Which of the Following Is a Problem with Studies That

Multiple Choice

Which of the following is a problem with studies that follow a single group of older people over time?

A) Differences between age groups cannot be compared.
B) A shift in the sex ratio due to the deaths of more men than women.
C) Differences between individual characteristics (such as ethnicity or social class) cannot be compared.
D) Lack of randomization invalidates many types of statistical analysis.