 Now in the previous lecture you saw how we relate area under a curve with probability or the p-value Now when we compare the means of two groups or even more The computer actually draws two types of graphs for us of which we can take the area under the curve One is called a z-distribution and one is called a t-distribution. With a z-distribution We really have to know what the population standard deviation is Most of the time we don't know that and we use the t-distribution Have a look at this