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from the first period, for example, it could artificially inflate the reliability estimate. When using this method, you should present evidence that this is not an issue. Alternate Forms method. This method, also referred to as parallel forms, is basically the same as the Test-Retest method, but with the use of different versions of the scale during each session. The use of different versions reduces the likelihood that the first administration of the scale influences responses to the second. The reliability of the scale can then be estimated by the correlation between the two scores. When using alternate forms, you should show that the administration of the first scale did not affect responses to the second and that the two versions of your scale are essentially the same. The use of this method is generally preferred to the TestRetest method. Split-Halves method. One difficulty with both the Test-Retest and the Alternate Forms methods is that the scale responses must be collected at two different points in time. This requires more work and introduces the possibility that some natural event might change the actual true score between the two administrations of the scale. In the Split-Halves method you only have respondents fill out your scale one time. You then divide your scale items into two sections (such as the even-numbered items and the odd-numbered items) and calculate a score for each half. You then determine the correlation between these two scores. Unlike the other methods, this correlation does not estimate your scale’s reliability. Instead, you get your estimate using the formula:
ρˆ =
2r 1+ r
where ρˆ is the reliability estimate and r is the correlation that you obtain.
Note that if you split your scale in different ways, you will obtain different reliability estimates. Assuming that there are no confounding variables, all split-halves should be centered on the true reliability. In general it is best not to use a first half/second half split of the questionnaire since respondents may become tired as they work through the scale. This would mean that you would expect greater variability in the score from the second half than in the score from the first half. In this case, your two measurements are not actually parallel, making your reliability estimate invalid. A more acceptable method would be to divide your scale into sections of odd-numbered and even-numbered items. Calculating reliability from internal consistency The other way to calculate reliability is to use a measure of internal consistency. The most popular of these reliability estimates is Cronbach's alpha. Cronbach's alpha can be obtained using the equation:
α=
Nr , 1 + r ( N − 1)