bluestar
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are they the same? Kaplan says yet but FA listed them as two enteies. who can explain this please?
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| PsychDr2B
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I was also surprised to see this confusion in the Behavioral Science Kaplan notes (2001 edition) as it lists the two biases in the same entry: Selection bias (sampling). I think this is an over-simplification on their part. Other resources make a clearer distinction. FA2004 does list them separately with two different defintions. This differentiation is verified in other sources, published even prior to Kaplan notes.
According to BRS Behavioral Science, 3rd Ed., 2000, Selection Bias and Sampling Bias are definitionally different forms of biases in research designs.
To paraphrase:
SELECTION BIAS: occurs when the subject CHOOSES whether to enter a drug group or a placebo group rather than being randomly assigned. OR, the investigator purposely CHOOSES to put a subject in a drug or placebo group.
SAMPLING BIAS: volunteer subjects in a study may not be representative of the population being studied, as a consequence, the results of the study may not be generizable to the entire population.
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| PsychDr2B
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Here is an example question from BRS Behavioral Science:
A study is designed to compare a new medication for Crohn's disease with a standard medication. Each of 50 Crohn's disease patients is allowed to decide which of these two treatment groups to join. The major reason that the result of this study may not be valid is because of...
A) selection bias
B) recall bias
C) sampling bias
D) difference in size of these two groups
E) the small number of patients in the study.
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| bluestar
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Now I can confidently select A, selection bias. Thanks for the explanation and sample question!
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| PsychDr2B
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CORRECT! Here is BRS Behavioral Science's explanation for this question:
A. The major reason that the results of this study are not valid is because of selection bias (i.e., the subjects were able to choose which group to go into). If very ill people were more likely to choose the standard treatment, people in the experimental treatment group (who were healthier to begin with) would have had a better outcome. In recall bias, knowledge of the presence of disorders alters the way subjects remember their histories. In sampling bias, subjects choose to be in a study because of factors which may be unrelated to the subject of the study, but which distinguish them from the rest of the population. A study can be valid even though two groups may be of different sizes or their is a small number of patients in the study.
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| mjl1717
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| bluestar
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thank you again, PsychDr!
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