Exploratory data analysis

Exploratory Data Analysis

Understand a dataset before trusting it with a model or a decision. The output of EDA is not a pile of statistics — it is a documented understanding: what the data is, what's wrong or surprising in it, and a set of explicit hypotheses to test next. Every step below feeds that document.

Workflow

1. Frame the goal and the grain

Before profiling anything, write down what decision or model this data will feed, and establish the grain: what does one row represent (an order? a customer? a customer-month?)? Duplicate keys and mixed grains invalidate every later statistic. If the eventual use is prediction, note when the prediction would be made — that timestamp drives the leakage checks in step 5.

2. Profile structurally

Column types, ranges, cardinality, and summary statistics for every column — cheaply, before any deep dive. Triage wide tables by metadata first (null rate, cardinality, type) and prioritize columns by relevance to the goal; don't profile 400 columns equally.

If the data doesn't fit in memory: push aggregations down to the warehouse (SQL), or use an out-of-core/lazy engine (DuckDB, Polars). When you sample, sample representatively — random or stratified, never head() (files are usually ordered by time or id) — and state the caveat that rare events and extreme outliers can be missed under sampling; verify those against the full data with targeted queries.

3. Map the missingness structure

Null counts are the start, not the answer. Distinguish:

• Random gaps — sporadic nulls, roughly uniform across segments.
• Structural missingness — a column populated only for some segment, after some date, or by some source system. Cross-tab null rates against segments and time to find these.
• Informative missingness — where the fact that a value is missing predicts the outcome (e.g., income missing for churned users). Flag it; it may be a feature, or it may be leakage.

Also check for disguised missing values: sentinel codes (0, -1, 999, "unknown", empty string) that aren't NULL.

4. Distributions and outliers

For key numeric columns: quantiles, histograms, and an outlier pass (IQR or similar). For categoricals: frequency tables and rare-level counts. Decide for each outlier cluster whether it is a data error, a unit mismatch, or a genuine tail — never drop outliers before classifying them; genuine tails are often the interesting part. Check skew before assuming any statistic (mean, std) is representative.

5. Target definition, balance, and leakage

If the data will feed a model:

• Define the target explicitly (e.g., "churn = no order in 90 days after cutoff") and check its class balance — a 2% positive rate changes the whole modeling and evaluation approach downstream.
• Hunt for leakage: any column recorded at-or-after the outcome (cancellation reason, final status, updated_at aggregates), and proxies that encode the outcome indirectly. Test: "would this value exist at prediction time?" If unsure, trace how the column is produced. A too-good correlation with the target is a leakage smell, not a win.

6. Relationships, not just profiles

Univariate profiles alone don't generate hypotheses. Examine bivariate structure: correlations among candidate features, and each candidate feature against the target (grouped rates, means by decile, simple cross-tabs). Note confounders — a feature↔target association may be explained by segment or time. This is where most real hypotheses come from.

7. Write the hypotheses and hand off

Close with a short findings document: data issues found, columns excluded and why (especially leakage suspects), and explicit hypotheses phrased as testable statements ("weekend signups churn more", "the price effect is driven by segment X") plus open questions for the data owners. Formal testing of those hypotheses is statistical-analysis territory; feature/training pipeline work is ml-pipeline-design.

Principles

• KISS: value counts and group-by rates beat clever visualizations for finding problems fast.
• YAGNI: profile to the goal — an exhaustive 400-column report nobody reads is not EDA.
• Honest accounting: record what you did NOT check (columns skipped, sample-size limits) in the findings doc.

Cross-skill boundaries

• Bad data in a pipeline or dashboard (freshness, contracts, dbt tests, lineage) → data-quality — that's an operational trust problem, not exploration of an unfamiliar dataset.
• Formal hypothesis testing, experiment design, significance → statistical-analysis.
• Turning the analysis into scheduled production code → notebook-to-production.
• Schema design for storing the data → data-modeling.