Using data collected between 1957 and 1978, from 15 samples of adults, Professor Rodgers (1982 Social Forces) found that (1) the average level of happiness reported by people under 65 years of age declined from 1957 to 1970. For this same group, the average level of happiness increased slightly from 1970 to 1978. (2) The average level of happiness reported by people age 65 and older increased from 1957 to 1978. A (the) variable(s) in this study is (are)