In the New York coupon experiment mentioned in your text, the experiment was conducted to test the effects of the face value of coupons on the likelihood of coupon redemption. Subjects were randomly assigned to 2 treatment groups. One group was offered 15-cent coupons and the other 50-cent coupons for four products. During the interviews, the respondents answered questions about which brands they used and how likely they were to cash coupons of the given face value the next time they shopped. In the preceding experiment, the test units were ________.