual examples that illustrate the data
- Expert interpretation
- Affected voices

**5. The implications**
- What does this mean going forward?
- What questions remain?
- What actions could result?

**6. The methodology box**
- Where did the data come from?
- How was it analyzed?
- What are the limitations?
- How can readers explore further?

### Methodology documentation template

```markdown
## How we did this analysis

### Data sources
[List all data sources with links and access dates]

### Time period
[Specify exactly what time period is covered]

### Definitions
[Define key terms and how you operationalized them]

### Analysis steps
1. [First step of analysis]
2. [Second step]
3. [Continue...]

### Limitations
- [Limitation 1]
- [Limitation 2]

### What we excluded and why
- [Excluded category]: [Reason]

### Verification
[How findings were verified/checked]

### Code and data availability
[Link to GitHub repo if sharing code/data]

### Contact
[How readers can reach you with questions]
```

## Data acquisition

### Public data sources

**Federal data sources**

*General:*
- **Data.gov** — Federal open data portal. Many datasets were removed between Feb 2025 and 2026; consult the [Harvard LIL Data.gov archive](https://lil.law.harvard.edu/blog/2025/02/06/announcing-data-gov-archive/) and the [Data Rescue Project](https://www.datarescueproject.org/) for preserved copies before assuming anything is still accessible.
- **Census Bureau** (census.gov) — Demographics, economic data. Many research pages were removed during the 2025 transition; the [End of Term Web Archive](https://eotarchive.org) holds snapshots.
- **BLS** (bls.gov) — Employment, inflation, wages. Following the 2025 funding lapse, the October 2025 Employment Situation release was canceled and the CPS October 2025 reference period is permanently uncollected. Check [revised release dates](https://www.bls.gov/bls/2025-lapse-revised-release-dates.htm) before relying on series continuity.
- **BEA** (bea.gov) — GDP, economic accounts.
- **FRED / Federal Reserve** (fred.stlouisfed.org) — Financial and macroeconomic data; expanded API access through 2026.
- **SEC EDGAR** — Corporate filings.

*Specific domains:*
- **EPA** (epa.gov/data) — Environmental data. At least 80 climate webpages were removed in Dec 2025, the endangerment finding was repealed Feb 12, 2026, and the Climate Change Indicators site was largely gutted. The [Environmental Data & Governance Initiative](https://envirodatagov.org) maintains mirrors.
- **FDA / openFDA** (open.fda.gov) — Drug approvals, recalls, adverse events.
- **CDC WONDER** — Health statistics. Many datasets were removed from data.cdc.gov after Jan 2025, partially restored under Doctors for America v. Trump (TRO Feb 11, 2025) but with altered terminology in some returns. The volunteer-run [RestoredCDC.org](https://restoredcdc.org/wonder.cdc.gov/) mirrors removed content.
- **NHTSA FARS / vPIC APIs** — Vehicle safety data.
- **DOT** — Transportation statistics.
- **FEC** — Campaign finance; 2025-2026 cycle data live.
- **USASpending.gov** — Federal contracts and grants; API v2 operational.

*Court records:*
- **CourtListener / RECAP** (courtlistener.com) — Free PACER alternative covering federal court filings; RECAP Search Alerts launched June 2025 ("Google Alerts for federal courts").
- **PACER** — Federal court filings; $0.10 per page, $30 per quarter waiver threshold.

*State and local:*
- State open data portals (search: "[state] open data")
- Tyler Data & Insights (formerly Socrata, rebranded May 2025) hosts many city and state portals
- OpenStreetMap, municipal GIS portals
- State comptroller and auditor reports

*International:*
- **Eurostat**, **OECD**, **World Bank Open Data**, **UN Data** — major comparative datasets, mostly stable through 2026.

*Specialized:*
- **NICAR Data Library** (IRE) — curated datasets, IRE members only.
- **IPUMS** (University of Minnesota) — free with account; canonical for harmonized microdata.
- **ICPSR** (University of Michigan) — social-science data archive.
- **ProPublica Data Store** — frozen; datasets only run through 2023.

*Federal-data preservation (use when source data has been removed):*
- [Data Rescue Project](https://www.datarescueproject.org) — citizen + library mirrors of removed federal data; more than 1,230 datasets across 85 offices as of Aug 2025.
- [End of Term Web Archive](https://eotarchive.org) — 500TB / 100M-page snapshot of federal sites at the 2024-2025 transition.
- Internet Archive Wayback Machine — useful for individual page-level recovery.

### Data request strategies

**Public records requests for datasets**

For request mechanics (templates, fee-waiver language, NJ OPRA, appeals, FOIA Improvement Act statutory citations), see the **foia-requests** skill. Data-specific guidance:

- Request databases, not just documents
- Ask for the data dictionary or schema
- Request in native format (CSV, SQL dump) — not PDFs or scanned printouts
- Specify field-level needs and any computed columns you want included
- For active datasets, ask the cadence (daily, monthly, quarterly) and request standing access if your reporting will continue

**Building your own dataset**

- Scraping public information (respect robots.txt, ToS, and rate limits)
- Crowdsourcing from readers
- Systematic document review
- Surveys with documented methodology

**Commercial data sources for newsrooms**

- LexisNexis, Refinitiv, Bloomberg
- Industry-specific databases (often via library proxy through your institution)

## Data cleaning and preparation

### Common data problems

```python
from typing import Any

import pandas as pd
import numpy as np
from rapidfuzz import fuzz
from itertools import combinations

# Inflation adjustment
import cpi
import wbdata

def standardize_name(name: Any) -> str | None:
    """Standardize name format to 'First Last'."""
    if pd.isna(name):
        return None
    name = str(name).strip().upper()
    # Handle "LAST, FIRST" format
    if ',' in name:
        parts = name.split(',')
        name = f"{parts[1].strip()} {parts[0].strip()}"
    return name

def parse_date(date_str: Any) -> pd.Timestamp | None:
    """Parse dates in various formats."""
    if pd.isna(date_str):
        return None

    formats = [
        '%m/%d/%Y', '%Y-%m-%d', '%B %d, %Y',
        '%d-%b-%y', '%m-%d-%Y', '%Y/%m/%d'
    ]

    for fmt in formats:
        try:
            return pd.to_datetime(date_str, format=fmt)
        except:
            continue

    # Fall back to pandas parser
    try:
        return pd.to_datetime(date_str)
    except:
        return None


def handle_missing(
    df: pd.DataFrame,
    thresh: int | None = None,
    per_thresh: float | None = None,
    required_col: str | None = None,
) -> pd.DataFrame:
    """Drop rows missing values in `required_col` if missingness exceeds either threshold."""
    if required_col is None or df.empty:
        return df
    if required_col not in df.columns:
        return df

    missing = df[required_col].isna().sum()

    if thresh is not None and missing >= thresh:
        return df.dropna(subset=[required_col]).reset_index(drop=True).copy()

    if per_thresh is not None and (missing / len(df) * 100) >= per_thresh:
        return df.dropna(subset=[required_col]).reset_index(drop=True).copy()

    return df


def handle_duplicates(df: pd.DataFrame, thresh: int | None = None) -> pd.DataFrame:
    """Drop duplicate rows when count exceeds `thresh`."""
    if thresh is not None and df.duplicated().sum() >= thresh:
        return df.drop_duplicates().reset_index(drop=True).copy()
    return df


def flag_similar_names(df: pd.DataFrame, name_col: str, threshold: int = 85) -> pd.DataFrame:
    """Flag rows that have potential duplicate names using vectorized comparison."""
    
    names = df[name_col].dropna().unique()
    
    # Use combinations() to avoid nested loop and duplicate comparisons
    dup_names: set[Any] = {
        name
        for name1, name2 in combinations(names, 2)
        if fuzz.ratio(str(name1).lower(), str(name2).lower()) >= threshold
        for name in (name1, name2)
    }
    
    df['has_similar_name'] = df[name_col].isin(dup_names)
    return df


def flag_outliers(series: pd.Series, method: str = 'iqr', threshold: float = 1.5) -> pd.Series:
    """Flag statistical outliers."""
    if method == 'iqr':
        Q1 = series.quantile(0.25)
        Q3 = series.quantile(0.75)
        IQR = Q3 - Q1
        lower = Q1 - threshold * IQR
        upper = Q3 + threshold * IQR
        return (series < lower) | (series > upper)
    elif method == 'zscore':
        z_scores = np.abs((series - series.mean()) / series.std())
        return z_scores > threshold



# use descriptive variable names and chain methods
data_clean = (pd

            # Load messy data — raw_data is a placeholder
            # Be sure to use the right reader for the filetype
            .read_csv('..data/raw/raw_data.csv')

            # DATA TYPE CORRECTIONS
            # Ensure proper types for analysis
            .assign(# Convert to numeric (handling errors)
                    amount = lambda x: pd.to_numeric(x['amount'], errors='coerce'),
                    
                    # Convert to categorical (saves memory, enables ordering)
                    status = lambda x: pd.Categorical(x['status']))
            
            .assign(
                    # INCONSISTENT FORMATTING
                    # Problem: Names in different formats
                    # e.g., "SMITH, JOHN" vs "John Smith" vs "smith john"
                    name_clean = lambda x: standardize_name(x['name']),

                    # DATE INCONSISTENCIES
                    # Problem: Dates in multiple formats
                    # e.g., "01/15/2024", "2024-01-15", "January 15, 2024", "15-Jan-24"
                    parse_date = lambda x: parse_date(x['date']),
                    
                    # OUTLIERS
                    # Identify potential data entry errors
                    amount_outlier = lambda x: flag_outliers(x['amount']),
                    
                    )
            
            # Fuzzy duplicates (similar but not identical)
            # Use record linkage or manual review
            .pipe(flag_similar_names, name_col='name_clean', threshold=85)

            # MISSING VALUES
            # Strategy depends on context — set required_col when you need to drop incomplete rows
            .pipe(handle_missing, required_col='amount', per_thresh=20.0)

            # DUPLICATES — Find and handle duplicates
            .pipe(handle_duplicates, thresh=1)
            
            .reset_index(drop=True)
            .copy())


```

### Data validation checklist

```markdown
## Pre-analysis data validation

### Structural checks
- [ ] Row count matches expected
- [ ] Column count and names correct
- [ ] Data types appropriate
- [ ] No unexpected null columns

### Content checks
- [ ] Date ranges make sense
- [ ] Numeric values within expected bounds
- [ ] Categorical values match expected options
- [ ] Geographic data resolves correctly
- [ ] IDs are unique where expected

### Consistency checks
- [ ] Totals add up to expected values
- [ ] Cross-tabulations balance
- [ ] Related fields are consistent
- [ ] Time series is continuous

### Source verification
- [ ] Can trace back to original source
- [ ] Methodology documented
- [ ] Known limitations noted
- [ ] Update frequency understood
```

## AI-assisted analysis: cautions

AI tools can speed up exploration, code generation, and pattern surfacing — but they have specific failure modes that journalists must guard against. *Mata v. Avianca* (2023, fabricated court citations sanctioned in federal court) and the Air Canada chatbot ruling (2024, hallucinated refund policy ruled binding on the airline) are the canonical cases of LLM fabrication treated as published fact.

### What LLMs reliably get wrong

- **Calculations at scale** — A model may produce a confident-looking sum, percentage, or rate that's off by 1-15%. Re-run any LLM-produced number in pandas, SQL, or R yourself before publishing.
- **Source citations** — Models hallucinate plausible URLs, paper titles, dataset names, and FOIA exemptions that don't exist. Verify every cited source by visiting it.
- **Dataset columns** — When asked to describe a dataset's structure, an LLM may invent columns that aren't there. Cross-check against the actual schema (`df.dtypes`, `df.columns.tolist()`).
- **Statistical reasoning** — LLMs confuse correlation with causation, conflate sample statistics with population parameters, and misapply tests. Treat any analytical claim as a hypothesis to verify, not a finding.

### Methodology disclosure

When AI was used in any stage (data cleaning, analysis, visualization, drafting), disclose it in the methodology box. Editors and readers need to know which steps had a human in the loop and which were automated.

- State the tool and version (e.g., "We used Claude 4.7 to draft the cleaning pipeline; the code was reviewed and run by [reporter]").
- State what was verified (e.g., "All numerical results were re-computed in pandas; all source citations were independently confirmed").
- State what was not verified (if relevant).

### Reproducibility

When using AI to generate analysis code, save the prompt, the model name, and the version alongside the code. AI-generated code is part of your methodology and should be reproducible by another reporter on the same data.

## Statistical analysis for journalism

### Basic statistics with context

```python
# Essential statistics for any dataset
def describe_for_journalism(df: pd.DataFrame, col: str) -> pd.DataFrame:
    """Generate journalist-friendly statistics."""
    stats = df[col].describe(percentiles=[0.25, 0.5, 0.75, 0.9, 0.99])
    
    # Add skewness to the describe() output
    stats['skewness'] = df[col].skew()
    
    return stats.to_frame(name=col)

# Example interpretation
stats = describe_for_journalism(salaries, 'salary')

print(f"""
ANALYSIS
---------------
We analyzed {stats['count']:,} salary records.

The median salary is ${stats['median']:,.0f}, meaning half of workers
earn more and half earn less.

The average salary is ${stats['mean']:,.0f}, which is
{'higher' if stats['mean'] > stats['median'] else 'lower'} than the median,
indicating the distribution is {'right-skewed (pulled up by high earners)'
if stats['skewness'] > 0 else 'left-skewed'}.

The top 10% of earners make at least ${stats['90th_percentile']:,.0f}.
The top 1% make at least ${stats['99th_percentile']:,.0f}.
""")
```

### Comparisons and context

```python
# Calculate change metrics for a column
def calculate_change(df: pd.DataFrame, col: str, periods: int = 1) -> pd.DataFrame:
    """Add change metrics to a DataFrame using built-in pandas methods.
    
    Args:
        df: Input DataFrame
        col: Column to calculate changes for
        periods: Number of rows to look back (1=previous row, 12=year-over-year for monthly)
    """
    return df.assign(
        absolute_change=df[col].diff(periods),
        percent_change=df[col].pct_change(periods) * 100,
        direction=np.sign(df[col].diff(periods)).map({1: 'increased', -1: 'decreased', 0: 'unchanged'})
    )

# Usage:
# changes = data_clean.pipe(calculate_change, 'revenue', periods=12) # Year-over-year for monthly data

# Per capita calculations (essential for fair comparisons)
def per_capita(value: float, population: float, multiplier: int = 100000) -> float:
    """Calculate per capita rate."""
    return (value / population) * multiplier  # Per 100,000 is standard

# Example: Crime rates
city_a = {'crimes': 5000, 'population': 100000}
city_b = {'crimes': 8000, 'population': 500000}

rate_a = per_capita(city_a['crimes'], city_a['population'])
rate_b = per_capita(city_b['crimes'], city_b['population'])

print(f"City A: {rate_a:.1f} crimes per 100,000 residents")
print(f"City B: {rate_b:.1f} crimes per 100,000 residents")
# City A actually has higher crime rate despite fewer total crimes!


def adjust_for_inflation(
    amount: float | pd.Series, 
    from_year: int | pd.Series, 
    to_year: int,
    country: str = 'US'
) -> float | pd.Series:
    """Adjust dollar amounts for inflation. Works with scalars or Series for .assign().
    
    Args:
        amount: Value(s) to adjust
        from_year: Original year(s) of the amount
        to_year: Target year to adjust to
        country: ISO 2-letter country code (default 'US'). US uses BLS data via cpi package,
                 others use World Bank CPI data (FP.CPI.TOTL indicator)
    """
    if country == 'US':
        # Use cpi package for US (more accurate, from BLS)
        if isinstance(from_year, pd.Series):
            return pd.Series([cpi.inflate(amt, yr, to=to_year) 
                            for amt, yr in zip(amount, from_year)], index=amount.index)
        return cpi.inflate(amount, from_year, to=to_year)
    else:
        # Use World Bank data for other countries
        cpi_data = wbdata.get_dataframe(
            {'FP.CPI.TOTL': 'cpi'}, 
            country=country
        )['cpi'].to_dict()
        
        from_cpi = pd.Series(from_year).map(cpi_data) if isinstance(from_year, pd.Series) else cpi_data[from_year]
        to_cpi = cpi_data[to_year]
        return amount * (to_cpi / from_cpi)

# Usage:
# adjust_for_inflation(100, 2020, 2024)  # US by default
# adjust_for_inflation(100, 2020, 2024, country='GB')  # UK
# df.assign(inf_adjust24=lambda x: adjust_for_inflation(x['amount'], x['year'], 2024, country='DE'))

# Always adjust when comparing dollars across years!
```

### Correlation vs causation

```markdown
## Reporting correlations responsibly

### What you CAN say
- "X and Y are correlated"
- "As X increases, Y tends to increase"
- "Areas with higher X also tend to have higher Y"
- "X is associated with Y"

### What you CANNOT say (without more evidence)
- "X causes Y"
- "X leads to Y"
- "Y happens because of X"

### Questions to ask before implying causation
1. Is there a plausible mechanism?
2. Does the timing make sense (cause before effect)?
3. Is there a dose-response relationship?
4. Has the finding been replicated?
5. Have confounding variables been controlled?
6. Are there alternative explanations?

### Red flags for spurious correlations
- Extremely high correlation (r > 0.95) with unrelated things
- No logical connection between variables
- Third variable could explain both
- Small sample size with high variance
```

## Data visualization

### Chart selection guide

```markdown
## Choosing the right chart

### Comparison
- **Bar chart**: Compare categories
- **Grouped bar**: Compare categories across groups
- **Bullet chart**: Actual vs target

### Change over time
- **Line chart**: Trends over time
- **Area chart**: Cumulative totals over time
- **Slope chart**: Change between two points

### Distribution
- **Histogram**: Distribution of one variable
- **Box plot**: Compare distributions across groups
- **Violin plot**: Detailed distribution shape

### Relationship
- **Scatter plot**: Relationship between two variables
- **Bubble chart**: Three variables (x, y, size)
- **Connected scatter**: Change in relationship over time

### Composition
- **Pie chart**: Parts of a whole (almost never use, max 5 slices, prefer donut charts)
- **Donut chart**: Parts of a whole
- **Stacked bar**: Parts of whole across categories
- **Treemap**: Hierarchical composition

### Geographic
- **Choropleth**: Values by region (use normalized data!)
- **Dot map**: Individual locations
- **Proportional symbol**: Magnitude at locations
```

### Exploratory interactive visualizations with Plotly Express

```python
import plotly.express as px
import plotly.graph_objects as go

# Set default template for all charts
px.defaults.template = 'simple_white'

def create_bar_chart(
    data: pd.DataFrame,
    title: str,
    source: str,
    x_val: str,
    y_val: str,
    x_lab: str | None = None,
    y_lab: str | None = None,
    text_col: str | None = None,
) -> go.Figure:
    """Create a bar chart with a source-credit footer."""
    fig = px.bar(
        data,
        x=x_val,
        y=y_val,
        text=text_col,
        title=title,
        labels={x_val: x_lab or x_val, y_val: y_lab or y_val},
    )
    fig.add_annotation(
        text=f"Source: {source}",
        showarrow=False,
        xref='paper', yref='paper',
        x=0, y=-0.15,
        xanchor='left', yanchor='top',
        font=dict(size=10, color='gray'),
    )
    return fig

# Example
fig = create_bar_chart(
    data,
    title='Annual widget production',
    source='Department of Widgets, 2024',
    x_val='year',
    y_val='widgets_prod',
    x_lab='Year',
    y_lab='Units produced',
)

fig.show()  # Interactive display
```

### Publication-ready automated data visualizations with Datawrapper
```python
import pandas as pd
import datawrapper as dw

# Authentication: Set DATAWRAPPER_ACCESS_TOKEN environment variable, 
# or read from file and pass to create()
with open('datawrapper_api_key.txt', 'r') as f:
    api_key = f.read().strip()

# read in your data
data = pd.read_csv('../data/raw/data.csv')

# Create a bar chart using the new OOP API
chart = dw.BarChart(
    title='My Bar Chart Title',
    intro='Subtitle or description text',
    data=data,

    # Formatting options
    value_label_format=dw.NumberFormat.ONE_DECIMAL,
    show_value_labels=True,
    value_label_alignment='left',
    sort_bars=True,  # sort by value
    reverse_order=False,

    # Source attribution
    source_name='Your Data Source',
    source_url='https://example.com',
    byline='Your Name',

    # Optional: custom base color
    base_color='#1d81a2'
)

# Create and publish (uses DATAWRAPPER_ACCESS_TOKEN env var, or pass token)
chart.create(access_token=api_key)
chart.publish()

# Get chart URL and embed code
print(f"Chart ID: {chart.chart_id}")
print(f"Chart URL: https://datawrapper.dwcdn.net/{chart.chart_id}")
iframe_code = chart.get_iframe_code(responsive=True)

# Update existing chart with new data (for live-updating charts)
existing_chart = dw.get_chart('YOUR_CHART_ID')  # retrieve by ID
existing_chart.data = new_df  # assign new DataFrame
existing_chart.title = 'Updated Title'  # modify properties
existing_chart.update()  # push changes to Datawrapper
existing_chart.publish()  # republish to make live

# Optional — Export chart as image
chart.export(filepath='chart.png', width=800, height=600)

#view chart
chart
```

### Avoiding misleading visualizations

```markdown
## Chart integrity checklist

### Axes
- [ ] Y-axis starts at zero (for bar charts)
- [ ] Axis labels are clear
- [ ] Scale is appropriate (not truncated to exaggerate)
- [ ] Both axes labeled with units

### Data representation
- [ ] All data points visible
- [ ] Colors are distinguishable (including colorblind)
- [ ] Proportions are accurate
- [ ] 3D effects not distorting perception

### Context
- [ ] Title describes what's shown, not conclusion
- [ ] Time period clearly stated
- [ ] Source cited
- [ ] Sample size/methodology noted if relevant
- [ ] Uncertainty shown where appropriate

### Honesty
- [ ] Cherry-picking dates avoided
- [ ] Outliers explained, not hidden
- [ ] Dual axes justified (usually avoid)
- [ ] Annotations don't mislead
```
## Working with geospatial data

### Geocoding data

#### U.S. Census Geocoder

**Best for:** U.S. addresses only. Returns Census geography (tract, block, FIPS codes) along with coordinates—essential for joining with Census demographic data.

**Pros:** Completely free with no API key required. Returns Census geographies (state/county FIPS, tract, block) that let you join with ACS/decennial Census data. Good match rates for standard U.S. addresses.

**Cons:** Limited to 10,000 addresses per batch. U.S. addresses only. Slower than commercial alternatives. Lower match rates for non-standard addresses (PO boxes, rural routes, new construction).

**Use when:** You need to geocode nicely formatted U.S. addresses or you don't have budget for a paid service. 

```python
# pip install censusgeocode
# (the older `censusbatchgeocoder` package on PyPI hasn't been updated since 2017
# and is unmaintained — use `censusgeocode` instead, which wraps the same Census
# batch endpoint and is actively maintained.)

import censusgeocode as cg
import pandas as pd

# DataFrame must have columns matching the *_col parameters below
# (defaults: id, address, city, state, zipcode). If your CSV uses
# different names like 'street' or 'zip', pass them via address_col=,
# zipcode_col=, etc. — internally these are renamed to the keys the
# Census API expects ('address', 'zip', etc.) before the request.
# (state and zipcode are optional but improve match rates)

def census_geocode(
    df: pd.DataFrame,
    id_col: str = 'id',
    address_col: str = 'address',
    city_col: str = 'city',
    state_col: str = 'state',
    zipcode_col: str = 'zipcode',
    chunk_size: int = 9999,
) -> pd.DataFrame:
    """
    Geocode a DataFrame using the U.S. Census batch geocoder.
    Automatically handles datasets larger than 10,000 rows by chunking.

    Returns DataFrame with the documented batch-endpoint fields:
        id, address, match, matchtype, parsed, tigerlineid, side, lat, lon

    The batch endpoint does not return state/county/tract FIPS codes.
    For census-geography output, use the per-address helper below
    (`cg.onelineaddress(addr, returntype='geographies')`) on the matched
    rows, or call `cg.address(...)` per row.
    """
    col_map = {id_col: 'id', address_col: 'address', city_col: 'city'}
    if state_col and state_col in df.columns:
        col_map[state_col] = 'state'
    if zipcode_col and zipcode_col in df.columns:
        col_map[zipcode_col] = 'zip'

    renamed_df = df.rename(columns=col_map)
    records = renamed_df.to_dict('records')

    if len(records) <= chunk_size:
        return pd.DataFrame(cg.addressbatch(records))

    all_results = []
    for i in range(0, len(records), chunk_size):
        chunk = records[i:i + chunk_size]
        print(f"Geocoding rows {i:,} to {i + len(chunk):,} of {len(records):,}...")
        try:
            all_results.extend(cg.addressbatch(chunk))
        except Exception as e:
            print(f"Error on chunk starting at {i}: {e}")
            for record in chunk:
                all_results.append({**record, 'match': False, 'lat': None, 'lon': None})

    return pd.DataFrame(all_results)

# Usage:
geocoded = (pd
              .read_csv('../data/raw/addresses.csv')
              .assign(id=lambda x: x.index)
              .pipe(census_geocode,
                    id_col='id',
                    address_col='street',
                    city_col='city',
                    state_col='state',
                    zipcode_col='zip'))

# Follow-up per-address lookup for census geographies (state/county/tract FIPS).
# The batch endpoint above does not return these — only lat/lon and match status.
def add_geographies(geocoded: pd.DataFrame) -> pd.DataFrame:
    """For each matched row, fetch census-geography FIPS via per-address API."""
    fips_rows = []
    for row in geocoded[geocoded['match']].itertuples():
        try:
            geo = cg.onelineaddress(row.address, returntype='geographies')
            block = geo[0]['geographies']['Census Blocks'][0]
            fips_rows.append({
                'id': row.id,
                'state_fips': block['STATE'],
                'county_fips': block['COUNTY'],
                'tract': block['TRACT'],
                'block': block['BLOCK'],
            })
        except (IndexError, KeyError):
            continue
    return geocoded.merge(pd.DataFrame(fips_rows), on='id', how='left')
```

#### Google Maps Geocoder

**Best for:** International addresses, high match rates, and messy/non-standard address formats.

**Pros:** Excellent match rates even for poorly formatted addresses. Works worldwide. Fast and reliable. Returns rich metadata (place types, address components, place IDs).

**Cons:** Costs money ($5 per 1,000 requests after free tier). Requires API key and billing account. Does not return Census geography—you'd need to do a separate spatial join.

**Use when:** You need to geocode international addresses, have messy address data that the Census geocoder can't match, or need the highest possible match rate and have budget for it.

```python
import googlemaps
from typing import Optional

def geocode_address_google(address: str, api_key: str) -> Optional[dict]:
    """
    Geocode address using Google Maps API.
    Requires API key with Geocoding API enabled.
    """
    gmaps = googlemaps.Client(key=api_key)
    result = gmaps.geocode(address)
    
    if result:
        location = result[0]['geometry']['location']
        return {
            'formatted_address': result[0]['formatted_address'],
            'lat': location['lat'],
            'lon': location['lng'],
            'place_id': result[0]['place_id']
        }
    return None

# Batch geocode a DataFrame
def batch_geocode(df: pd.DataFrame, address_col: str, api_key: str) -> pd.DataFrame:
    gmaps = googlemaps.Client(key=api_key)
    
    results = []
    for address in df[address_col]:
        try:
            result = gmaps.geocode(address)
            if result:
                loc = result[0]['geometry']['location']
                results.append({'lat': loc['lat'], 'lon': loc['lng']})
            else:
                results.append({'lat': None, 'lon': None})
        except Exception:
            results.append({'lat': None, 'lon': None})
    
    return pd.concat([df, pd.DataFrame(results)], axis=1)
```

### Geopandas
```python
import geopandas as gpd
import pandas as pd
from shapely.geometry import Point

# Read data from various formats
gdf = gpd.read_file('data.geojson')                    # GeoJSON
gdf = gpd.read_file('data.shp')                         # Shapefile
gdf = gpd.read_file('https://example.com/data.geojson') # From URL
gdf = gpd.read_parquet('data.parquet')                  # GeoParquet (fast!)

# Transform DataFrame with lat/lon to GeoDataFrame
df = pd.read_csv('locations.csv')
geometry = [Point(xy) for xy in zip(df['longitude'], df['latitude'])]
gdf = gpd.GeoDataFrame(df, geometry=geometry)

# Set CRS (Coordinate Reference System)
# EPSG:4326 = WGS84 (standard latitude, longitude)
gdf = gdf.set_crs('EPSG:4326')

# Transform to different CRS (for area/distance calculations, use projected CRS)
gdf_projected = gdf.to_crs('EPSG:3857')  # Web Mercator, for distance in meters

# Basic spatial operations

#Find the area of a shape
gdf['area'] = gdf_projected.geometry.area 

#Find the center of a shape
gdf['centroid'] = gdf.geometry.centroid

#Draw a 1km boundary around a point
gdf['buffer_1km'] = gdf_projected.geometry.buffer(1000) #when set to CRS 3857

# Spatial join: find points within polygons
points = gpd.read_file('points.geojson')
polygons = gpd.read_file('boundaries.geojson')
joined = gpd.sjoin(points, polygons, predicate='within')

# Dissolve: merge geometries by attribute
dissolved = gdf.dissolve(by='state', aggfunc='sum')

# Export to various formats
gdf.to_parquet('output.parquet')          # GeoParquet (recommended)
gdf.to_file('output.geojson', driver='GeoJSON') #for tools that dont support GeoParquet
```

### Geo-Visualization with `.explore()`, `lonboard` and Datawrapper

#### `.explore()`

**Best for:** Quick exploration and prototyping during data analysis.

**Pros:** Built into GeoPandas—method is available on any GeoDataFrame. Great for exploratory data analysis—checking that your data looks right, exploring spatial patterns, and iterating quickly on map designs.

**Cons:** Becomes slow with large datasets (>100k features). Limited customization compared to dedicated mapping libraries. Requires extra dependencies to be installed.

**Use when:** You're in the middle of analysis and want to quickly visualize your GeoDataFrame without switching tools.

**Required dependencies:**
```bash
pip install folium mapclassify matplotlib
```
- `folium` - Required for `.explore()` to work at all (renders the interactive map)
- `mapclassify` - Required when using `scheme=` parameter for classification (e.g., 'naturalbreaks', 'quantiles', 'equalinterval')
- `matplotlib` - Required for colormap (`cmap=`) support

```python
import geopandas as gpd
# folium, mapclassify, and matplotlib must be installed but don't need to be imported
# geopandas imports them automatically when you call .explore()

# Basic interactive map (uses folium under the hood)
gdf.explore()

# Choropleth map with customization
# (requires mapclassify for scheme parameter)
gdf.explore(
    column='population',           # Column for color scale
    cmap='YlOrRd',                 # Matplotlib colormap
    scheme='naturalbreaks',        # Classification scheme (needs mapclassify)
    k=5,                           # Number of bins
    legend=True,
    tooltip=['name', 'population'],  # Columns to show on hover
    popup=True,                    # Show all columns on click
    tiles='CartoDB positron',      # Background tiles
    style_kwds={'color': 'black', 'weight': 0.5}  # Border style
)
```

#### `lonboard`

**Best for:** Large datasets and high-performance visualization in Jupyter notebooks.

**Pros:** GPU-accelerated rendering via deck.gl can handle millions of points smoothly. Excellent interactivity—pan, zoom, and hover work fluidly even with massive datasets. Native support for GeoArrow format for efficient data transfer.

**Cons:** Requires separate installation (`pip install lonboard`). Styling options are more technical (RGBA arrays, deck.gl conventions). 

**Use when:** You have large point datasets (crime incidents, sensor readings, business locations) or need smooth interactivity with 100k+ features.

```python
import geopandas as gpd
from lonboard import viz, Map, ScatterplotLayer, PolygonLayer

# Quick visualization (auto-detects geometry type)
viz(gdf)

# Custom ScatterplotLayer for points
layer = ScatterplotLayer.from_geopandas(
    gdf,
    get_radius=100,
    get_fill_color=[255, 0, 0, 200],  # RGBA
    pickable=True
)
m = Map(layer)
m

# PolygonLayer with color based on column
from lonboard.colormap import apply_continuous_cmap
import matplotlib.pyplot as plt

colors = apply_continuous_cmap(gdf['value'], plt.cm.viridis)
layer = PolygonLayer.from_geopandas(
    gdf,
    get_fill_color=colors,
    get_line_color=[0, 0, 0, 100],
    pickable=True
)
Map(layer)
```

#### Datawrapper

**Best for:** Publication-ready choropleth and proportional symbol maps for articles and reports.

**Pros:** Beautiful, professional defaults out of the box. Generates embeddable, responsive iframes that work in any CMS. Readers can interact (hover, click) without running any code. Accessible and mobile-friendly. Easy to update data programmatically for updating data.

**Cons:** Requires a Datawrapper account (free tier available). Limited to Datawrapper's supported boundary files—you can't bring arbitrary geometries. Less flexibility for custom visualizations.

**Use when:** You need a polished map for publication. Ideal for choropleth maps showing statistics by region (unemployment by state, COVID cases by county, election results by district). Your audience will view the map in a browser, not a notebook.

Unlike `.explore()` or `lonboard`, you don't pass raw geometry—instead you match your data to Datawrapper's built-in boundary files using standard codes (FIPS, ISO, etc.).

```python
import datawrapper as dw
import pandas as pd

# Read API key
with open('datawrapper_api_key.txt', 'r') as f:
    api_key = f.read().strip()

# Prepare data with location codes that match Datawrapper's boundaries
# For US states: use 2-letter abbreviations or FIPS codes
# For countries: use ISO 3166-1 alpha-2 codes
df = pd.DataFrame({
    'state': ['AL', 'AK', 'AZ', 'AR', 'CA'],  # State abbreviations
    'unemployment_rate': [4.9, 3.2, 7.1, 4.2, 5.8]
})

# Create a choropleth map
chart = dw.ChoroplethMap(
    title='Unemployment Rate by State',
    intro='Percentage of labor force unemployed, 2024',
    data=df,

    # Map configuration
    basemap='us-states',           # Built-in US states boundaries
    basemap_key='state',           # Column in your data with location codes
    value_column='unemployment_rate',

    # Styling
    color_palette='YlOrRd',        # Color scheme
    legend_title='Unemployment %',

    # Attribution
    source_name='Bureau of Labor Statistics',
    source_url='https://www.bls.gov/',
    byline='Your Name'
)

# Create and publish
chart.create(access_token=api_key)
chart.publish()

# Get embed code for your article
iframe = chart.get_iframe_code(responsive=True)
print(f"Chart URL: https://datawrapper.dwcdn.net/{chart.chart_id}")

# Update with new data (for live-updating maps)
new_df = pd.DataFrame({...})  # Updated data
existing_chart = dw.get_chart('YOUR_CHART_ID')
existing_chart.data = new_df
existing_chart.update()
existing_chart.publish()
```

**Available Datawrapper basemaps include:**
- `us-states`, `us-counties`, `us-congressional-districts`
- `world`, `europe`, `africa`, `asia`
- Country-specific maps (e.g., `germany-states`, `uk-constituencies`)

### Learning resources

- **NICAR** — Annual data journalism conference hosted by Investigative Reporters and Editors (IRE).
- **Knight Center for Journalism in the Americas** — Free MOOCs in English, Spanish, and Portuguese ([journalismcourses.org](https://journalismcourses.org/)).
- **Data Journalism Handbook 2: Towards a Critical Data Practice** (Bounegru and Gray) — current canonical edition ([datajournalism.com/read/handbook/two](https://datajournalism.com/read/handbook/two)).
- **Flowing Data** ([flowingdata.com](https://flowingdata.com)) — Nathan Yau's examples and tutorials.
- **The Pudding** ([pudding.cool](https://pudding.cool)) — examples of editorial data essays.
- **Sigma Awards** ([sigmaawards.org](https://sigmaawards.org/)) — annual awards, hosted by GIJN since 2024.
- **Related skills** — for fact-checking computed numbers see **fact-check-workflow**; for FOIA-ing datasets see **foia-requests**; for verifying social-media-derived data see **source-verification**; for OSINT and platform-API context see **social-media-intelligence**.

---

## Skill metadata

| Field | Value |
|-------|-------|
| version | 1.1.0 |
| created | 2025-12-26 |
| updated | 2026-05-08 |
| author | Joe Amditis |
| domain | journalism, data |
| complexity | intermediate |