Ecological Study – Interventional Studies (Study Designs)

00:01 So having said that, the last kind of study design we're going to talk about this lecture is the ecological study. It isn't really an experiment. It's probably a kind of observational study as well, but very often it's spoken about in the same breath as an experiment.

00:16 The defining characteristic of an ecological study is that the unit of analysis is a population or a group and not an individual. Here is an example, populations with high proportions of pets are also populations with low frequencies of asthma. Think about that for a second.

00:35 That will lead you to probably surmise that owning a pet would reduce your chances of having asthma. The problem is you can't really make that conclusion. You can't say that an individual living in one of these towns with high amounts of pets is likely to not have asthma. We can't make predictions about individuals based upon these population averages. This is what we call the ecological fallacy. An ecological fallacy is when we infer something about an individual based upon observations about a group, so what we learn from an ecological study cannot be used to make conclusions about individual behavior. I would say the ecological fallacy is the foundation of racism. You observe behavior of a group and you assume the individuals from that group must also have the same characteristics or behaviors. Another example, it has been observed that the greater the proportion of immigrants in a given US state, the lower the average literacy level of that state. You might conclude then that immigrants have a lower level of literacy. It's not true. Individual immigrant residents are more literate than the native residents, what's going on? A number of scenarios, I encourage you to investigate further if this interests you, but the important observation here is that observations made about a population cannot be extrapolated to individual characteristics from that population.

01:56 So let's go over what we've learned. What distinguishes an experiment from an observational study, do you remember? Well in an experiment the researcher, you perhaps, manipulates a variable in the study, such as deciding who gets the intervention and who gets the placebo.

02:13 Maybe that decision is made by the flip of a coin, but you were involved in doing the flipping at the very least. What's considered the gold standard of evidence? The RCT is, because of its controlled nature. We can control extraneous variables and control confounding quite well, so it has a high degree of internal validity, hence a high degree of causal proof.

02:37 Why is random allocation important? Well because it allows for the equal distribution of biasing factors, not just the ones you know about, especially the ones you don't know about. And again that improves internal validity. Why is blinding important? Well blinding is important to control for certain biases, especially the Rosenthal and Hawthorne effects. And how is a quasi-experiment different from an RCT? It lacks randomization, that's the 'R' part. And how is a natural experiment different from an RCT? Do you remember? Allocation in a natural experiment is determined by an external force, like a natural disaster or a law of some kind. And what's the distinguishing trait of an ecological study?