ConQuest Community
Help desk => Questions and Answers => Topic started by: Scherge on March 06, 2021, 09:13:08 AM

Hi conquest community,
I can't find the error in my data file. Can somebody please check this? The error message statement is:
"No data for books 1, item 8, estimation cannot proceed. Data is needed for a constraint
You will have to recode the data before estimation can proceed"
However with all other data files such as gender and others the DIF Analysis was fine.
Recoding with
recode (1,2,3,4,5,6) (1,1,2,2,3,3) ! books 1;
to shrink the amount of answers didn't help.
Many thanks in advance,
Scherge

Hi Scherge,
The model you have specified:
item  book + item*book;
requests an item parameter for each item and book combination. That is, you have 8 levels of “book”, and therefore each traditional item is generalised to have a distinct item parameter for each book (in your case, the “book” main effect is the average shift in difficulty across the books, and each item*book effect represents the deviation from the main book effect for this item and book combination).
For this to be an identified model, there will need to be at least two valid responses for each item and book interaction (one in the zero category, and one in another category – this is because the identification constraint on the item*book effect is that the parameter for the last item within each book is the negative sum of the other parameters within the book. If you only have one valid response within that book then the parameter cannot be calculated (and in fact the design matrix for the model is misspecified).
I have attached the itanal for the model:item – book;
You will see that you have only one response to book 1. This model is not identified. If you remove booklet 1, you can estimate this model (show file attached). Just note that you are not modelling the steps/categories within the items in this model. Each item score is simply multiplied by the person theta in the probability function and this is quite a restrictive specification.

Thanks again, dan c.
I reduced the number of book categories by half and it works fine.
Have a nice start into the new week,
Scherge