Item-total correlation
Encyclopedia
The item-total correlation test arises in psychometrics
in contexts where a number of tests or questions are given to an individual and where the problem is to construct a useful single quantity for each individual that can be used to compare that individual with others in a given population
. The test is used to see if any of the tests or questions ("items") do not have responses that vary in line with those for other tests across the population. The summary measure would be an average of some form, weighted where necessary, and the item-correlation test is used to decide whether or not responses to a given test should be included in the set being averaged. In some fields of application such a summary measure is called a scale
.
It is supposed that the result for a particular test on a given individual is initially used to produce a score, where the scores for different tests have a similar range across individuals. An overall measure for an individual would be constructed as the average of the scores for a number of different tests. A check on whether a given test behaves similarly to the others is done by evaluating the Pearson correlation
(across all individuals) between the scores for that test and the average of the scores of the remaining tests that are still candidates for inclusion in the measure. In a reliable measure, all items should correlate well with the average of the others.
A small item-correlation provides empirical evidence
that the item is not measuring the same construct measured by the other items included. A correlation value less than 0.2 or 0.3 indicates that the corresponding item does not correlate very well with the scale overall and, thus, it may be dropped.
Psychometrics
Psychometrics is the field of study concerned with the theory and technique of psychological measurement, which includes the measurement of knowledge, abilities, attitudes, personality traits, and educational measurement...
in contexts where a number of tests or questions are given to an individual and where the problem is to construct a useful single quantity for each individual that can be used to compare that individual with others in a given population
Statistical population
A statistical population is a set of entities concerning which statistical inferences are to be drawn, often based on a random sample taken from the population. For example, if we were interested in generalizations about crows, then we would describe the set of crows that is of interest...
. The test is used to see if any of the tests or questions ("items") do not have responses that vary in line with those for other tests across the population. The summary measure would be an average of some form, weighted where necessary, and the item-correlation test is used to decide whether or not responses to a given test should be included in the set being averaged. In some fields of application such a summary measure is called a scale
Scale analysis (statistics)
In statistics, scale analysis is a set of methods to analyse survey data, in which responses to questions are combined to measure a latent variable. These items can be dichotomous or polytomous...
.
The test
An item-total correlation test is performed to check if any item in the set of tests is inconsistent with the averaged behaviour of the others, and thus can be discarded. The analysis is performed to purify the measure by eliminating ‘garbage’ items prior to determining the factors that represent the construct; that is, the meaning of the averaged measure.It is supposed that the result for a particular test on a given individual is initially used to produce a score, where the scores for different tests have a similar range across individuals. An overall measure for an individual would be constructed as the average of the scores for a number of different tests. A check on whether a given test behaves similarly to the others is done by evaluating the Pearson correlation
Pearson product-moment correlation coefficient
In statistics, the Pearson product-moment correlation coefficient is a measure of the correlation between two variables X and Y, giving a value between +1 and −1 inclusive...
(across all individuals) between the scores for that test and the average of the scores of the remaining tests that are still candidates for inclusion in the measure. In a reliable measure, all items should correlate well with the average of the others.
A small item-correlation provides empirical evidence
Empirical research
Empirical research is a way of gaining knowledge by means of direct and indirect observation or experience. Empirical evidence can be analyzed quantitatively or qualitatively...
that the item is not measuring the same construct measured by the other items included. A correlation value less than 0.2 or 0.3 indicates that the corresponding item does not correlate very well with the scale overall and, thus, it may be dropped.