SS - Statistical Studies Design Lesson

Statistical Studies Design Lesson

Sample Size

Statistical studies are designed with carefully selected measures that ensure (within error margins) that, if the sample is well selected and the study is well designed and conducted, the mean and other measures of the sample are likely to be similar to the corresponding measures of the population being studied. Sometimes, if the population is small (such as high school seniors in a small town), it may be possible that the sample studied is the entire population. However, often a sample is a smaller subset of a population (such as a research question that might target the entire population of high school seniors in a state or in the nation).

Blind Study/Placebo Effect

With a blind study, participants do not know whether they are receiving the treatment or the placebo. This is crucial for trying to control the placebo effect. Using a placebo is worthless if the participants know they are getting it.

There is often a concern that if researchers know who is getting the placebo, it may affect their ability to fairly assess a treatment. For example, researchers may subconsciously check for improvement more carefully. A physical therapist may push the patient who is receiving the treatment to work harder, while not encouraging the control group patient as much. In a double-blind study, only a third person not involved in the assessment knows the participants who are receiving the treatment and those who are in the control group. After all data have been collected, this person then identifies each patient file as treatment or control.

Revising Spud Potato Chips Sample

Recall the Spud Potato Chips scenario from earlier. You hypothesized that the true mean weight of bags of Spud's might be less than the 28.3 grams advertised on the bags. Discuss and make some notes on how you might collect a sample of bags to test your hypothesis. Remember that the sample should be representative of the population.

What do you mean by "the population of Spud Potato Chips" that you are interested in testing? The population of interest is all bags of potato chips, not just those being sold at your school. 

If we only test the bags at the school, it is likely that all of those bags came from the same shipment. Perhaps that shipment was undersized for some reason (for example, faulty equipment that has since been fixed). We should at least select bags from various places around town. The best possible scenario is testing bags from around the country.

 

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