Data Analysis

Reference Materials and Scripts

• Execute profile_data.py with a data file path to get a quick profile of any CSV, Excel, or JSON dataset — it reports shape, types, missing values, stats, and value distributions. Run with --help for usage.
• Load statistical-tests-guide.md when choosing statistical tests — it has a decision matrix for test selection, effect size interpretation tables, and sample size guidelines.

Understand the Data

Before analysis, get your bearings:

1. Source and context: Where did this data come from? How was it collected? What time period does it cover?
2. Schema: What are the columns/fields? What do they represent? What are the data types?
3. Scale: How many rows/records? What's the granularity? (Per user? Per day? Per transaction?)
4. Known issues: Is the data known to be incomplete, biased, or have quality problems?

# First look at any dataset
import pandas as pd

df = pd.read_csv("data.csv")  # or read_excel, read_json, etc.
print(f"Shape: {df.shape}")
print(f"\nColumn types:\n{df.dtypes}")
print(f"\nFirst rows:\n{df.head()}")
print(f"\nMissing values:\n{df.isnull().sum()}")
print(f"\nBasic stats:\n{df.describe()}")

Analysis Workflow

Step 1: Clean and Validate

Data quality determines analysis quality. Don't skip this.

Handle Missing Values

• Count them first: What percentage of each column is missing?
• Understand why: Are they random? Systematic? (e.g., optional fields vs data collection failures)
• Choose a strategy and document it:
  • Drop rows: When missing data is rare and random (less than 5%)
  • Impute with median/mode: When missing data is moderate and the distribution is known
  • Flag as separate category: When missingness itself is informative
  • Leave as-is: When the analysis method handles nulls natively

# Document your decisions
missing_pct = df.isnull().sum() / len(df) * 100
print("Missing data percentage per column:")
print(missing_pct[missing_pct > 0].sort_values(ascending=False))

Handle Outliers

• Detect: Use IQR method, z-scores, or domain knowledge
• Investigate: Are they errors or legitimate extreme values?
• Document your decision: Keep, cap, or remove — and explain why

# IQR method for outlier detection
Q1 = df['value'].quantile(0.25)
Q3 = df['value'].quantile(0.75)
IQR = Q3 - Q1
outliers = df[(df['value'] < Q1 - 1.5 * IQR) | (df['value'] > Q3 + 1.5 * IQR)]
print(f"Found {len(outliers)} outliers ({len(outliers)/len(df)*100:.1f}%)")

Validate Data Types and Ranges

• Dates should be dates, numbers should be numbers
• Check for impossible values (negative ages, future dates, percentages over 100)
• Verify categorical values are consistent (watch for "USA", "US", "United States")

Step 2: Explore

Start broad, then focus on what's interesting.

Descriptive Statistics

Always start here — understand the basics before going deeper.

# Numerical columns
print(df.describe())

# Categorical columns
for col in df.select_dtypes(include='object').columns:
    print(f"\n{col}: {df[col].nunique()} unique values")
    print(df[col].value_counts().head(10))

Distributions

Understanding shape matters for choosing the right tests.

import matplotlib.pyplot as plt

# Distribution of key metrics
fig, axes = plt.subplots(1, 3, figsize=(15, 5))
for i, col in enumerate(['metric_a', 'metric_b', 'metric_c']):
    df[col].hist(ax=axes[i], bins=30)
    axes[i].set_title(col)
    axes[i].axvline(df[col].median(), color='red', linestyle='--', label='median')
    axes[i].legend()
plt.tight_layout()
plt.savefig("distributions.png")

Correlations and Relationships

Look for patterns between variables.

# Correlation matrix for numerical columns
corr = df.select_dtypes(include='number').corr()
print("Strong correlations (|r| > 0.5):")
for i in range(len(corr.columns)):
    for j in range(i+1, len(corr.columns)):
        if abs(corr.iloc[i, j]) > 0.5:
            print(f"  {corr.columns[i]} vs {corr.columns[j]}: {corr.iloc[i,j]:.3f}")

Trends Over Time

If the data has a time dimension, always look at trends.

# Time series analysis
df['date'] = pd.to_datetime(df['date'])
daily = df.groupby('date')['metric'].agg(['mean', 'count'])
daily['mean'].plot(figsize=(12, 4), title='Daily Average')
plt.savefig("trend.png")

Step 3: Analyze

Choose the right method for the question.

Comparison Questions ("Is A different from B?")

Use the right statistical test:

• Two groups, continuous outcome: t-test (if normal) or Mann-Whitney U (if not)
• Multiple groups: ANOVA (if normal) or Kruskal-Wallis (if not)
• Two groups, categorical outcome: Chi-squared test
• Before/after with same subjects: Paired t-test or Wilcoxon signed-rank

Always report:

• Sample sizes for each group
• Effect size (not just p-value) — Cohen's d, odds ratio, or percentage difference
• Confidence intervals
• Practical significance, not just statistical significance

from scipy import stats

# Example: comparing two groups
group_a = df[df['variant'] == 'A']['metric']
group_b = df[df['variant'] == 'B']['metric']

# Check normality first
_, p_normal_a = stats.shapiro(group_a.sample(min(5000, len(group_a))))
_, p_normal_b = stats.shapiro(group_b.sample(min(5000, len(group_b))))

if p_normal_a > 0.05 and p_normal_b > 0.05:
    stat, p_value = stats.ttest_ind(group_a, group_b)
    test_name = "t-test"