- (continued) Therefore, z = 0.344/0.0713 = 4.69. A z
table can be used to find that the
two-tailed probability value for a z of 4.69 is less than 0.0001.

If n_{1}= n_{2}then p is simply the average of p_{1}and p_{2}:

The two p values are averaged since the computations are based on the assumption that the null hypothesis is true. When the null hypothesis is true, both p_{1}and p_{2}are estimates of the same value of π. The best estimate of π is then the average of the two p's.

Naturally, if one p is based on more observations than the other, it should be counted more heavily. The formula for p when n_{1}≠ n_{2 }is:

The computations for the example on early childhood intervention are:

=