An animal rescue agent wanted to estimate the true proportion of all animals in shelters that are adopted each month. To do so, she selects a random sample of 100 animals that were in shelters last month and determines that the 95% confidence interval for the true proportion of animals adopted is between 0.12 and 0.24. This interval has a margin of error of 0.06. Which of the following can be accounted for by the margin of error?