No measurements were made on their final adult heights. Nonresponse can cause bias in surveys because non-respondents often tend to behave differently from people who respond. Nonsampling errors are often bigger than the random sampling errors in surveys. Slight changes in the wording of questions can make a measurable difference to survey results. People will sometimes answer a question differently for different interviewers.
Randomization ensures that both samples are representative of their respective populations.
The precision is determined by the ratio of the sample size to the total population size. A smaller proportion of the American population has been chosen. Therefore, a particular person has a smaller chance of being selected in America than in Canada. A potential stratification variable for both countries could be location - eastern, middle, or western continental. Random digit dialing to select people for the survey could induce biases in the results if the characteristic of interest for the survey is related to income.