Split-half testing measures consistency by:
Compute the difference between half-test scores.Split test into two halves and create half-test scores.Just keep in mind that although Cronbach's Alpha is equivalent to the average of all possible split half correlations we would never actually calculate it that way. The figure shows several of the split-half estimates for our six item example and lists them as SH with a subscript. Because we measured all of our sample on each of the six items, all we have to do is have the computer analyze the random subsets of items and compute the resulting correlations. In saying we compute all possible split-half estimates, it doesn't mean that each time we go and actually measure a new sample! Instead, we calculate all split-half estimates from the same sample. Cronbach's Alpha is the geek equivalent to the average of all possible split-half estimates (although that's not how we do it.) If we compute one split-half reliability and then randomly divide the items into another set of split halves and recompute, we can keep doing this until we have computed all possible split half estimates of reliability. estimate of SEM for different test lengths can be obtained using.
where L = the number of times longer the new test will be.used to estimate how much a test's reliability will increase when the length of the test is increased by adding parallel items.Step 3: Adjust the half-test reliability using the Spear-man-Brown formula Step 2: Compute a Pearson r between scores on the two halves of the test. Step 1: Divide the test into equivalent halves. Cronbach's alpha is a less conservative estimate of reliability than test/retest. In the end, your computer output generates one number for Cronbach's alpha - and just like a correlation coefficient, the closer it is to one, the higher the reliability estimate of your instrument. In short, Cronbach's alpha splits all the questions on your instrument every possible way and computes correlation values for them all (we use a computer program for this part). For example, you could write two sets of three questions that measure the same concept (say class participation) and after collecting the responses, run a correlation between those two groups of three questions to determine if your instrument is reliably measuring that concept. Internal consistency estimates reliability by grouping questions in a questionnaire that measure the same concept. Instructions, time limits, assertation examples formed should be equal. The range and level of difficulty of the items should also be equal. The test should contain the same number of items and items should be express in the same form and should cover the same type of content.
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