Data Quality Skill

Systematic approach to diagnosing and fixing data quality problems.

Data Quality Process

Define & Identify → Detect & Quantify → Clean & Rectify → Measure & Verify

1. Define: Understand data context, business rules, quality requirements
2. Detect: Profile data, find glitches (missing, duplicates, outliers, violations)
3. Clean: Apply appropriate repair strategies
4. Measure: Validate repairs, quantify improvement

Quick Reference

Problem	Script	Key Function
Data overview	data_profiling.py	profile_dataframe(df)
Find quality issues	data_profiling.py	detect_glitches(df)
Missing values	missing_data.py	analyze_missing(df)
Imputation	missing_data.py	impute_mean/median/regression()
Duplicates	duplicate_detection.py	find_duplicates(df, cols)
Deduplication	duplicate_detection.py	deduplicate(df, cols)
Outliers	anomaly_detection.py	detect_anomalies(df)
Constraint check	constraint_checking.py	validate_constraints(df, rules)
String matching	similarity_metrics.py	jaro_winkler_similarity()

Workflow

Step 1: Profile the Data

from scripts.data_profiling import profile_dataframe, detect_glitches, generate_quality_report

# Quick overview
print(generate_quality_report(df))

# Detailed profile
profile = profile_dataframe(df)

# Find issues
glitches = detect_glitches(df)

Step 2: Analyze Specific Issues

Missing Data:

from scripts.missing_data import analyze_missing, test_mcar

analysis = analyze_missing(df)
# Check if safe to delete rows
mcar_test = test_mcar(df, 'column_with_missing', ['other_cols'])

Duplicates:

from scripts.duplicate_detection import find_duplicates, cluster_duplicates

matches = find_duplicates(df, ['name', 'email'], threshold=0.85)
clusters = cluster_duplicates(matches)

Outliers:

from scripts.anomaly_detection import detect_anomalies, iqr_outliers

# Multi-column summary
anomalies = detect_anomalies(df, method='iqr')

# Single column detail
result = iqr_outliers(df, 'price', multiplier=1.5)

Constraints:

from scripts.constraint_checking import validate_constraints

constraints = [
    {'type': 'unique', 'columns': ['id']},
    {'type': 'not_null', 'columns': ['name', 'email']},
    {'type': 'fd', 'determinant': ['id'], 'dependent': ['name']},
    {'type': 'domain', 'column': 'age', 'min_value': 0, 'max_value': 150},
]
results = validate_constraints(df, constraints)

Step 3: Clean the Data

Handle Missing:

from scripts.missing_data import impute_median, impute_regression, listwise_deletion

# Simple: median for numeric
df_clean = impute_median(df, 'age')

# Better: regression-based
df_clean = impute_regression(df, 'income', ['age', 'education'])

# If MCAR confirmed
df_clean = listwise_deletion(df)

Remove Duplicates:

from scripts.duplicate_detection import deduplicate

df_clean, summary = deduplicate(
    df, 
    columns=['name', 'email', 'address'],
    threshold=0.8,
    merge_strategy='most_complete'
)
print(f"Reduced from {summary['original_rows']} to {summary['final_rows']} rows")

Handle Outliers:

# Cap extreme values
q01, q99 = df['col'].quantile([0.01, 0.99])
df['col'] = df['col'].clip(q01, q99)

# Or remove
df_clean = df[~detect_anomalies(df)['col']['outlier_indices']]

Step 4: Validate

Re-run profiling and constraint checks on cleaned data to verify improvements.

References

For deeper understanding:

• references/dimensions.md: Data quality dimensions (accuracy, completeness, etc.)
• references/glitch_taxonomy.md: Types of data glitches and detection approaches
• references/repair_strategies.md: Detailed repair and cleaning strategies

Key Concepts

Data Quality = Fit for Use

• Free of defects
• Has features needed for the task
• Right information, right place, right time

Missing Data Mechanisms:

• MCAR: Missing Completely At Random (safe to delete)
• MAR: Missing At Random (imputation may work)
• MNAR: Missing Not At Random (most problematic)

Constraints:

• Functional Dependency: X → Y means X uniquely determines Y
• Referential Integrity: foreign keys reference valid primary keys
• Domain Constraints: values within allowed set/range

Entity Resolution:

• Blocking reduces O(n²) to O(n·window)
• Similarity metrics: Jaro-Winkler (names), Levenshtein (typos), Jaccard (sets)
• Cluster by transitive closure, merge by strategy

Similarity Metrics Comparison

Metric	Best For	Example
Jaro-Winkler	Names, short strings	"Robert" vs "Rupert"
Levenshtein	Typos, edit distance	"recieve" vs "receive"
Jaccard	Token/word comparison	"John Doe" vs "Doe, John"
Q-gram	Fuzzy substring matching	Partial matches