If you create a consideration Video campaign and want to measure it through a Brand Lift study,when should you setup that study to get optimal control and exposed groups?

Setting up a Brand Lift study before the campaign serves any impressions is crucial for optimal control and exposed groups. This allows the system to randomly assign users to either the exposed group (who will see your ads) or the control group (who will not) before any exposure to your campaign. If you wait until after impressions have started, some users will have already seen your ads, contaminating the control group and making it difficult to accurately measure the true lift in brand metrics. Waiting until the campaign ends or two weeks after it starts would completely invalidate the study's ability to establish a baseline and measure the impact of the ads.