It is hard to measure creative performance or compare the creative performance if all the creatives were not live at the same time (start to end). The problem which comes is the cookie window. The creatives which are live for a longer period of time have the advantage of a longer cookie window and the creatives which are live for a shorter period of time have the big disadvantage. This is a bigger issue if one of the creative is live only during towards the end of the campaign.
Looking at the example below, the creative C, was live during the last 4 days of the campaign and thus does not have the cookie window advantage which creative A and B do.
Thus, it is recommended to think through the methodology based on the cookie window before starting the analysis. One solution is that measure the results 30 days beyond the campaign end date, then all the creatives will have similar cookie windows. However, this solution is not applicable in all situations specially if there is a big seasonality issue of the product example: Christmas candy canes, holiday decorations, customized products for a special occasion like Mother 's Day etc.