Originally Posted by

**Gwayne**
But I would like to see the maths behind the "expected number of hits before n hits in a row" formula. I've been trying to work on something similar and I get different results: at least for n=2 and a=0.5 I get 4 hits as Expectancy value for the time until 2 hits in a row. Your formula gives 6 hits.

Either I'm doing something wrong, or you are including the 2 hits in a row in your number, or I'm taking the "expected" a bit too literally as the mathematical Expectancy Value, while you mean slightly different.

Could you give your derivation of that formula please?