I have data for an application's installs per day and their ranking in the store for the following day. Each user/installation can be further broken down into two categories - those that installed it because of an ad, and those that installed it on their own (e.g., by word of mouth).
From this data, I want to know if increasing the number of ads would increase installations and thus ranking. I know rank is a terrible statistic to tie it to (since it's not an independent variable; e.g., it's tied to the performance of everyone else's applications in the store), but for the sake of comparison, I have already run a linear regression for the days where the number of ad-driven installs made up for <5% of all installs on a given day.
Given this, which statistical test should I run next to filter out the non-ad-driven installs' effect on the ranking? If I used the wrong metrics (e.g., I should be measuring change in rank against change in installs per day), please let me know.
(And before anyone asks, this is not homework. And on top of that, I know which tests to run after this step.)