In many games, there are clear rules for making a "skill check", but the rules for how skill checks will really work in the game in terms of who makes what roles how often in what situations are not fully specified and are left up to the GM. In fourth edition Dungeons & Dragons, it is not always clear what method the game master should choose to have the party make “group” skill checks, in which anyone could potentially make the check and the entire group will benefit. Since I'm planning to put one of my adventures up on my sidebar, I thought I would describe how I've been playing group skill checks.
The method I have chosen is to have every player roll a single skill roll against the listed DC of the group skill check, and if any player succeeds in the skill check, the party succeeds in the skill check. The typical DC is 20 for a level 1 check. Here are the reasons I chose this method:
1. I decided I wanted having skills to be very useful even if you don't have the highest skill level in the party. In many games I've played, having a skill is pretty much useless if someone else in the party has a higher skill level than you. This isn't much fun if you are the one with the second-best skill. This is especially so in D&D, now that the party will have more total skills than the total number of skills in the game, so some of the skills will definitely be doubled up. Also, in D&D you basically have no control over how good you are in a skill unless you take rare feats like skill focus; if you take a skill, your ability and that skill depends on your statistics, and your statistics depend on your class. It is already not very efficient for a fighter to take a skill he won't be very good in, like Streetwise; why compound the problem by guaranteeing the skill won't be useful if the party has a higher charisma character who has taken the skill. Hence, I did not want to allow only the highest skill character to try the challenge.
2. Also, when I first saw the rules, I noticed that the bonus for training in a skill is rather small compared to what you would expect in reality or fiction. If the check is so hard that you have only at 50% chance of succeeding even with training, you still have a 25% chance of succeeding even without training. I decided that this is part of the “fun is better than simulation” aspect of the new D&D rules, and wanted to reflect this spirit in the use of my rules by allowing everyone to participate in the skill check, allowing the barbarian to occasionally get lucky and know more about arcane knowledge than the wizard.
3. The game has rules for “aid another”, but I decided not to use those rules. First, the rule is broken because the DC is fixed, meaning that once you reach a certain level the aid action becomes automatic and your level of skill no longer means anything at all - an incompetent helps just as much as an expert. This is a minor quibble because it could easily be fixed by setting the aid DC to equal the skill DC – 10. My real objection is that even if you do this, the skill level of the aiding players has a rather small effect; the skill bonus of the lead player is 10x as important as that of anyone else.
4. In some campaigns, only characters who declare that they are trying the skill get to roll. But if I'm going to allow every character to make a skill roll, I think it is much more fair to simply have everyone do so rather than give bonuses to players who talk more.
5. DC 20 gives the average party about a 70% chance of making the roll, and I like making the characters successful more often than they are unsuccessful. And this isn't one of these "by average difficulty I mean easy" averages, this is an average difficulty. Characters are certainly highly successful in combat, they should have a good shot in noncombat too. I never understood games where any task interesting enough that a hero would want to do it, is so hard he has about a 25% chance of success. That just isn't a lot of fun.
Also, DC 20 is a nice number because it means that checks are generally not impossible, a one point difference in you skill almost always makes a difference. A skill bonus of 0 is better than a bonus of -1 and worse than a bonus of +1. This wouldn't be the case with DC 24, since the roll would be impossible in all 3 cases.
At high levels these various numbers won't work out too well anymore, but I'm not worried about high levels yet. I haven't yet figured out a good formula for how quickly the skill checks should rise. Clearly skill checks rise more slowly than hit rolls, since most of them are based on statistics that do not rise every 4 levels, and magic items that improve skill checks are fairly rare. But characters occasionally get utility powers or possibly feats that improve skill checks, so figuring out the expected rate of progression could take a fair amount of analysis.