# exactly k consecutive heads, n tosses

What is the expected number of strings of exactly k consecutive heads if a fair coin is tossed n times?

My current answer is $${n-1\choose k} (\frac{1}{2})^{(k-1)}$$

Is this correct?

A possible string : HTHHHTHH. This has 3 strings for 2 consecutive heads. The string for 3 Hs, is also 2 strings for 2 consecutive Hs.

• How did you get this result ? If you toss a coin 5 times, how many strings with the length of k=3 are possible ? – callculus Oct 16 '14 at 18:39
• 4 choose 3 locations possible, each with (1/2)^(k-1) probability of being the kth Head in the string – mathguy Oct 16 '14 at 18:42
• I have only 3. $hhhtt-thhht-tthhh$ The formula would be $n-k+1$ for the first part. – callculus Oct 16 '14 at 18:45
• I should give a possible string, HTHHHTHH. This has 3 strings for 2 consecutive heads. The string for 3 Hs, is also 2 strings for 2 consecutive Hs. – mathguy Oct 16 '14 at 18:48
• The formula for the current answer, ${n-1\choose k}(1/2)^{(k-1)}$, is clearly wrong when $k=1$. – Barry Cipra Oct 16 '14 at 19:06

The probability of tossing $k$ consecutive heads in $k$ tosses is simply $(1/2)^k$. There are $n-k+1$ possible starting positions (if we start counting after the $n-k+1$th toss, we do not have enough tosses for $k$ consecutive heads). Thus, the expected number is simply $\frac{n-k+1}{2^k}$.
Edit: To make this clearer, for $i = 1, \dots, n-k+1$, let $X_i$ be an indicator function such that $X_i = 1$ if there are $k$ consecutive heads starting at position $i$ and $0$ otherwise. Then $P(X_i = 1) = 1/2^k$. The total number $T$ of $k$ consecutive heads is $$T = X_1 + \dots + X_{n-k+1}.$$ Thus, $E[T] = (n-k+1)E[X_1] = \frac{n-k+1}{2^k}.$
• Important to note: Of course $X_i$ and $X_{i+1}$ are not independent, but as long as we are only concerned with the expected value, we couldn't care less. The exact distribution for $T$ is apparently more complicated. – Hagen von Eitzen Jan 17 '17 at 7:25