Is there a difference between an index score and a composite score?

Question

Following on from my previous question on forming scale scores, my sample has 100 people who have answered 10 likert scale questions each on two academic subjects: Maths and English.

The likert scale is a 5 point scale, going from not important to absolute importance.

I have aggregated the scores:

• The maximum score a person can get is 40 (which is 10 responses x 4 which is the maximum score on the likert scale e.g. 4 = absolute importance).

• The minimum is 0 (which is 10 responses x 0 which is the minimum score on the likert scale e.g. 0 = not important)

If a person gets 20 points (total of 10 likert scale responses for Maths) and 30 points (total of 10 likert scale responses for English), my questions are:

1. Is 20 or 30 a composite score or an index score? (given that they are both based on subjective weighting aligned with the likert scale response)

2. Can I do a correlation analysis on the two scores? (given they are composite/index scores, hence represent a latent variable)

Many thanks

Answer 1

Composite score, scale score, and index score are terms that are often used interchangeably. As to your question 2, I'm not sure what makes you hesitate to use correlation; the usual assumptions involving correlation should apply here. Otherwise, you'll find more information at this page on this site

Answer 2

A scale score is a subtest score. A composite is quite literally a composition of scores. A composite and an index score are generally interchangeable terms. They are usually reported in Standard Scores but can be in T-scores, Z-scores, etc. Standard Scores and scaled scores are not equal interval scales. For comparison purposes you may wish to use percentile rankings. A Psychometric Conversion Table is freely available on the Internet.

On a Likert-like scale, which is a forced-choice scale, the scores are simply "total raw score" or "total score". Your content validity will tell you more about what, if anything, about these scores is comparable.

RC Wilcox, MEd, MS, PsyS.