version: "1.0.0" name: Data analysis description: Explore, clean, and draw conclusions from a dataset: profiling it, handling missing and malformed values, aggregating, and reporting findings honestly. Use when handed a CSV, spreadsheet, or query result and asked what it shows.

Data analysis

Profile before you analyse

Every wrong analysis starts with an assumption about the data that was never checked:

df.shape
df.dtypes
df.head(20)
df.isna().sum()
df.describe(include="all")
for c in df.select_dtypes("object"): print(c, df[c].nunique(), df[c].unique()[:8])
df.duplicated().sum()

What you are looking for: numeric columns typed as object (the tell for numbers stored as text), dates as strings, categoricals with near-duplicate values ("NY", "ny", "New York"), impossible values (negative ages, future dates), and how much is missing.

Missing data

Find out why it is missing before deciding what to do. Missing at random and missing because the sensor was offline call for different handling. Dropping rows silently changes the population you are describing.

Say what you did. "Dropped 1,204 rows (12%) with no timestamp" belongs in the output, not just in the code.

Aggregation

• Check group sizes before comparing group means. A "group" with three rows is

noise, and it will produce the most extreme number in your table.

• Prefer the median for anything with a tail: income, latency, session length.

A mean latency is almost always the wrong number to quote.

• After a join, check the row count. Growth means a many-to-many you did not

expect; shrinkage means keys that did not match.

Reporting honestly

• Correlation is not causation, and the confounder is usually obvious once

named. Name it.

• Give the denominator. "40% increase" from 5 to 7 is not a finding.
• Report the uncertainty, or at least the sample size.
• State what would change the conclusion. That is what makes it useful rather

than decorative.

• If the data cannot answer the question, say so. That is a legitimate and

frequently correct result.

Plotting

Label the axes, include units, and start bar charts at zero. Use a chart to show a shape (trend, distribution, comparison); use a table for exact values. Do not use both for the same numbers.