In CTT, the Cronbach’s a statistic measures the internal consistency, or reliability, of an instrument. Values
range between 0 and 1 and are highest when the variance of items is small compared to the variance of total test
scores, with values over 0.70 generally accepted as indicating a reliable instrument. P-values measure the
difficulty of each item. An item’s P-value is the proportion of students that got the item correct. Here, however,
we report item difficulty as D, the proportion of students that got the item incorrect so that higher values indicate
more difficult items. With this adaptation, difficulty values lower than 0.20 indicate items that might be too easy
and values higher than 0.80 indicate items that might be too hard (Schlingman et al. 2012). The point-biserial
index, rpb, measures an item’s ability to discriminate between high ability students and low ability students. A
value over 0.30 generally indicates that students’ scores on that item are well-correlated with their total test
scores.
In CTT, the Cronbach’s a statistic measures the internal consistency, or reliability, of an instrument. Valuesrange between 0 and 1 and are highest when the variance of items is small compared to the variance of total testscores, with values over 0.70 generally accepted as indicating a reliable instrument. P-values measure thedifficulty of each item. An item’s P-value is the proportion of students that got the item correct. Here, however,we report item difficulty as D, the proportion of students that got the item incorrect so that higher values indicatemore difficult items. With this adaptation, difficulty values lower than 0.20 indicate items that might be too easyand values higher than 0.80 indicate items that might be too hard (Schlingman et al. 2012). The point-biserialindex, rpb, measures an item’s ability to discriminate between high ability students and low ability students. Avalue over 0.30 generally indicates that students’ scores on that item are well-correlated with their total testscores.
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