l43 - Group Comparisons: Differences in Composition Versus...

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Group Comparisons: Differences in Composition Versus Differences in Models and Effects—Page 1 Group Comparisons: Differences in Composition Versus Differences in Models and Effects Overview. This is the first of a series of handouts that will deal with techniques for comparing groups. This initial handout notes that, when comparing groups, it is important to realize that groups can differ in two ways: There can be compositional differences between groups. Specifically, means on the IVs may differ between groups. The effects of IVs can differ between groups. A variable might have a stronger effect on one group than it does on the other. Indeed, the direction of an effect may even differ between groups. The model that describes one group may be very different from the model that describes another. For example, blacks may have lower levels of education and less job experience than do whites. As a result, they may tend to have lower levels of income, even if the effects of education and job experience are the same for both groups. Simple T-tests or ANOVA tests can determine whether there are significant compositional differences between groups. Or, blacks may have similar levels of education and job experience, but the effects of these variables may be less for them, e.g. a year of education is worth less to a black than it is to a white. As a result, blacks may tend to have lower incomes than comparable whites. Compositional, or mean, differences between groups on the IVs may suggest that differences on the DVs are “justified”, e.g. blacks earn less than whites because they are less educated; women earn less than men because they are concentrated in lower-paying occupations, or have less continuous service with the same company. Of course, one must then ask what produced the compositional differences. It may be, for example, that race is a cause of education and job experience; that is, race may be an indirect cause of income, because race affects education and job experience which in turn affect income. Differences in effects raise questions about why those differences exist. If blacks benefit less from education than whites, is this perhaps because of discrimination? Or do other factors need to be considered in the model? It is important to keep compositional differences and differences in effects separate. Researchers will sometimes confuse the two, muddling the discussion of why group differences exist. In particular, researchers sometimes focus a lot on their models, and overlook how important compositional factors can be in explaining group differences. Differences in Composition. Returning again to our hypothetical data from 400 whites and 100 blacks: the following t-tests and descriptive statistics reveal that blacks have lower levels of education and job experience than do whites. These differences are all highly significant. These lower levels of education and job experience probably are part of the reason that black income is also lower than white income.
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l43 - Group Comparisons: Differences in Composition Versus...

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