When you are optimising a display campaign for the first few times, if you have mobile devices on, providing you aren’t doing manual placements, you will find most likely your ads are appearing on mobile apps which with the assistance of analytics perform poorly to be polite (100% bounce rate).
If you are targeting by keyword or interest on display this is particularly relevant. Mobile apps are not quite ready for advertising in my opinion and the above video shows how to exclude all mobile apps without switching mobiles off completely.
For easy copy & paste, enter www.adsenseformobileapps.com into your display campaigns placement exclusions and your ads will no longer show on mobile apps. This will be relief to anyone who has spent hours excluding bad automatic placements and if you have found this before you have had to go through this experience, well done! You have learnt from my hours of pain without having to go through it yourself!