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Cross-sectional research is a type of non-experimental research.  We employ cross sectional research methods when we want to compare two or more pre-existing groups of people.  The independent variable is not manipulated, nor is there random assignation of participants to the groups.  An example would be a researcher who wants to compare the memory ability of people who regularly eat a balanced diet, according to the Canada Food Guide 2019, versus those who do not. As it would not be ethical to randomly assign participants to the unhealthy eating group, we would be required to compare pre-existing groups of healthy and non-healthy eaters; however, it is important to note that there is a danger of introducing a selection bias to the research, because the groups may differ in other ways. For example, the healthy food eating group may also be more likely to exercise and get more sleep, both of which increase memory function. We would not know then what the effect of healthy eating is, in isolation, upon memory ability, because there may be other variables (e.g. exercise, sleep) that factor into memory ability.