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Starting an Exploratory Data Analysis

Lesson 1: Starting an Exploratory Data Analysis

What is EDA?

EDA is an initial investigation of a dataset using summaries, tables, charts, and questions.

EDA does

EDA doesn't

Reveal structure and quality issuesProve causes
Generate hypothesesReplace formal testing
Guide which analysis to runProduce final conclusions on its own

A repeatable checklist

Stage

Check

pandas example

StructureRows, columns, namesdf.shape, df.columns
TypesCorrect data typesdf.dtypes
MissingnessGaps per columndf.isna().sum()
DuplicatesRepeated rows or keysdf.duplicated().sum()
UniquenessDistinct valuesdf.nunique()
Numeric summaryMin, max, mean, quartilesdf.describe()
CategoriesFrequencies, odd labelsdf['col'].value_counts()
Univariate chartsDistribution of each key variableHistograms, bar charts
Bivariate checksRelationships between variablesScatter plots, grouped summaries
Time checksGaps, spikes, trendsLine chart of daily counts

Order: broad checks → focused questions.

Example

Customer churn dataset:

Step

What to inspect

OverallChurn rate (e.g., 18% of customers left)
By contract typeMonthly vs annual churn
By tenureDo newer customers churn more?
By monthly chargesDistribution for churned vs retained customers
Missing valuesWhich fields have gaps?

Remember: differences between customers who left and stayed don't prove why they left.

How it works

  • Move from broad checks to focused questions.
  • Keep a running EDA log:
    • Surprising values
    • Assumptions made
    • Checks still needed
    • Questions for the data owner
  • Revisit early assumptions as you learn more.

Helpful tools: automated profiling libraries such as ydata-profiling can generate a quick overview report. Review the results critically; don't accept them blindly.

Common mistakes

  • Confirmation bias: searching only for patterns that support an initial belief.
  • Ignoring alternatives: not testing whether another factor explains the pattern.
  • Not checking subsets: a pattern in the whole dataset may disappear or reverse in segments.
  • Skipping EDA entirely and jumping to modelling.

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