/senior-data-scientist

name: "senior-data-scientist"
description: "Design experiments and analyze tabular data: A/B tests, causal inference, feature engineering, model evaluation. Use whenever the user asks how to set up or read an A/B test, whether a change caused a lift, which features to build, or how good a model is."

Senior Data Scientist

World-class senior data scientist skill for production-grade AI/ML/Data systems.

Core Workflows

1. Design an A/B Test

import numpy as np
from scipy import stats

def calculate_sample_size(baseline_rate, mde, alpha=0.05, power=0.8):
    """
    Calculate required sample size per variant.
    baseline_rate: current conversion rate (e.g. 0.10)
    mde: minimum detectable effect (relative, e.g. 0.05 = 5% lift)
    """
    p1 = baseline_rate
    p2 = baseline_rate * (1 + mde)
    effect_size = abs(p2 - p1) / np.sqrt((p1 * (1 - p1) + p2 * (1 - p2)) / 2)
    z_alpha = stats.norm.ppf(1 - alpha / 2)
    z_beta = stats.norm.ppf(power)
    n = ((z_alpha + z_beta) / effect_size) ** 2
    return int(np.ceil(n))

def analyze_experiment(control, treatment, alpha=0.05):
    """
    Run two-proportion z-test and return structured results.
    control/treatment: dicts with 'conversions' and 'visitors'.
    """
    p_c = control["conversions"] / control["visitors"]
    p_t = treatment["conversions"] / treatment["visitors"]
    pooled = (control["conversions"] + treatment["conversions"]) / (control["visitors"] + treatment["visitors"])
    se = np.sqrt(pooled * (1 - pooled) * (1 / control["visitors"] + 1 / treatment["visitors"]))
    z = (p_t - p_c) / se
    p_value = 2 * (1 - stats.norm.cdf(abs(z)))
    ci_low = (p_t - p_c) - stats.norm.ppf(1 - alpha / 2) * se
    ci_high = (p_t - p_c) + stats.norm.ppf(1 - alpha / 2) * se
    return {
        "lift": (p_t - p_c) / p_c,
        "p_value": p_value,
        "significant": p_value < alpha,
        "ci_95": (ci_low, ci_high),
    }

# --- Experiment checklist ---
# 1. Define ONE primary metric and pre-register secondary metrics.
# 2. Calculate sample size BEFORE starting: calculate_sample_size(0.10, 0.05)
# 3. Randomise at the user (not session) level to avoid leakage.
# 4. Run for at least 1 full business cycle (typically 2 weeks).
# 5. Check for sample ratio mismatch: abs(n_control - n_treatment) / expected < 0.01
# 6. Analyze with analyze_experiment() and report lift + CI, not just p-value.
# 7. Apply Bonferroni correction if testing multiple metrics: alpha / n_metrics

2. Build a Feature Engineering Pipeline

import pandas as pd
import numpy as np
from sklearn.pipeline import Pipeline
from sklearn.preprocessing import StandardScaler, OneHotEncoder
from sklearn.impute import SimpleImputer
from sklearn.compose import ColumnTransformer

def build_feature_pipeline(numeric_cols, categorical_cols, date_cols=None):
    """
    Returns a fitted-ready ColumnTransformer for structured tabular data.
    """
    numeric_pipeline = Pipeline([
        ("impute", SimpleImputer(strategy="median")),
        ("scale",  StandardScaler()),
    ])
    categorical_pipeline = Pipeline([
        ("impute", SimpleImputer(strategy="most_frequent")),
        ("encode", OneHotEncoder(handle_unknown="ignore", sparse_output=False)),
    ])
    transformers = [
        ("num", numeric_pipeline, numeric_cols),
        ("cat", categorical_pipeline, categorical_cols),
    ]
    return ColumnTransformer(transformers, remainder="drop")

def add_time_features(df, date_col):
    """Extract cyclical and lag features from a datetime column."""
    df = df.copy()
    df[date_col] = pd.to_datetime(df[date_col])
    df["dow_sin"] = np.sin(2 * np.pi * df[date_col].dt.dayofweek / 7)
    df["dow_cos"] = np.cos(2 * np.pi * df[date_col].dt.dayofweek / 7)
    df["month_sin"] = np.sin(2 * np.pi * df[date_col].dt.month / 12)
    df["month_cos"] = np.cos(2 * np.pi * df[date_col].dt.month / 12)
    df["is_weekend"] = (df[date_col].dt.dayofweek >= 5).astype(int)
    return df

# --- Feature engineering checklist ---
# 1. Never fit transformers on the full dataset — fit on train, transform test.
# 2. Log-transform right-skewed numeric features before scaling.
# 3. For high-cardinality categoricals (>50 levels), use target encoding or embeddings.
# 4. Generate lag/rolling features BEFORE the train/test split to avoid leakage.
# 5. Document each feature's business meaning alongside its code.

3. Train, Evaluate, and Select a Prediction Model

from sklearn.model_selection import StratifiedKFold, cross_validate
from sklearn.metrics import make_scorer, roc_auc_score, average_precision_score
import xgboost as xgb
import mlflow

SCORERS = {
    "roc_auc":  make_scorer(roc_auc_score, needs_proba=True),
    "avg_prec": make_scorer(average_precision_score, needs_proba=True),
}

def evaluate_model(model, X, y, cv=5):
    """
    Cross-validate and return mean ± std for each scorer.
    Use StratifiedKFold for classification to preserve class balance.
    """
    cv_results = cross_validate(
        model, X, y,
        cv=StratifiedKFold(n_splits=cv, shuffle=True, random_state=42),
        scoring=SCORERS,
        return_train_score=True,
    )
    summary = {}
    for metric in SCORERS:
        test_scores = cv_results[f"test_{metric}"]
        summary[metric] = {"mean": test_scores.mean(), "std": test_scores.std()}
        # Flag overfitting: large gap between train and test score
        train_mean = cv_results[f"train_{metric}"].mean()
        summary[metric]["overfit_gap"] = train_mean - test_scores.mean()
    return summary

def train_and_log(model, X_train, y_train, X_test, y_test, run_name):
    """Train model and log all artefacts to MLflow."""
    with mlflow.start_run(run_name=run_name):
        model.fit(X_train, y_train)
        proba = model.predict_proba(X_test)[:, 1]
        metrics = {
            "roc_auc":  roc_auc_score(y_test, proba),
            "avg_prec": average_precision_score(y_test, proba),
        }
        mlf