The Effects of Luck

In GURPS, the Luck advantage has a unique effect on the core mechanic: when you use Luck, you roll three times and take the best result (generally the lowest if you're making a Success Roll). What does this mean for the lucky character?

Well, it means that on average you have a +2.5 to your effective skill. That is, the average result on 3d6 is 10.5, while the average result on the lowest of 3 rolls of 3d6 is 7.99. But the averages don't tell the whole story because of the beauty of the bell curve.

The following graph shows the probability of rolling a given result or less on a normal Success Roll versus one with Luck.

You can see how much faster the Luck roll tops out compared to a normal roll. Basic Set considers a skill of 14 to be expert level. A normal roll will succeed against a Skill 14 90.74% of the time. With Luck, you'll hit that likelihood at between Skill 10 and Skill 11. Skill 16 on a normal roll is equivalent to Skill 12 with Luck. So in the range of many starting character's skills, it's really more like a +4.

Looking at it from the other direction, if you're using Luck to make up for a very low skill (perhaps a default), you'll succeed against a Skill 6 25.29% of the time, versus only 9.26% of the time normally.

And the beauty of the advantage lies in the fact that you don't have to decide to use it until after you've seen the result of your first roll! So keep this in mind the next time you're wondering if Luck is really worth 15 points.