Answer to Question 1
A comparison of the major strengths and limitations is presented in Table 5.5 in the text. Essentially, a cohort study may provide complete data on the cases and stages of a disease in which calculations and comparisons can be made between those who were exposed and not exposed. These studies tend to be expensive and take many years to complete, thus increasing the amount of non-response data and limiting the control of confounding variables. Case-control studies are relatively quick and inexpensive studies that are an excellent way to study rare diseases and diseases with long latency periods. Oftentimes, existing records can be utilized on a relatively few study subjects. Limitations of this type of study include that the mechanism of the disease is not studied, with validation of data difficult to achieve.
Answer to Question 2
Two possible explanations for incorrect or invalid results include:
They are biased due to a systematic error in measuring one or more outcome variables or there were systematic differences in the populations studied.
The results are due to chance and do not represent the true state of affairs; that is, the observations made arose from random variations within the sample.
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