Data Science Expert
Expert guidance for data science, analytics, statistical modeling, and data visualization.
Core Concepts Data Analysis Exploratory Data Analysis (EDA) Data cleaning and preprocessing Feature engineering Statistical inference Time series analysis A/B testing Machine Learning Supervised learning (classification, regression) Unsupervised learning (clustering, PCA) Model selection and validation Feature importance Hyperparameter tuning Ensemble methods Data Visualization Matplotlib, Seaborn, Plotly Statistical plots Interactive dashboards Storytelling with data Best practices for visualization Color theory and accessibility Data Cleaning and EDA import pandas as pd import numpy as np import matplotlib.pyplot as plt import seaborn as sns from typing import Dict, List
class DataCleaner: """Clean and preprocess data"""
def __init__(self, df: pd.DataFrame):
self.df = df.copy()
self.cleaning_log = []
def handle_missing_values(self, strategy: str = 'drop',
fill_value=None) -> pd.DataFrame:
"""Handle missing values"""
missing_before = self.df.isnull().sum().sum()
if strategy == 'drop':
self.df = self.df.dropna()
elif strategy == 'fill':
if fill_value is not None:
self.df = self.df.fillna(fill_value)
else:
# Fill numeric with median, categorical with mode
for col in self.df.columns:
if self.df[col].dtype in ['float64', 'int64']:
self.df[col].fillna(self.df[col].median(), inplace=True)
else:
self.df[col].fillna(self.df[col].mode()[0], inplace=True)
missing_after = self.df.isnull().sum().sum()
self.cleaning_log.append(f"Missing values: {missing_before} -> {missing_after}")
return self.df
def remove_duplicates(self) -> pd.DataFrame:
"""Remove duplicate rows"""
before = len(self.df)
self.df = self.df.drop_duplicates()
after = len(self.df)
self.cleaning_log.append(f"Duplicates removed: {before - after}")
return self.df
def remove_outliers(self, columns: List[str],
method: str = 'iqr',
threshold: float = 1.5) -> pd.DataFrame:
"""Remove outliers"""
before = len(self.df)
for col in columns:
if method == 'iqr':
Q1 = self.df[col].quantile(0.25)
Q3 = self.df[col].quantile(0.75)
IQR = Q3 - Q1
lower = Q1 - threshold * IQR
upper = Q3 + threshold * IQR
self.df = self.df[(self.df[col] >= lower) & (self.df[col] <= upper)]
elif method == 'zscore':
z_scores = np.abs(stats.zscore(self.df[col]))
self.df = self.df[z_scores < threshold]
after = len(self.df)
self.cleaning_log.append(f"Outliers removed: {before - after}")
return self.df
class EDA: """Exploratory Data Analysis"""
def __init__(self, df: pd.DataFrame):
self.df = df
def summary_stats(self) -> pd.DataFrame:
"""Generate summary statistics"""
return self.df.describe(include='all').T
def correlation_analysis(self, method: str = 'pearson') -> pd.DataFrame:
"""Calculate correlation matrix"""
numeric_cols = self.df.select_dtypes(include=[np.number]).columns
return self.df[numeric_cols].corr(method=method)
def plot_distributions(self, columns: List[str] = None):
"""Plot distributions of numeric columns"""
if columns is None:
columns = self.df.select_dtypes(include=[np.number]).columns
n_cols = len(columns)
n_rows = (n_cols + 2) // 3
fig, axes = plt.subplots(n_rows, 3, figsize=(15, 5*n_rows))
axes = axes.flatten()
for idx, col in enumerate(columns):
sns.histplot(self.df[col], kde=True, ax=axes[idx])
axes[idx].set_title(f'Distribution of {col}')
plt.tight_layout()
return fig
def plot_correlation_heatmap(self):
"""Plot correlation heatmap"""
corr = self.correlation_analysis()
plt.figure(figsize=(12, 10))
sns.heatmap(corr, annot=True, fmt='.2f', cmap='coolwarm',
center=0, square=True, linewidths=1)
plt.title('Correlation Heatmap')
return plt.gcf()
Feature Engineering from sklearn.preprocessing import StandardScaler, LabelEncoder, OneHotEncoder from sklearn.feature_selection import SelectKBest, f_classif, mutual_info_classif
class FeatureEngineer: """Engineer features for machine learning"""
def __init__(self, df: pd.DataFrame):
self.df = df.copy()
self.transformers = {}
def create_interaction_features(self, col1: str, col2: str) -> pd.Series:
"""Create interaction features"""
self.df[f'{col1}_x_{col2}'] = self.df[col1] * self.df[col2]
return self.df[f'{col1}_x_{col2}']
def create_polynomial_features(self, col: str, degree: int = 2) -> pd.DataFrame:
"""Create polynomial features"""
for d in range(2, degree + 1):
self.df[f'{col}_pow_{d}'] = self.df[col] ** d
return self.df
def bin_numeric_feature(self, col: str, n_bins: int = 5,
strategy: str = 'quantile') -> pd.Series:
"""Bin numeric features"""
self.df[f'{col}_binned'] = pd.qcut(self.df[col], q=n_bins,
labels=False, duplicates='drop')
return self.df[f'{col}_binned']
def encode_categorical(self, col: str, method: str = 'onehot') -> pd.DataFrame:
"""Encode categorical variables"""
if method == 'label':
le = LabelEncoder()
self.df[f'{col}_encoded'] = le.fit_transform(self.df[col])
self.transformers[col] = le
elif method == 'onehot':
dummies = pd.get_dummies(self.df[col], prefix=col, drop_first=True)
self.df = pd.concat([self.df, dummies], axis=1)
return self.df
def scale_features(self, columns: List[str],
method: str = 'standard') -> pd.DataFrame:
"""Scale numeric features"""
if method == 'standard':
scaler = StandardScaler()
elif method == 'minmax':
from sklearn.preprocessing import MinMaxScaler
scaler = MinMaxScaler()
self.df[columns] = scaler.fit_transform(self.df[columns])
self.transformers['scaler'] = scaler
return self.df
def select_features(self, X: pd.DataFrame, y: pd.Series,
k: int = 10,
method: str = 'f_classif') -> List[str]:
"""Select top k features"""
if method == 'f_classif':
scorer = f_classif
elif method == 'mutual_info':
scorer = mutual_info_classif
selector = SelectKBest(scorer, k=k)
selector.fit(X, y)
selected_features = X.columns[selector.get_support()].tolist()
return selected_features
Time Series Analysis from statsmodels.tsa.seasonal import seasonal_decompose from statsmodels.tsa.stattools import adfuller from statsmodels.tsa.arima.model import ARIMA
class TimeSeriesAnalyzer: """Analyze time series data"""
def __init__(self, data: pd.Series, freq: str = 'D'):
self.data = data
self.freq = freq
def decompose(self, model: str = 'additive'):
"""Decompose time series"""
result = seasonal_decompose(self.data, model=model, period=30)
return {
'trend': result.trend,
'seasonal': result.seasonal,
'residual': result.resid
}
def test_stationarity(self) -> dict:
"""Test for stationarity using Augmented Dickey-Fuller"""
result = adfuller(self.data.dropna())
return {
'adf_statistic': result[0],
'p_value': result[1],
'critical_values': result[4],
'is_stationary': result[1] < 0.05
}
def make_stationary(self, method: str = 'diff') -> pd.Series:
"""Make series stationary"""
if method == 'diff':
return self.data.diff().dropna()
elif method == 'log':
return np.log(self.data)
elif method == 'log_diff':
return np.log(self.data).diff().dropna()
def fit_arima(self, order: tuple = (1, 1, 1)):
"""Fit ARIMA model"""
model = ARIMA(self.data, order=order)
fitted_model = model.fit()
return {
'model': fitted_model,
'aic': fitted_model.aic,
'bic': fitted_model.bic,
'summary': fitted_model.summary()
}
def forecast(self, model, steps: int = 30) -> pd.Series:
"""Generate forecast"""
return model.forecast(steps=steps)
A/B Testing from scipy import stats
class ABTest: """Conduct A/B tests"""
def __init__(self, control: np.ndarray, treatment: np.ndarray):
self.control = control
self.treatment = treatment
def ttest(self) -> dict:
"""Two-sample t-test"""
statistic, p_value = stats.ttest_ind(self.control, self.treatment)
# Calculate confidence interval for difference
diff_mean = self.treatment.mean() - self.control.mean()
se_diff = np.sqrt(self.control.var()/len(self.control) +
self.treatment.var()/len(self.treatment))
ci_lower = diff_mean - 1.96 * se_diff
ci_upper = diff_mean + 1.96 * se_diff
return {
't_statistic': statistic,
'p_value': p_value,
'mean_control': self.control.mean(),
'mean_treatment': self.treatment.mean(),
'difference': diff_mean,
'ci_95': (ci_lower, ci_upper),
'significant': p_value < 0.05
}
def proportion_test(self, conversions_control: int,
conversions_treatment: int) -> dict:
"""Test difference in proportions"""
n_control = len(self.control)
n_treatment = len(self.treatment)
p_control = conversions_control / n_control
p_treatment = conversions_treatment / n_treatment
p_pooled = (conversions_control + conversions_treatment) / (n_control + n_treatment)
se = np.sqrt(p_pooled * (1 - p_pooled) * (1/n_control + 1/n_treatment))
z = (p_treatment - p_control) / se
p_value = 2 * (1 - stats.norm.cdf(abs(z)))
return {
'conversion_rate_control': p_control,
'conversion_rate_treatment': p_treatment,
'lift': (p_treatment - p_control) / p_control * 100,
'z_statistic': z,
'p_value': p_value,
'significant': p_value < 0.05
}
Best Practices Data Analysis Always explore data before modeling Check data quality and missing values Understand variable distributions Look for correlations and relationships Document data cleaning steps Validate assumptions Feature Engineering Create domain-specific features Test feature importance Avoid data leakage Use cross-validation for validation Document feature transformations Keep features interpretable Visualization Choose appropriate plot types Use clear labels and titles Consider color accessibility Avoid chartjunk Tell a story with data Make visualizations reproducible Anti-Patterns
❌ Not exploring data before modeling ❌ Ignoring data quality issues ❌ Data leakage in feature engineering ❌ Over-engineering features ❌ Misleading visualizations ❌ Not documenting analysis steps ❌ Ignoring business context
Resources Pandas: https://pandas.pydata.org/ NumPy: https://numpy.org/ Scikit-learn: https://scikit-learn.org/ Seaborn: https://seaborn.pydata.org/ Plotly: https://plotly.com/python/