It depends on what you want from your ads. If your goal is to get more sales or actions, focusing on CPA makes more sense because it shows how much you’re paying for each result, like a signup or purchase. But if you want your ad to be seen more often or win better placements, eCPM can help with that. eCPM tells you how much you're spending for every thousand impressions, which is useful when trying to understand how competitive your bid is. Some ad networks even use eCPM to decide which ads to show more often. So if your CPA is good but you’re not getting enough traffic, checking your eCPM might help. How do you usually set your bids?