Data from IDC and App Annie suggested that both Apple and Google are likely to see their app store revenues doubling within the next four years, reports Re/code. Based on Apple’s 2014 total revenue of $10B, this would suggest that Apple is likely to hit total revenue of $20B in 2018, with developers seeing $14B of it.

The study also suggested that the trend toward freemium apps, which derive their income from in-app purchases, will continue–though this model is more common on Android than iOS … 

“I would consider this projection conservative,” App Annie CEO Bertrand Schmitt said in an interview, noting that the company has consistently been revising its forecasts up over the past two years as revenue has exceeded expectations.

Advertising revenue is important for both platforms, said IDC, accounting for 68% of US mobile app revenue last year.