Measuring groups is both an art and a science. A well-practiced shooter will usually shoot groups that behave according to the laws of statistics. When that is so, the group sizes can be expressed as probabilities.
For the most part, you can get a really good idea of what your rifle and load are doing by firing three, ten-shot groups. The groups can be measured center-to-center or outside-to-outside, but keep it all the same. You can use three, five-shot groups, but the groups will give an average that is about 70% of the size of the ten-shot groups (95% of the time), and that is why the ten-shot groups are a better measure of long-term performance.
Groups are usually measured by casual shooters as center-to-center, but other methods give a better measurement of long-term performance. Outside measurement and baseline measurement do better, but are tough to correlate with your buddies' measurements for bragging rights.
While this is simplifying it too much, do this: make sure that the average of the three, ten-shot groups falls inside of your desired target area. For example, if you have a 1 minute X-ring, then the average of you groups should fall inside 1 minute of angle. If you deer has a heart that is 4 or 5 inches and you expect to shoot it at 200 yards at the most, then you should have a group that will shoot into 5 inches at 200 yards. That will be for a 95% confidence limit.