Validate Data

Workflow

1. Review Methodology and Assumptions

Examine:

• Question framing: Is the analysis answering the right question? Could the question be interpreted differently?
• Data selection: Are the right tables/datasets being used? Is the time range appropriate?
• Population definition: Is the analysis population correctly defined? Are there unintended exclusions?
• Metric definitions: Are metrics defined clearly and consistently? Do they match how stakeholders understand them?
• Baseline and comparison: Is the comparison fair? Are time periods, cohort sizes, and contexts comparable?

2. Run the Pre-Delivery QA Checklist

Work through the checklist below — data quality, calculation, reasonableness, and presentation checks.

3. Check for Common Analytical Pitfalls

Systematically review against the detailed pitfall catalog below (join explosion, survivorship bias, incomplete period comparison, denominator shifting, average of averages, timezone mismatches, selection bias).

4. Verify Calculations and Aggregations

Where possible, spot-check:

• Recalculate a few key numbers independently
• Verify that subtotals sum to totals
• Check that percentages sum to 100% (or close to it) where expected
• Confirm that YoY/MoM comparisons use the correct base periods
• Validate that filters are applied consistently across all metrics

Apply the result sanity-checking techniques below (magnitude checks, cross-validation, red-flag detection).

5. Assess Visualizations

If the analysis includes charts:

• Do axes start at appropriate values (zero for bar charts)?
• Are scales consistent across comparison charts?
• Do chart titles accurately describe what's shown?
• Could the visualization mislead a quick reader?
• Are there truncated axes, inconsistent intervals, or 3D effects that distort perception?

6. Evaluate Narrative and Conclusions

Review whether:

• Conclusions are supported by the data shown
• Alternative explanations are acknowledged
• Uncertainty is communicated appropriately
• Recommendations follow logically from findings
• The level of confidence matches the strength of evidence

7. Suggest Improvements

Provide specific, actionable suggestions:

• Additional analyses that would strengthen the conclusions
• Caveats or limitations that should be noted
• Better visualizations or framings for key points
• Missing context that stakeholders would want

8. Generate Confidence Assessment

Rate the analysis on a 3-level scale:

Ready to share -- Analysis is methodologically sound, calculations verified, caveats noted. Minor suggestions for improvement but nothing blocking.

Share with noted caveats -- Analysis is largely correct but has specific limitations or assumptions that must be communicated to stakeholders. List the required caveats.

Needs revision -- Found specific errors, methodological issues, or missing analyses that should be addressed before sharing. List the required changes with priority order.

Output Format

## Validation Report

### Overall Assessment: [Ready to share | Share with caveats | Needs revision]

### Methodology Review
[Findings about approach, data selection, definitions]

### Issues Found
1. [Severity: High/Medium/Low] [Issue description and impact]
2. ...

### Calculation Spot-Checks
- [Metric]: [Verified / Discrepancy found]
- ...

### Visualization Review
[Any issues with charts or visual presentation]

### Suggested Improvements
1. [Improvement and why it matters]
2. ...

### Required Caveats for Stakeholders
- [Caveat that must be communicated]
- ...

Pre-Delivery QA Checklist

Run through this checklist before sharing any analysis with stakeholders.

Data Quality Checks

• Source verification: Confirmed which tables/data sources were used. Are they the right ones for this question?
• Freshness: Data is current enough for the analysis. Noted the "as of" date.
• Completeness: No unexpected gaps in time series or missing segments.
• Null handling: Checked null rates in key columns. Nulls are handled appropriately (excluded, imputed, or flagged).
• Deduplication: Confirmed no double-counting from bad joins or duplicate source records.
• Filter verification: All WHERE clauses and filters are correct. No unintended exclusions.

Calculation Checks

• Aggregation logic: GROUP BY includes all non-aggregated columns. Aggregation level matches the analysis grain.
• Denominator correctness: Rate and percentage calculations use the right denominator. Denominators are non-zero.
• Date alignment: Comparisons use the same time period length. Partial periods are excluded or noted.
• Join correctness: JOIN types are appropriate (INNER vs LEFT). Many-to-many joins haven't inflated counts.
• Metric definitions: Metrics match how stakeholders define them. Any deviations are noted.
• Subtotals sum: Parts add up to the whole where expected. If they don't, explain why (e.g., overlap).

Reasonableness Checks

• Magnitude: Numbers are in a plausible range. Revenue isn't negative. Percentages are between 0-100%.
• Trend continuity: No unexplained jumps or drops in time series.
• Cross-reference: Key numbers match other known sources (dashboards, previous reports, finance data).
• Order of magnitude: Total revenue is in the right ballpark. User counts match known figures.
• Edge cases: What happens at the boundaries? Empty segments, zero-activity periods, new entities.

Presentation Checks

• Chart accuracy: Bar charts start at zero. Axes are labeled. Scales are consistent across panels.
• Number formatting: Appropriate precision. Consistent currency/percentage formatting. Thousands separators where needed.
• Title clarity: Titles state the insight, not just the metric. Date ranges are specified.
• Caveat transparency: Known limitations and assumptions are stated explicitly.
• Reproducibility: Someone else could recreate this analysis from the documentation provided.

Common Data Analysis Pitfalls

Join Explosion

The problem: A many-to-many join silently multiplies rows, inflating counts and sums.

How to detect:

-- Check row count before and after join
SELECT COUNT(*) FROM table_a;  -- 1,000
SELECT COUNT(*) FROM table_a a JOIN table_b b ON a.id = b.a_id;  -- 3,500 (uh oh)

How to prevent:

• Always check row counts after joins
• If counts increase, investigate the join relationship (is it really 1:1 or 1:many?)
• Use COUNT(DISTINCT a.id) instead of COUNT(*) when counting entities through joins

Survivorship Bias

The problem: Analyzing only entities that exist today, ignoring those that were deleted, churned, or failed.

Examples:

• Analyzing user behavior of "current users" misses churned users
• Looking at "companies using our product" ignores those who evaluated and left
• Studying properties of "successful" outcomes without "unsuccessful" ones

How to prevent: Ask "who is NOT in this dataset?" before drawing conclusions.

Incomplete Period Comparison

The problem: Comparing a partial period to a full period.

Examples:

• "January revenue is $500K vs. December's $800K" -- but January isn't over yet
• "This week's signups are down" -- checked on Wednesday, comparing to a full prior week

How to prevent: Always filter to complete periods, or compare same-day-of-month / same-number-of-days.

Denominator Shifting

The problem: The denominator changes between periods, making rates incomparable.

Examples:

• Conversion rate improves because you changed how you count "eligible" users
• Churn rate changes because the definition of "active" was updated

How to prevent: Use consistent definitions across all compared periods. Note any definition changes.

Average of Averages

The problem: Averaging pre-computed averages gives wrong results when group sizes differ.

Example:

• Group A: 100 users, average revenue $50
• Group B: 10 users, average revenue $200
• Wrong: Average of averages = ($50 + $200) / 2 = $125
• Right: Weighted average = (100*$50 + 10*$200) / 110 = $63.64

How to prevent: Always aggregate from raw data. Never average pre-aggregated averages.

Timezone Mismatches

The problem: Different data sources use different timezones, causing misalignment.

Examples:

• Event timestamps in UTC vs. user-facing dates in local time
• Daily rollups that use different cutoff times

How to prevent: Standardize all timestamps to a single timezone (UTC recommended) before analysis. Document the timezone used.

Selection Bias in Segmentation

The problem: Segments are defined by the outcome you're measuring, creating circular logic.

Examples:

• "Users who completed onboarding have higher retention" -- obviously, they self-selected
• "Power users generate more revenue" -- they became power users BY generating revenue

How to prevent: Define segments based on pre-treatment characteristics, not outcomes.

Other Statistical Traps

• Simpson's paradox: Trend reverses when data is aggregated vs. segmented
• Correlation presented as causation without supporting evidence
• Small sample sizes leading to unreliable conclusions
• Outliers disproportionately affecting averages (should medians be used instead?)
• Multiple testing / cherry-picking significant results
• Look-ahead bias: Using future information to explain past events
• Cherry-picked time ranges that favor a particular narrative

Result Sanity Checking

Magnitude Checks

For any key number in your analysis, verify it passes the "smell test":

Cross-Validation Techniques

1. Calculate the same metric two different ways and verify they match
2. Spot-check individual records -- pick a few specific entities and trace their data manually
3. Compare to known benchmarks -- match against published dashboards, finance reports, or prior analyses
4. Reverse engineer -- if total revenue is X, does per-user revenue times user count approximately equal X?
5. Boundary checks -- what happens when you filter to a single day, a single user, or a single category? Are those micro-results sensible?

Red Flags That Warrant Investigation

• Any metric that changed by more than 50% period-over-period without an obvious cause
• Counts or sums that are exact round numbers (suggests a filter or default value issue)
• Rates exactly at 0% or 100% (may indicate incomplete data)
• Results that perfectly confirm the hypothesis (reality is usually messier)
• Identical values across time periods or segments (suggests the query is ignoring a dimension)

Documentation Standards for Reproducibility

Analysis Documentation Template

Every non-trivial analysis should include:

## Analysis: [Title]

### Question
[The specific question being answered]

### Data Sources
- Table: [schema.table_name] (as of [date])
- Table: [schema.other_table] (as of [date])
- File: [filename] (source: [where it came from])

### Definitions
- [Metric A]: [Exactly how it's calculated]
- [Segment X]: [Exactly how membership is determined]
- [Time period]: [Start date] to [end date], [timezone]

### Methodology
1. [Step 1 of the analysis approach]
2. [Step 2]
3. [Step 3]

### Assumptions and Limitations
- [Assumption 1 and why it's reasonable]
- [Limitation 1 and its potential impact on conclusions]

### Key Findings
1. [Finding 1 with supporting evidence]
2. [Finding 2 with supporting evidence]

### SQL Queries
[All queries used, with comments]

### Caveats
- [Things the reader should know before acting on this]

Code Documentation

For any code (SQL, Python) that may be reused:

"""
Analysis: Monthly Cohort Retention
Author: [Name]
Date: [Date]
Data Source: events table, users table
Last Validated: [Date] -- results matched dashboard within 2%

Purpose:
    Calculate monthly user retention cohorts based on first activity date.

Assumptions:
    - "Active" means at least one event in the month
    - Excludes test/internal accounts (user_type != 'internal')
    - Uses UTC dates throughout

Output:
    Cohort retention matrix with cohort_month rows and months_since_signup columns.
    Values are retention rates (0-100%).
"""

Version Control for Analyses

• Save queries and code in version control (git) or a shared docs system
• Note the date of the data snapshot used
• If an analysis is re-run with updated data, document what changed and why
• Link to prior versions of recurring analyses for trend comparison

Examples

/validate-data Review this quarterly revenue analysis before I send it to the exec team: [analysis]

/validate-data Check my churn analysis -- I'm comparing Q4 churn rates to Q3 but Q4 has a shorter measurement window

/validate-data Here's a SQL query and its results for our conversion funnel. Does the logic look right? [query + results]

Tips

• Run /validate-data before any high-stakes presentation or decision
• Even quick analyses benefit from a sanity check -- it takes a minute and can save your credibility
• If the validation finds issues, fix them and re-validate
• Share the validation output alongside your analysis to build stakeholder confidence