DIF command parameter "items2test" malfunctioning when provided character vector of itemnames.

[jbuncher]

The "items2test" parameter in the DIF command currently doesn't function correctly when passed a character vector of the items to be tested. Rather than testing the items with the names given, it tests the first n items, where n is the number of items given in the "items2test" list.

I think this is due to an issue in the "which" command used in line 279 in DIF.R: if(is.character(items2test)) items2test <- which(items2test %in% itemnames)

The %in% command returns a vector that lists if each element in items2test is in itemnames. So, if you want to test items 13 and 15 out of a 20-item set, and the item names for 13 and 15 are in itemnames, the %in% command returns "TRUE TRUE". The which command then looks at which indices of that vector are T/F, and will return "1 2", so the first two items of the 20 will be tested for DIF.

If switched (itemnames % items2test), then %in% will check if each of the itemnames is in the list of items2test, and return a T/F vector for those. In the example above, it would return a 20-element vector with items 13 and 15 as TRUE, and the "which" command would return "13 15", selecting the right elements to test for DIF.

[philchalmers]

Ugh, thanks. A rather silly implementation bug, but glad that it's been patched.