Skill: Compare Datasets

Purpose

Compare metrics, findings, and patterns across two or more connected datasets. Helps identify cross-dataset patterns (e.g., "conversion funnel behavior is similar across both product lines") and dataset-specific anomalies.

When to Use

• User says /compare-datasets or "compare across datasets"
• After analyzing multiple datasets, to find commonalities
• When the user asks "is this pattern unique to this dataset?"

Invocation

/compare-datasets — compare active dataset with all others /compare-datasets {id1} {id2} — compare two specific datasets /compare-datasets metric={name} — compare a specific metric across datasets

Instructions

Step 0: Feasibility Check

Before running the full 6-step workflow, verify the comparison is possible:

1. Check minimum requirements:

   • At least 2 datasets must be connected (check .knowledge/datasets/)
   • If only 1 dataset: "Only one dataset connected. Use /connect-data to add another."

2. For data-intensive comparisons (e.g., retention, conversion funnels):

   • Verify temporal span: Each dataset needs sufficient history (e.g., 3-6 months for retention)
   • Verify sample size: Small datasets (<30 rows) may not produce meaningful comparisons
   • Verify schema compatibility: Key fields must exist in both datasets

3. If comparison is BLOCKED:

   • State why upfront: "Cannot compare retention — Dataset B has only 27 days of data vs. Dataset A's 12 months. Retention requires 3-6 months minimum."
   • Provide decision criteria: "We can compare when Dataset B has [specific requirement]."
   • Skip to Step 6 (Present Results) with a diagnostic report instead of attempting analysis

Rationale: This saves time and avoids misleading partial comparisons. Better to diagnose "why we can't compare" upfront than to discover data gaps in Step 4.

Step 1: Identify Datasets to Compare

1. Read .knowledge/datasets/ to enumerate all connected datasets.
2. If specific datasets are named, validate they exist.
3. If no datasets specified, use active + all others.
4. List datasets with their connection status (connected vs. not connected).

Step 2: Load Metric Dictionaries

For each dataset:

1. Read .knowledge/datasets/{id}/metrics/index.yaml
2. Build a union of all metric IDs across datasets
3. Identify shared metrics (same ID or same name) vs. dataset-specific metrics
4. If all metric dictionaries are empty: State this explicitly — "No formal metric definitions found in either dataset. Comparison will rely on schema-based inference."

Why this matters: Knowing upfront that metric dictionaries are empty helps explain why definitions might differ (they were never formalized).

Step 3: Compare Shared Metrics

For each metric that exists in 2+ datasets:

1. Load the metric YAML from each dataset
2. Compare: definition match? (same formula, same unit)
3. Compare: typical range overlap? (do the datasets have similar baselines?)
4. Compare: guardrails alignment? (are thresholds consistent?)
5. Flag discrepancies: "conversion_rate is defined differently in {dataset_a} vs {dataset_b}"

If no formal metrics exist: Reconstruct from schema and document the inferred definition. Show the reconstruction in the output so the user understands what was compared.

Step 4: Compare Analysis History

For each dataset:

1. Read .knowledge/analyses/index.yaml
2. Extract key findings from recent analyses
3. Look for cross-dataset patterns:
   • Same finding appearing in multiple datasets
   • Opposite findings (metric up in one, down in another)
   • Same root cause identified independently

Edge case handling:

• If Dataset A has analysis history but Dataset B does not, state this: "Dataset B has no analysis history yet. Comparison limited to schema and metric definitions."
• If neither dataset has analysis history, document this and focus on structural comparison (schemas, metric definitions).

Step 5: Generate Cross-Dataset Observations

Write findings to .knowledge/global/cross_dataset_observations.yaml:

---
comparison_date: YYYY-MM-DD
datasets_compared: [dataset_a, dataset_b]
comparison_type: [full | metric-specific | feasibility-blocked]

shared_patterns:
  - pattern: "Mobile conversion gap"
    datasets: [dataset_a, dataset_b]
    magnitude: "Desktop ~40% higher than mobile in both"
    confidence: HIGH  # HIGH | MEDIUM | LOW

divergences:
  - metric: revenue_per_user
    dataset_a_value: $313.64
    dataset_b_value: $113.98
    explanation: "Different business models (retail vs. wholesale) or dataset maturity"
    confidence: MEDIUM

metric_alignment:
  - metric: conversion_rate
    aligned: false
    discrepancy: "Different column names (order_total vs. total_amount)"

blockers:
  - issue: "Insufficient temporal span in Dataset B"
    severity: CRITICAL
    resolution: "Wait 3-6 months for data maturity"

suggested_investigations:
  - "Investigate why revenue_per_user differs by 175%"
  - "Align conversion_rate definitions across datasets"
  - "Profile analytics_prod and company_postgres to check for device tracking"

Write this file to disk with the Write tool; it is the persistent record of the comparison.

Why this file matters: It creates a persistent record of cross-dataset insights that future analyses can reference. If Dataset B matures later, we can re-run the comparison and see how findings evolved.

Step 6: Present Results

Display a comparison table in your response to the user:

Cross-Dataset Comparison: {dataset_a} vs {dataset_b}

Comparison Status: [COMPLETE | PARTIAL | BLOCKED]
[If BLOCKED or PARTIAL, explain why upfront]

Shared Metrics: {N} ({M} with matching definitions)
Metric Discrepancies: {list}

Shared Patterns:
  - {pattern description} (seen in both datasets, confidence: HIGH/MEDIUM/LOW)
  - [If sample size > 30 per group, report statistical significance: "p < 0.001" (highly significant)]

Divergences:
  - {metric} is {direction} in {dataset_a} but {direction} in {dataset_b}
  - Explanation: {why this might be real vs. artifact}
  - [If applicable, estimate business impact: "Fixing this could recover ~$X/month or Y conversions"]

Data Quality Issues:
  - {Any temporal inconsistencies, sample size issues, schema gaps}

Suggested Next:
  - "Investigate why {pattern} differs between datasets"
  - "Align {metric} definitions across datasets"
  - "Wait for Dataset B to mature before re-comparing" [if applicable]

Statistical Significance Guidance: When comparing metrics across datasets, report statistical significance where applicable:

• Conversion rates: Use chi-squared test if sample size > 30 per group
• Means (AOV, session duration): Use t-test or Mann-Whitney U test
• Reporting format: "p < 0.001" (highly significant), "p = 0.023" (significant at 0.05 level), or "p = 0.23" (not significant)
• Insufficient sample: If N < 30, state "Sample too small for significance test — treat results as directional only"

Business Impact Estimation: For shared friction points or significant divergences, estimate business impact when possible:

• Conversion lift: "Fixing mobile experience could add 40+ conversions/month" (based on session volume × gap × baseline conversion)
• Revenue impact: "Recovering high-value cart abandonment = ~$10K/month" (lost conversions × avg cart value)
• Use conservative ranges rather than point estimates to account for uncertainty

Comparison Status Line: Include this upfront. If BLOCKED or PARTIAL, explain why immediately so the user knows whether to trust the findings.

Edge Cases

Only 1 dataset connected

Standard response: Cannot compare — suggest connecting another via /connect-data.

Pragmatic alternative: If the active dataset has natural segments (product categories, regions, user cohorts), offer to compare those segments instead:

"Only 1 dataset is connected, but it has two main product categories (Electronics and Clothing). Would you like me to compare checkout conversion between these categories instead?"

This interpretation is valid when:

• User says "compare datasets" or "compare across product lines"
• Only 1 actual dataset exists in .knowledge/datasets/
• The dataset has clear categorical dimensions that represent distinct business units

Rationale: Comparing segments within a dataset can answer the same business question (e.g., "is this pattern universal or product-specific?") even when multiple datasets don't exist.

No shared metrics

Report this — datasets may serve different purposes. Focus comparison on analysis history and schema structure instead.

No analysis history

Compare schemas and metric definitions only. State: "Neither dataset has analysis history yet. This comparison is limited to structural alignment (schemas, metric definitions)."

Many datasets (>5)

Compare pairwise with the active dataset only. List all datasets but only run the 6-step workflow for active vs. each other dataset. Present results in a summary table at the end.

Insufficient data for comparison

Example: User asks to compare retention curves but Dataset B has only 27 days of data vs. Dataset A's 12 months.

Response:

1. Diagnose the blocker upfront (Step 0)
2. State why comparison is impossible: "Retention curves require 3-6 months of data. Dataset B has 27 days."
3. Skip analysis steps and go straight to diagnostic report
4. Provide decision framework: "We can compare when Dataset B has [X months] of data and [Y sample size]."
5. Still write the cross_dataset_observations.yaml with the comparison_type: feasibility-blocked flag

Rationale: Better to clearly state "we can't compare yet" than to produce misleading partial results.

Data quality issues

Example: Dataset B has future-dated records, customer ID mismatches across tables, or temporal inconsistencies.

Response:

1. Flag the data quality issue in Step 0 or Step 1
2. Include a "Data Quality Issues" section in the output
3. Provide diagnostic queries the user can run to investigate (e.g., SELECT order_date, TYPEOF(order_date) FROM orders LIMIT 10;)
4. Recommend fixing data quality before re-attempting comparison

Rationale: Data quality issues can make comparisons meaningless. Better to diagnose and fix the source data than to compare bad data.