<b>If today's sale was caused by an ad seen three weeks ago, which report ever catches it?</b>

<b>If today's sale was caused by an ad seen three weeks ago, which report ever catches it?</b>

The honest answer for most click-based attribution: none. Advertising carryover is a systematic blind spot, and ignoring it skews every channel comparison.

<b>The concept</b>

Adstock (or carryover) is the idea that advertising's effect doesn't vanish at impression — it decays over days or weeks. A brand video seen Monday can influence a purchase the following month. The decay is often summarized by a <i>half-life</i>: the time for the effect to fall to half its initial strength.

<b>Why attribution misses it</b>

Multi-touch and last-click models are bounded by the lookback window and biased toward touches near the conversion. Long-carryover, upper-funnel media (TV, brand video, audio) routinely produce effects that mature <i>outside</i> the window entirely — so the model assigns their downstream sales to whatever lower-funnel touch happened to close the gap. The closer didn't create the demand; the long-decay channel did.

<b>The nuance</b>

Adstock is also where naive MMM goes wrong in the other direction — fit too long a half-life and you let a channel claim credit for sales it didn't cause; too short and you erase its real lagged effect. The decay parameter is doing heavy lifting and is rarely estimated carefully.

<b>What to actually do</b>

— Recognize that any channel with long carryover is structurally under-credited by click attribution; don't judge brand media on last-click ROAS.
— Estimate adstock half-lives empirically within MMM rather than defaulting to vendor presets.
— Use long-window incrementality tests to capture lagged effects path models can't see.

Bottom line for practitioners: carryover is the reason upper-funnel media always looks weak in attribution and strong in experiments. The effect is real and lagged — measure it with methods that respect time, not snapshots that don't.