Flags a meaningful week-over-week fall in impression share - the percentage of available auctions your ad actually showed in. It compares complete weeks only, so a half-finished current week never creates a fake drop. When one fires, check share lost to rank versus lost to budget in the campaign to point yourself at the right fix - and note Budget-limited campaign covers the budget half directly.
Defaults & settings
Week-over-week drop - the relative fall versus the previous complete week that triggers it. Default 20%, adjustable 5-50%.
Minimum prior IS - don't alert if the prior week's share was already below this level, where movement is just noise. Default 10%, adjustable 1-50%.
Checks daily; 72-hour cooldown.
Example. A campaign falls from 54% to 39% impression share - a 28% relative drop against a healthy prior week - and is flagged with both weeks' figures.