What is the difference between inter- and intra-individual differences?

Inter-individual differences are differences that are observed between people, whereas intra-individual differences are differences that are observed within the same person when they are assessed at different times or in different situations. An easy way to remember this is that intra-individual differences occur within the same person. Good examples of inter-individual differences are gender, age, ethnic background, anxiety levels or attachment style. Attention or effort are good examples of intra-individual differences. 

Some factors can vary both between and within individuals dependent on the context, so it is important to consider the full experiment when deciding if something is an inter-individual difference or an intra-individual difference. Let's take effort as an example. Some people are going to put in more effort than others. This could be for many reasons - perhaps because they have a greater interest in the test. In this situation effort is an inter-individual difference. Alternatively, if we tested the same person multiple times, they may put in more effort in the first testing session and less in the later testing session - perhaps because they are more tired. In this case, the effort someone puts in is an intra-individual difference. Because a single factor could be both an inter-individual difference and an intra-individual difference, telling the difference can be difficult but if you look at the context as a whole this will help.  We can go through some more examples if that would be helpful?