Return on Ad Spend (ROAS) indicates the revenue generated from a specific mobile ad campaign and it measures the effectiveness of it. It is crucial for mobile advertisers to determine and track the most efficient sources that bring the most quality app users. 

If ROAS of an ad campaign is negative, it means the ad campaign requires more cost than the revenue generated from acquired users. In this case, the ad campaign should be optimized or stopped. To decide on if a mobile ad campaign should continue running, it should be ROAS positive. To make a mobile ad campaign ROAS positive, all campaign elements including creatives, ad types selected, app store pages, CTAs, and target audiences should be optimized. 

Within ROAS calculation, all the revenue coming from in-app purchases and in-app advertising is taken into consideration.  

ROAS= Total Revenue Generated from an Ad Campaign / Total Cost of the Mobile Ad Campaign