In call and high-ticket funnels the sale can come weeks after the first ad click, after a webinar, several emails and a sales call. Last-click models credit the email that reminded someone to book; first-click models may credit an organic visit from months ago.
The scientific model answers the question media buyers actually ask: which paid ad brought this person in? It gives 100% of the credit to the first paid ad click (paid search, paid social, display or native) within the scientific window. If there is none, it falls back to the last non-direct touch. The window is set separately from the lookback, so a short lookback with a 30-day scientific window shows both the ad that closed and the ad that started it.