building-baselines

name	building-baselines
description	Builds the transparent rule-based baseline every model must beat — a hand-written score with reason codes, ranked output, and precision@K evaluation. Use before training any model, or when someone reports model results with nothing to compare against.

Building baselines

A model without a baseline is a number without a meaning. The baseline is a rule a human can read — and its job is to be honestly beatable.

Build it in this order

1. Say the rule in plain words first. "A page is worth reviewing if it used to get traffic, it's getting old, and its position is slipping." If you can't say it, you can't code it.

2. Code it as a transparent score. Multiply/add simple conditions; no fitted weights:

stale   = (df["days_since_update"] >= 180).astype(int)
visible = (df["impressions"] >= 500).astype(int)
df["score"] = stale * visible * df["impressions"]     # readable on purpose

3. Attach reason codes. Every scored item carries WHY it scored ("stale_but_visible", "position_slipping"). Reason codes are what make a ranked list trustworthy to a human.

4. Rank and evaluate at K. For "which ones first?" problems the honest metric is precision@K: of the top K the rule flags, how many were actually right?

def precision_at_k(scores, labels, k):
    import numpy as np
    order = np.argsort(-np.asarray(scores))
    return np.asarray(labels)[order[:k]].mean()

Always print the base rate next to it (labels.mean()): precision@50 of 0.60 means little until you know whether random picking gives 0.55 or 0.10.

5. Review the top of the list by hand. The top 20 is where bad logic shows itself. For each: the action, the reason code, a confidence note, and what would make it wrong.

Also useful

• A dummy baseline (predict the majority class / mean) is the floor below the floor — one line with sklearn's DummyClassifier, and surprisingly clarifying.
• Keep the baseline frozen once the model work starts. Moving the goalposts mid-game convinces nobody, including you.

How to verify

• The rule fits in three sentences a non-engineer understands.
• precision@K is computed on the SAME data slice and labels the model will use later.
• The top-20 hand review found at least one weak pick — if it found none, look harder.