Issues, concepts, and methods in the study of older adults. Diversity in older adults: the word diversity is well suited to the older population. Not only are older people different from one another, but they are more different from one another than are individuals in any other age group. Age effects are the consequences of being given chronological age. Cohort effects are the consequences of having been born in a given year and having grown up during a particular time period. A cohort effect exists if these people have some factor that distinguishes them from people who turned 65 or older at an earlier date. Tim-of-measurement effects are confounds that arise because events at an exact point in time can have a specific effect on variable being studied over time. For example, time of measurement could affect the results of studies assessing post-traumatic stress disorder is holocaust survivors if one of the assessments occurs shortly after 9/11.