The Goliath Barbarian, with it's reroll attack ability, poses a rather interesting number crunching problem when analyzing the statistics of a warband, just how good is it's ability? Clearly the piece is worth more than the +10/+5 (20) that is listed on the card, but just how much more?
This is also useful to star wars players, since you can spend a force point to re-roll an attack roll you missed on.
To answer this question, I am going to resort to what amounts to applied elementary probability and statistics, and if it makes your head hurt just skip down to the chart. I call it elementary because I don't do this stuff for a living and it makes sense to me, therefore it can't be all that difficult. What I did was convert the "roll needed" to hit to a probability of hitting, at a simple 0.05 probability per side that can hit. Then I squared that probability to get the odds that I will miss twice. Once on the normal attack and once on the re-roll. The rest of the probability, split basically 50/50 is the odds I hit on the normal roll or the re roll. But I just calculated those together. Then I figured what an equivalent "hit number" would be for those odds, the difference between the two is the benefit from using the re-roll. Here is a chart summarizing the numbers.
Armed with this information we have our answer, about +3/+4 on most attacks. Against a Large Silver dragon it's like having a +14/+5 attack (+13/+5 if buffed). Against an Orc Champion it's +10/+10 (+13 on a single attack), against an Ogre Ravager it's +10/+10 or +14 on a single attack. Against a Frenzied Bezerker it's +10/+8, not so special there. Between the primary and secondary attacks it is most effective against ACs between 15 and 23, i.e. most of the beaters!
Here is a quick comparison chart for a Goliath re-rolling an attack against a particular AC.