Determine the principal reason for the unreliability of the survey results:
In order to determine how American undergraduate college students feel about eliminating spring break in order to finish spring term a week early, a survey was conducted. 200 undergraduate students from the University of Miami (FL) were interviewed. Both of the interviewers hired to conduct the survey were told to interview 25 freshmen, 25 sophomores, 25 juniors, and 25 seniors. Of the 200 students interviewed, $20 \%$ were in favor of the elimination of spring break, $70 \%$ were opposed, and $10 \%$ had no opinion.
The results of this survey are unreliable primarily because of
the absence of a control group.
sample (selection) bias only.
nonresponse bias only.
both sample (selection) bias and non-response bias.
None of the above
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Final Answer: The principal reason for the unreliability of the survey results is \(\boxed{\text{sample (selection) bias only}}\).
Step 1 ：The question is asking for the main reason why the survey results might be unreliable. The survey was conducted among 200 undergraduate students from the University of Miami, with an equal number of students from each year of study. The results showed that 20% were in favor of eliminating spring break, 70% were opposed, and 10% had no opinion.
Step 2 ：The options given are: the absence of a control group, sample (selection) bias only, nonresponse bias only, both sample (selection) bias and non-response bias, and none of the above.
Step 3 ：A control group is not necessary in a survey study, so the absence of a control group is not a valid reason for the unreliability of the survey results.
Step 4 ：Sample (selection) bias occurs when the sample is not representative of the population. In this case, the sample only includes students from the University of Miami, which may not be representative of all American undergraduate college students. Therefore, there is a possibility of sample (selection) bias.
Step 5 ：Nonresponse bias occurs when the respondents who chose not to respond have different opinions from those who did respond. In this case, there is no information given about nonresponse bias.
Step 6 ：Therefore, the most likely reason for the unreliability of the survey results is sample (selection) bias.
Step 7 ：Final Answer: The principal reason for the unreliability of the survey results is \(\boxed{\text{sample (selection) bias only}}\).
