Situational Luck or Roll Once Luck

Luck in GURPS is often used for characters that are experts in their field. It can emulate the gunman who has unusual experience finding the perfect shot across the bar, the computer hacker that sees their favourite algorithm in play or even the martial artist who knows the enemy's style from their morning study. These heroes have an advantage because they work so hard that sometimes it pays off all at once. When Luck is used to simulate this expertise, players are left to the fate of a second dice roll, even though the normal way to deal with advantageous situations is a positive modifier.

So how much do you have to add to a dice roll to have the same chances as rolling twice? Turns out, the answer is...

Situational Luck [varies]: add +2 to effective skill before rolling. If that roll is a critical success, the luck is not 'used up' and the cooldown is instant (i.e. this ability can be used again straight away). Frequency and points cost as Luck (p. B66)

Why does it work?

(If you get queasy thinking about maths and graphs, you should probably skip this section!)

Situational Luck works by assuming that players use Luck to increase their probability of success by getting at least one of two successes. But there is more than one way to turn the odds: a +2 to a roll increases the chances of success by about the same as rolling twice.

How much luckier situational luck is based on the roll it's applied to

As you can see from the graph above, the difference in raw probability from rolling twice or adding a +2 modifier is pretty close. At skill levels 10+, Situational Luck disfavours the player a little, but not enough for concern. At skill levels 9 or less, the player gains a significant advantage with Situational Luck but since you have to declare it before you roll, I figure the chances balance out.

But what about Serendipity?

The experienced GURPser may notice that this advantage is quite similar to the Serendipity advantage. The key difference is that with Situational Luck, the player decides when it applies and gives a fixed bonus (unlike Serendipity where the GM decides everything).

So, can Situational Luck be used in place of Luck?

Yes!

It has some key differences to Luck, but if the GM understands these caveats then it shouldn't be a problem.

Conclusion

Situational Luck is a nice drop in replacement for Luck. It has caveats (some of which have not been discussed here): favours the player at low skill levels, etc. but overall if the flavour of Luck you are after is making the character better instead of luckier, Situational Luck is a step in the right direction.

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