okay
@YoungArt this is a very smart question.
because at the end it's all about optimization.
first of all you start from the large factors who has the big impact on your campaign, like landing pages and placements .
that's the first thing you start optimizing in your campaigns.
in the landing pages you look at the ctr, and the placements you look at the ctr as well.
cus some times you launch a campaign and your test budget get sucked by one placement only and that happened with me lot of times, so during the test always look at the placements and the landing pages, any factor with bad ctr kill it.
after that you wanna look into the offer and see if the offer getting clicks with conversion or no conversions at all.
-1) if the offer is getting conversion, means carry on the optimization process means send more traffic and go to optimize the other factors like browsers, carriers(if you are targeting many), os's, os's versions,of course look at the placements always,... ETC,
-2) if you are not getting any conversions means you will have to replace the offer with an other offer in the same geo and the same vertical, or kill the campaign.
hopefully i helped, feel free to ask if you still have Questions.