Hi Everybody, I have just been introduced to R and have used the ltm package for calibrating items under IRT. I have been able to get the item parameters using the fit function under different models. I am using the 2PL model. I have got the parameters for some 300 items from 15 different tests using the function fit.
Now I need to automatically generate tests such that they have atleast a certain level of information. Thus I have created a data frame with item id, and information at 20 different points/ability levels. I have to pick questions from the calibrated item set. Right now I am just picking up 30 questions at random from the bank and adding the information at these 20 points. But I also need to plot the information function. Till now I have been using the plot function defined in the package ltm but it needs an object which inherits from ltm. However I don't know how to create this object as I don't have any response pattern for the set of 30 questions randomly picked from the bank as it hasn't been used in any test yet. How can I do this? Also how can I plot the ICC for any item if I just have the item parameters, difficulty and discrimination but not the fit object? Is there someway to plot the curve using the formula given under 2 PL IRT model and the values for a and b? Thanks and regards, -- Megha Student, Chennai Mathematical Institute Chennai (India) ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
