Four minus one does NOT equal two. You have THREE options to choose from.
But sorry Shadowcat, your math is wrong.
You can’t just add up the figures the way you did and end up with an accurate probability in this situation.
Assuming your math was correct, and assuming no dual-class characters, then you’d have a 40% chance of getting a class missing. Which shows that you forgot to sanity-check your math.
You calculated the chance of a single specific class being missing at ~10%
You didn’t account for the fact that this figure applies across the spectrum, not individually for each class.
There’s a 10% chance that your team will be missing one class, regardless of what that class is, assuming that each class is totally independent. So with the possibility of dual-spec characters, that 10% is further reduced.
The calculation that gave your 37.6% figure is a reasonable approximation of what percentage of 10% you need to subtract from itself to account for the dual-spec characters though, assuming a 25% chance of the player going dual-spec. And the slightly-higher-than accurate number will allow it to also account for the occasional 3- and 4-class players.
Giving a final result of…
6.24% chance of missing ONE class.
NOTE: Because of the chance of one or two classes being extremely popular at first, the actual probability of this situation may be significantly elevated early on - possibly close to Shadowcat’s calculation in the 35 - 40% range, because the variance in numbers between the characters will be past the threshold allowed by a mathematical calculation where equal or near-equal popularity is assumed between the different options. Over time, this imbalance will correct itself, though, as people move from the “cool” factor to more practical builds for their playstyle.
(Oh, and for reference, I haven’t studied statistics in the past 11 years, but when I was 17, I passed my final exam for the year with just under 70% - I went into the exam without a calculator)