 This is an introduction to degrees of freedom. The easiest way to understand the concept is to think about calculating an average or a mean. Presume that you have four scores with a mean of 50. That means that the sum of those four scores has to come up to 200. So let's choose our first score. Well, it can be just anything we want. Let's say it's 70. So our running total at the moment is 70. Our second score can be anything we want. We're totally free to choose our second score. So let's say it's a 20, which brings our total to 90. Again, we're totally free to choose our third score, say 45, which brings us up to 135. And at this point, our freedom stops. We cannot have any choice on that last number. It has to be 65 because the sum has to come up to 200. So essentially, when we're doing the average of four scores, we have three degrees of freedom. If we had ten scores, we'd have nine degrees of freedom. We're totally free for the first nine numbers, but the tenth number is forced on us. In general, when computing an average, if you have n scores, you have n minus one degrees of freedom. This same logic carries over when we're doing something like the related t-test, where we're calculating the average of the differences between the before and after conditions. Since we have n items or n people who are participating in the experiment, that means we have n minus one degrees of freedom when calculating the t-statistic for related samples. And the same logic applies when we're talking about the t-statistic for related samples. Here, we have two different averages that we're computing. The average for group A and the average group for group B. For group A, we're going to have n sub A minus one degrees of freedom. And for group B, we're going to have n sub B minus one degrees of freedom. So when we're calculating the t-statistic for unrelated samples, we have n sub A plus n sub B minus two degrees of freedom. Degrees of freedom calculation can become a lot more complicated than this, but the general idea is the same. It's how many numbers can you choose freely before you are forced to choose that last number?