The random variable $Z$, which for a fair coin describes the number of occurrences of ``heads'' in four rolls, also has the expected value 2 and analogously $P ( Z = 2 ) = \frac { 3 } { 8 }$. Calculate the variance of $Z$, compare it with the variance of $Y$, and based on this describe a qualitative difference in the probability distributions of $Z$ and $Y$.