One reasonable reading of this literature would be that there are age-
related declines in some aspects of Extraversion and increases in Agree -
ableness but that most other aspects of personality remain constant
across the lifespan. But many researchers in the field would be skepticalabout this conclusion. They would point out that cross-sectional find -
ings may result from many causes besides maturation. One alternative
interpretation is provided by sampling bias.
Sampling Bias
Virtually all psychological research makes use of sampling and statisti -
cal inference in reaching its conclusions. We do not give personality
tests to all 80-year-olds and all 30-year-olds; we give them to some fewwhom we can persuade to take them. Yet the conclusions we are inter -
ested in are not limited to the group we happen to have measured but
concern aging in general or 80-year-olds in general. A great deal ofacute mathematical thinking has gone into the development of statisti-cal tests that allow us to determine whether the differences we see be-tween two random samples are real or simply a chance outcome of theparticular group of people we happened to examine. In general, thelarger the sample, the better the estimate of the true score for the popu-lation, the whole group about which we want to make generalizations.Samples of about 2,000 are routinely used to estimate the feelings of250 million Americans on such issues as the energy crisis or the likeli-hood of an economic depression.
Sampling itself, then, is not a problem for science. The trouble co -
mes in the requirement that the sample be random . If we could go
through the records of the U.S. Census Bureau, selecting every 100,000thcitizen, we would be in a good position to make the best of a cross-sectional study . But samples tend to be quite different in real life. Inpractice, we find a sample by asking the cooperation of a senior citizens’group, or advertising in the paper, or asking our friends if they or theirparents would participate in our study . This is sometimes called haphaz -
ard sampling , since it is neither really random nor exactly systematic.
In the early days of gerontological research, some dreadfully flawed
conclusions were reached because of this problem. Old people were se -
lected from nursing homes, since there were few senior citizens’ clubs
in those days; young people, of course, came from the colleges at whichthe researchers worked. The so-called age differences that emerged from62 PERSONALITY IN ADULTHOOD