A psychology professor wants to know whether study skills are related to memory quality in current first-year students at her small college. Participants in the study (first-year students at her college) complete an online memory task. The students are first shown a list of 60 words. Next they are shown a list that includes 10 words that were on the original list. Then they are asked to identify the words on the second list that appeared on the original list. She uses the percentage of words that were correctly recognized on the original list as her measure of memory quality. She also asks the students to report several characteristics such as their age, their gender, and a self-assessment of their study skills. Each of the 1,000 first-year students (543 males and 457 females) at her school volunteers to participate. The professor chose 100 students at random to complete the memory task and answer the questions. The average percentage of words that were correctly recognized on the original list was 68%. The professor infers that if all 1,000 first-year students had completed the study, the results would show that an average of 68% (plus or minus sampling error) of the words were correctly recognized as being on the original list.
Which of the following are variables in the study?
a. The students, percentage of words that were correctly recognized on the original list
b. The students' self-assessments of their study skills
c. The 100 students
d. The 1,000 students