We say that the design of a study is biased if
a. the sample isn’t perfectly representative of the population.
b. you get a sample result that differs from the true value.
c. it is very likely to underestimate or very likely to overestimate the value you want to know.
d. the correlation is greater than 1 or less than –1.
e. an observational study was used when an experiment would have been feasible.