Author Topic: Traditional item analysis / covariance matrix not positive definite  (Read 1095 times)

jonmerl

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Hello,

I just started working with Conquest (4.5.2). I ran a model with one dimension and a model with two dimensions.

Regarding the descriptive statistics, the frequencies of right and wrong answers (0,1) are slightly different than the frequencies in the data file. Often a difference of one case per item.
I am not sure if I did a mistake with the import or in the syntax.

Moreover, the twodimensional model breaks up. Error message: "Covariance matrix is not positive definite". Is this a problem of the syntax or is it that this model can not be estimated with this data.

I would be very grateful for your help.
 
Thank you,
Jonas

« Last Edit: January 17, 2021, 10:58:28 PM by dan_c »

dan_c

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Re: Traditional item analysis / covariance matrix not positive definite
« Reply #1 on: January 16, 2021, 10:31:53 PM »
Hi Jonas,

your issues is with the data file you have crated. It is encoded UTF8 with a byte order marker (BOM). Early versions of ConQuest 4 did not support this encoding. I would recommend creating files with a BOM - it is not part of the Unicode standard, and only seems to appear with MS Excel when exporting files.

ConQuest 5 will attempt to strip the BOM when reading the file. I would encourage you to upgrade: https://shop.acer.edu.au/acer-conquest-5.html

Note that in ConQuest 5, you can read and write CSV files directly and do not need to create a text (.dat) or SPSS file.

Attached is a converted data file (I did this with VS Code - which is free to use and an excellent text editor) and output files. Note the issue with the covariance matrix is still present in your 2D model, I would guess it is to do with the very high correlation between your dimensions (>0.95), the very narrow variance (~0.1-0.3) and small sample size. Note the model deviance implies the 1D model is a better fit anyway.

« Last Edit: January 17, 2021, 10:58:40 PM by dan_c »

jonmerl

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Re: Traditional item analysis / covariance matrix not positive definite
« Reply #2 on: January 17, 2021, 12:41:03 PM »
Thank you very much for your helpful and quick response.
This forum is really great.

Just one question:
Is it possible to remove the data files from the posts, so the problem and solution would still be comprehensible for the community, only the data is not available. This would be a bit calming for me.

Thank you for your response.
Best wishes.

dan_c

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Re: Traditional item analysis / covariance matrix not positive definite
« Reply #3 on: January 17, 2021, 10:59:05 PM »
of course - I have removed the attachments.