Final answer:
Using a random sample for their survey, the student council at Harvest High School is most likely achieving internal validity, as this method reduces confounding variables and selection effects, thus reflecting the true level of school spirit.
Step-by-step explanation:
The student council at Harvest High School used a random sample to measure school spirit, implying that every student had an equal chance of being selected for the survey. Given this information, the results of the survey most likely have internal validity. Internal validity refers to the extent to which the survey accurately reflects or assesses the specific concept that the researchers aim to measure, which in this case is the school spirit among students. By using a random sample, the student council minimizes other forms of validity, such as face validity, which is the appearance of the survey being valid to respondents and laypeople. By avoiding selection effects and confounding variables, the results are more likely to represent the true level of school spirit at Harvest High School.