I've been reading a biology textbook, and it's stated that
If the probability of an outcome is indicated by $a^n$, this means that $n+1$ independent events, each occurring randomly and with comparable probability per unit of time, must take place before the ultimate outcome--in this case a diagnosed tumor--is achieved.
I've been trying to figure out why this is, because I thought that the number of independent events would be $n$. For example, the probability of all heads in a coin flipping trial of flipping $n$ coins is $0.5^n$, that means that n independent events (each coin flip) have happened. Am I missing something?
Thanks in advanced for your help!