You may have noticed a few paragraphs back that I casually slipped in the statement that some scale had already been determined to be a valid instrument. How do we know that the scale is measuring weight? Maybe it’s measuring something else. How can we be sure? We have to make concepts up to study them, so there is no direct way to evaluate the validity of an instrument for measuring a concept. Ultimately, we are left to decide, on the basis of our best judgment, whether an instrument is valid or not.
We are helped in making that judgment by some tests for face validity, content validity, construct validity, and criterion validity.
Establishing face validity involves simply looking at the operational indicators of a concept and deciding whether or not, on the face of it, the indicators make sense. On the face of it, asking people ‘‘How old were you when you were toilet trained?’’ is not a valid way to get at this kind of information. A paper-and-pencil test about the rules of the road is not, on the face of it, a valid indicator of whether someone knows how to drive a car. But the paper-and-pencil test is probably a valid test for determining if an applicant for a driver’s license can read road signs. These different instruments—the road test and the paper-and-pencil test—have face validity for measuring different things.
Boster (1985) studied how well the women of the Aguaruna Jivaro in Peru understood the differences among manioc plants. He planted some fields with different varieties of manioc and asked women to identify the varieties. This technique, or instrument, for measuring cultural competence has great face validity,most researchers would agree that being able to identify more varieties of manioc is a valid indicator of cultural competence in this domain.
Boster might have simply asked women to list as many varieties of manioc as they could. This instrument would not have been as valid, on the face of it, as having them identify actual plants that were growing in the field. There are just too many things that could interfere with a person’s memory of manioc names, even if they were really competent about planting roots, harvesting them, cooking them, trading them, and so on.
Face validity is based on consensus among researchers: If everyone agrees that asking people ‘‘How old are you’’ is a valid instrument for measuring age, then, until proven otherwise, that question is a valid instrument for measuring age.