Compare These With Those These children seem ideal for cross-sectional research—they are school children of both sexes and many ethnicities. Their only difference seems to be age, so a study might conclude that 6-year-olds raise their hands but 16-year-olds do not. But any two groups in cross-sectional research may differ in ways that are not obvious—perhaps income, national origin, or culture—and that may be the underlying reason for any observed age differences.