Working with two group means requires a few extra steps. We need to know the relationship between the two groups. Are the groups independent or dependent? Another important step is to understand the variability of the two groups (our old friend variability comes into play again!). If the groups have unequal variance we may have to account for that in our hypothesis test.
Let's take a look at independence and dependence first.
Independent and Dependent Samples Section
- Independent Sample
- The samples from two populations are independent if the samples selected from one of the populations have no relationship with the samples selected from the other population.
- Dependent Sample
- The samples are dependent (also called paired data) if each measurement in one sample is matched or paired with a particular measurement in the other sample. Another way to consider this is how many measurements are taken off of each subject. If only one measurement, then independent; if two measurements, then paired. Exceptions are in familial situations such as in a study of spouses or twins. In such cases, the data is almost always treated as paired data.
These notes will first work through independent groups and then proceed to dependent groups.