The first decision a researcher must make is whether to test a group of people at a single moment in time or whether to test a group of people several times, across many days, weeks or even years. Imagine you need to make this decision. What are the pros and cons of each choice?
If you decide that you are going to study a group of people just one time, in a single session, that research design is called cross-sectional research. An obvious pro, or advantage, to cross-sectional research is that it is relatively easy and inexpensive. It's also easier to do your study using a large group of people because you only need them for a single session. It's easier to get people to agree to be in a study if it only takes a single hour or single day. However, a con, or disadvantage, to cross-sectional research is that you cannot study whether people change over time. That's where the second type of research comes into play.
If you decide that you want to study a certain group of people over multiple sessions, or over an extended period of time, that research design is called longitudinal research. You can remember the name 'longitudinal' because it starts with the word 'long,' meaning a long period of time. The advantages and disadvantages of longitudinal research are the exact opposite of cross-sectional research. With longitudinal research, you can follow people over time, which means that you can see how their thoughts or behaviors change across different locations, when they are around different people or simply as they age.