<b>Where did the 40-20-40 attribution split come from — and is there any science in it?</b>

<b>Where did the 40-20-40 attribution split come from — and is there any science in it?</b>

A fair question, because U-shaped and W-shaped models are everywhere and almost no one can defend the weights.

<b>The models</b>

— <i>U-shaped (position-based):</i> 40% to first touch, 40% to last touch, 20% spread across the middle.
— <i>W-shaped:</i> adds a weighted bump for the lead-creation or opportunity stage, common in long B2B cycles.
— <i>Time-decay:</i> credit grows exponentially toward touches nearer the conversion, governed by a half-life you pick.

<b>The uncomfortable truth</b>

These weights are heuristics, not measurements. The 40-20-40 split encodes a <i>belief</i> — that introduction and closing matter most — but it is not estimated from your data. Two practitioners can pick different half-lives and produce opposite channel rankings from identical paths.

<b>The nuance</b>

That doesn't make rule-based models worthless. They are transparent, stable, and impose a deliberate prior. A time-decay model with a sensibly chosen half-life can be a reasonable default for a short, impulse-driven funnel. The error is treating a chosen heuristic as a discovered fact.

<b>What to actually do</b>

— Pick a rule-based model as a <i>communication and stability</i> layer, and document that the weights are assumptions.
— If you want the weights to reflect reality, you need a data-driven model or an experiment — not a prettier curve.

Bottom line for practitioners: position-based models are opinions made of arithmetic. Useful as a transparent default, dangerous when mistaken for evidence about how your customers actually behave.