Indexes

Definition

Indexes are based on multiple items, each of which taps a dimension of some variable. Unlike scales, which are based on items of varying intensity, items used in indexes are treated as having the same intensity. Each taps a different aspect or component of a variable.

Development of an index follows the same process described earlier for construction of scales, beginning with the definition of the concept to be measured. As with scale construction, the next step is to select indicators for use in constructing the index.

Selecting indicators

An example should help in understanding how an index is constructed. Imagine that we wanted to develop an index for socio-economic status of families. We define the concept as it is commonly used; as the generally perceived ranking of families from high to low within the community. We further decide that we will use three indicators of socio-economic status: (1) years of schooling of the male heads of households; (2) the occupations of the heads; and (3) the kind of material used in the construction of their houses.

Other indicators could have been used. We might, for example, have decided to use the square meters of floor space in houses or the wealth of the heads of households. For practical reasons, we decide to use the kind of building material instead of floor space. An interviewer can easily see and record what kind of material the house is made of, whereas getting floor space would require a lot of questions or some form of measurement. By using what the house is made of, we also avoid imposing further on respondents. Also, we realize that getting accurate data on wealth will be difficult. Respondents will resent being asked about their wealth and probably will refuse to answer or will give false replies. Asking about schooling and occupation is less threatening and are facts that most persons will provide.

Also, in selecting items, we also try to select those that will produce the greatest amount of variation in scores. As we said earlier, variation is necessary, for without variation there is no point in using an indicator. In building an index, therefore, look for indicators for which there is considerable variation. Our three indicators, we think, will provide wide variation.

We also have to demonstrate that the selected indicators are valid measures of socio-economic status. Unidmensionality and validity can be established in several ways. One approach is to argue that the items are logical or reasonable measures of the concept and that concept only. Such claims of face or logical validity can be made by the researcher, but it is better to produce stronger support by having other competent persons review the items. Agreement by a panel of reviewers strengthens claims for face validity. (Other ways of estimating validity are described in Chapter 6). Let's say we did this and found that all judges agreed that the three indicators were, in fact, reasonable indicators of socio-economic status. We would then be ready to develop the index. This begins with writing the three indicators we have chosen.

Writing items

We describe how to write items in Chapter 10, as part of questionnaire development, so we won't go into detail here. Let's say we are using a questionnaire to collect our data and decided to use the following three items:

1. What is the main occupation of the head of this household?
2. How many years of schooling did the head of this household complete?
3. The house is made of:
   • combination of cardboard, cloth, tin, etc.
   • mud bricks
   • concrete blocks
   • fired bricks
   • other, describe: _______________________

The first two items are open-ended; the interviewer would record what the respondents gave as their answers. The third item includes four response categories with an open-ended response. Together, these categories meet the criteria of being mutually exclusive, but we added the category of "other" to ensure that the attributes of house construction would also meet the criterion of exhaustiveness. We might be fairly sure the four categories cover all the kinds of house construction, but we added the "other" just in case some other material might be found. On the blank line following "other," the interviewer would record a description of any additional   form of house construction. The three items would then be included in our questionnaire to collect data for constructing the index.

Identifying attributes

Our next decision would be to assign numerical values to each attribute or category making up each indicator. To achieve greater variability, researchers generally use three to five categories for an indicator. In our illustration, however, for simplicity, we use only two categories for each variable. These categories are simply described as "low" or "high" for each item. We would examine each occupation and place each in a high or low category, based on ratings of social prestige of occupations in the community where we are doing our study. A panel of judges could be used to establish the breaking point between low and high prestige occupations. A dividing point would be established for years of schooling as well. We might combine the first two attributes for house construction to form the low category and the other two to form the high attribute for this indicator. "Other" responses would be placed in either category: depending on how similar they are to one category or the other.