Content validity is achieved when an instrument has appropriate content for measuring a complex concept, or construct. If you walk out of a test and feel that it was unfair because it tapped too narrow a band of knowledge, your complaint is that the test lacked content validity.
Content validity is very, very tough to achieve, particularly for complex, multidimensional constructs. Consider, for example, what’s involved in measuring a concept like strength of ethnic identity among, say, second-generation Mexican Americans. Any scale to assess this has to have components that deal with religion, language, socioeconomic status, sense of history, and gastronomy.
Religion: Mexican Americans tend to be mostly Roman Catholic, but a growing number of Mexicans are now Protestants. The migration of a few million of these converts to the United States over the next decade will have an impact on ethnic politics—and ethnic identity—within the Mexican American population.
Language: Some second-generation Mexican Americans speak almost no Spanish; others are completely bilingual. Some use Spanish only in the home; others use it with their friends and business associates.
Socioeconomic status: Many Mexican Americans are poor (about 31% of Hispanic households in the United States have incomes below $25,000 a year), but many others are well off (about 20% have incomes above $75,000 a year) (SAUS 2010:table 674). People with radically different incomes tend to have different political and economic values.
Sense of history: Some so-called Mexican Americans have roots that go back to before the British Pilgrims landed at Plymouth Rock. The Hispanos (as they are known) of New Mexico were Spaniards who came north from the Spanish colony of Mexico. Their selfdescribed ethnic identity is quite different from recent immigrants from Mexico.
Gastronomy: The last refuge of ethnicity is food. When language is gone (Spanish, Yiddish, Polish, Gaelic, Greek, Chinese . ..), and when ties to the ‘‘old country’’ are gone, burritos, bagels, pirogis, corned beef, mousaka, and lo mein remain. For some second- generation Mexican Americans, cuisine is practically synonymous with identity,for others it’s just part of a much larger complex of traits.
A valid measure of ethnic identity, then, has to get at all these areas. People’s use of Spanish inside and outside the home and their preference for Mexican or Mexican American foods are good measures of some of the content of Mexican American ethnicity. But if these are the only questions you ask, then your measure of ethnicity has low content validity. (See Cabassa  and Cruz et al.  on acculturation scales for Hispanics in the United States.)
‘‘Life satisfaction” is another very complex variable, composed of several concepts— like ‘‘having sufficient income,’’ ‘‘a general feeling of well-being,” and “satisfaction with level of personal control over one’s life.’’ In fact, most of the really interesting things that social scientists study are complex constructs, things like ‘‘quality of life,’’ “socioeconomic class,’’ ‘‘ability of teenagers to resist peer pressure to smoke,’’ and so on.