Editing Dark Heresy Second Edition (section)

====Characteristic Bonuses and Penalties====
By default, the 10 characteristics are all 20 + 2d10, so 22-40, with an average of 31 (most GMs will let you generate your 10 scores, then allocate them, but RAW, you generate them one at a time, and live with what you are given).  You can, instead, use point buy, which gives you 25 in every characteristic and then 60 discretionary to distribute, with a maximum of 40; if you distribute your points evenly, you will have exactly 31 in every characteristic.

A characteristic bonus works differently for each model.  If you are rolling, a characteristic with a bonus is 20 + 3d10 keep the two highest, which means your two characteristics with a bonus become, on average, 33.475 rather than 31, or a net bonus of +2.475 to each (4.95 total).  By contrast, the one with a penalty is 20 + 3d10 keep the two lowest, which drops it by 2.475 on average (typically to 28.525).  

If you are using point buy, instead each of your bonuses adds 5 to the relevant characteristic and your penalty subtracts 5 (giving you the only legal way to start with a characteristic at 20).