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Comparing the Normal Ogive and Logistic Item Characteristic Curves

​
scaling constant D
1.
item parameters
discrimination a
1.
difficulty b
0.
guessing c
0.
In item response theory, the relationship between a latent ability (
θ
) and the probability of a correct response (
P
) on a test item is modelled by an item characteristic curve. This Demonstration plots the item characteristic curve of a single dichotomous item under two different models: the normal ogive model and the logistic model. The parameters
a
,
b
, and
c
represent item properties related to discrimination, difficulty, and guessing. The constant
D
is used to scale the logistic curve. Notice that the two curves are nearly identical when
D≈1.7
.
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