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Internal Consistency Reliability


Also known as item homogeneity, this method for assessing a test’s reliability involves determining the extent to which, if people score well on one item, they also score well on the other test items. If each of the test’s items were a perfect measure of reasoning ability, that is to say the score the person obtained on the items was not influenced by any random error, then the only factor that would determine whether a person was able to answer each item correctly would be the item’s difficulty. As a result, each person would be expected to answer all the easier test items correctly, up until the point at which the items became too difficult for them to answer. In this way, the extent to which respondents’ scores on each item are correlated with their scores on the other test items, can be used to estimate the test’s reliability.


The most commonly used internal consistency measure of reliability is Cronbach’s (1960) alpha coefficient. If the items on a scale have high intercorrelations with each other, then the test is said to have a high level of internal consistency (reliability) and the alpha coefficient will be high. Thus a high alpha coefficient indicates that the test’s items are all measuring the same thing, and are not greatly influenced by random measurement error. A low alpha coefficient on the other hand suggests that either the scale’s items are measuring different attributes, or that the test’s scores are affected by significant random error. If the alpha coefficient is low this indicates that the test is not a reliable measure, and is therefore of little practical use for assessment and selection purposes.








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