A group of biologists wants to estimate the abundance of barrel cactus in a desert. They divide the desert into a grid of 100 rectangular areas but exclude 10 of those areas because they are difficult to access. The biologists then measure the density of cactus in a randomly selected sample of 40 of the 90 accessible areas.
a. What was the sampling frame in this study, and how did it differ from the population of interest?
b. Explain why the problem called "using the wrong sampling frame" might lead to a biased estimate of the abundance of cactus in the desert.