Shield Expertise level 5 has a bonus of 36+30, 25% chance of being applied
averages are given with the standard deviation
Knife (8-12), stat pane damage listed as 16-20
Shield Expertise 0 = 12 to 20 range, 15.9 +- 2.4 average
Shield Expertise 5 = 12 to 56 range, 24.5 +- 15.7 average
Breaking down the level 5 values into groups based on if the skill added damage;
none = 12 to 20 range, 15.8 +- 2.5 average
bash = 48 to 56 range, 51.6 +- 2.5 average, 24.4% hit rate
average damage added = 35.8
Short Sword (18-22), stat pane damage listed as 26-30
Shield Expertise 0 = 22 to 30 range, 25.4 +- 2.6 average
Shield Expertise 5 = 22 to 66 range, 34.4 +- 15.8 average
Breaking down the level 5 values into groups based on if the skill added damage;
none = 22 to 30 range, 25.9 +- 2.7 average
bash = 58 to 66 range, 62.6 +- 2.8 average, 23.3% hit rate
average damage added = 36.7
Apparently the damage bonus is not evenly distributed (which would result in an expected average damage bonus of 51), but is influenced by agility, difference in character levels, or some other factor. Given the number of data values, the observed hit rate matches the specified 25%. The above numbers are the result of killing Jake several times with a level 32 female survivor on the hard difficulty level.