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There are six basic types of epidemiological study designs, and they can each be distinguished using certain criteria.
The first criterion for deciding which study design to use is whether you have individual or group data.
For example, let’s say we want to know how many people out of 100 people had migraines in the past year.
Now, with individual data, we have information about each person, so we can tell whether or not each of the 100 people had a migraine.
So, let’s say that 9 people had migraines. If we have individual data, we can look at the individual characteristics for each of the 9 people that had migraines, like their sex, age, race, or past history of migraines, and we can compare them to the people that didn’t have migraines.
On the other hand, if we have group data, we don’t actually know which specific individuals out of the 100 people had migraines.
So even though we know that 9 people had them, we don’t know which 9 people they were or any of their individual characteristics.
Now, ecological studies are a type of study design that uses group data to figure out if there is a potential association between two variables.
For example, let’s say you want to figure out if people who sleep less are more likely to get migraines.
And perhaps you have information about average sleep duration for populations in ten different cities.
You could plot this information on a graph with average sleep duration on the x-axis and the prevalence of migraines—which is the number of people that suffer from migraines, per 100,000 people—on the y-axis.
The thing is, we can’t actually say that getting less sleep causes migraines, since we don’t have information about each individual in each city.
Ecological studies are helpful for making hypotheses, though, that can later be tested using individual-level studies.
And, in general, individual-level studies are considered stronger than ecological studies, because knowing individual characteristics can help us determine what risk factors are associated with certain diseases.
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