Apart from calculating whether each item measuring a construct is in accordance with the other items measuring the same construct, it is also necessary to calculate the reliability of all items combined when measuring the construct. In the past, the split-half reliability was calculated. For the split-half reliability all items are subdivided into two sets. A total score is computed for each set and then the correlation between both sets is calculated. If the items in both sets measure the same construct, there should be a high correlation between the tests. The correlation (and hence split-half reliability) is considered high if it is .70 or higher.
The disadvantage of the split-half reliability is that the correlation that is found depends on which items are placed in which set. If you subdivide the items a little differently, it may result in a different split-half reliability. Because of this reason, we recently calculate more often the ‘Chronbach’s alpha coefficient’.
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