The setting is: Interest Based Ads. If you are doing micro niche, I would strongly advice you to turn this setting off! After disabling this setting, I have seen an increase in CTR, one channel even close to double (from 15% to 26%). This experiment is conducted without any other changes to the ads.
To change the setting, go to your google adsense acount under account settings, click edit the interest based ads and choose the disable option.
I have also attach 2 screen shots showing some of the improvements before and after.
Total Average CTR (on 8 channels) = 5.59%
Total Average CTR (on 8 channels) = 8.3%
Edit: I removed the actual screen shot showing my adsense account due to the advice of the kind Warriors =)
Before you go and change the settings, please understand this sharing is based on my personal observations. And I highly encourage to only change this settings for your adsense account if you are targeting micro niches.
Anyone has similar experience to share?