A study was done by calling land-line telephones, and those conducting the survey are concerned about the possibility of undercoverage, since some people don't own a phone or only own a cell phone. Which of the following is the best way for them to correct for this source of bias?
a. use a lower confidence level, such as 80%
b. use a higher confidence level, such as 99%
c. take a larger sample
d. use a t-interval instead of a z-interval
e. throw this sample out and start over again with a better sampling method.