Chapter 7: Constructing Composite Measures: Scales and Indexes

Introduction

In previous chapters you became familiar with the language of research; particularly, the use of indicators and their attributes for measuring the dimensions of concepts.   These discussions were based on the use of individual items, as in questionnaires. In this chapter, we expand the ways researchers measure indicators by showing how responses to sets of items can be combined to form composite measures of indicators.

Composite measures are based on a simple idea: A number of indicators provide a better measure of a concept than a single one. In composite measurement, responses to two or more items are combined to provide a single score or measure for the indicator.

Every exam you have taken is a kind of composite measure. On your last exam, suppose you answered 18 of 20 questions correctly. Your instructor gave you a grade by assigning 1 point for each correct answer and 0 for incorrect answers and then added the correct points to get your grade of 18 on the exam. The 20 questions formed a composite measure of your knowledge of the material covered in that course. Your grade of 18 was your score on the exam. Throughout this chapter, you will learn how this logic is applied in the development of more complex composite measures.

Kinds of composite measures

Scales and indexes are the main forms of composite measures. Each is based on assigning numbers to responses to the items making up the index or scale. These numbers are added together to generate the score for the composite measure. The score is then used in place of the individual responses.

The terms scales and indexes are frequently used interchangeably. This causes some confusion. In general, scales are used for measuring the intensity of feelings, attitudes, or beliefs of persons. Indexes are used to measure variables such as socio-economic status or various health or other social indicators.

Essential features

Scales and indexes are alike in several important ways. Each provides measurements at the ordinal, interval, or ratio levels, depending on the level of measurement used for the items.   Construction of indexes and scales is also based on similar requirements. First, all the items making up the index or scale should measure the same thing. In technical terms, the items should be unidimensional or one-dimensional in content. Going back to the exam illustration, you would expect all the test items to be related to material covered in your course. Questions about the content of other courses would violate the unidimensional requirement, and you would rightly object to this error. In addition, responses to the items used in a scale or index have to be mutually exclusive and exhaustive. Mutually exclusive means that respondents can give one and only answer; exhaustive means that response categories must allow for all possible answers.