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Item Information Function for 3PLM
IIF3PLM(a, b, c, theta)
Returns a numeric vector representing the item information at each ability level theta. The information is calculated as: \(I(\theta) = \frac{a^2(1-P(\theta))(P(\theta)-c)^2}{(1-c)^2P(\theta)}\)
slope parameter
location parameter
lower asymptote parameter
ability parameter