Author Topic: DIF report question  (Read 530 times)

chiehyu

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DIF report question
« on: September 10, 2016, 06:25:54 PM »
I am doing a DIF analysis for a test with polytomous items. My DIF analysis compared the item function between two years. My question is which part should I use to report the DIF?

I followed the tutorial and wrote these commands, containing two analyses as it said. I know the tutorial running this for comparing the step structure, but both would generate “TERM 3: item*year”. Should I report the “TERM 3: item*year” from the first model (item-year+item*year+year*item*step) or the one from second model (item-year+item*year+item*step)?

Here are my commands:

Title ASQSEBR DIF 36m;
datafile \Users\Tom Chen\Desktop\asqsebr_36m_dif.dat;
format responses 1-30 year 31;
set warnings=no;
codes 2,1,0;
model item-year+item*year+year*item*step;
labels << \Users\Tom Chen\Desktop\asqse_labels_36m.txt;
estimate ! iterations=10000,method=quadrature,nodes=15;
show!table=1:2 >>dif36m_a.shw;
reset;

datafile \Users\Tom Chen\Desktop\asqsebr_36m_dif.dat;
format responses 1-30 year 31;
set warnings=no;
codes 2,1,0;
model item-year+item*year+item*step;
labels << \Users\Tom Chen\Desktop\asqse_labels_36m.txt;
estimate ! iterations=10000,method=quadrature,nodes=15;
show!table=1:2 >>dif36m_b.shw;

Eveline Gebhardt

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Re: DIF report question
« Reply #1 on: October 05, 2016, 08:55:38 AM »
Hello

The second model is correct. The item*year term will give you the DIF estimates.

(I'd use the order: item + item*step - year + item*year)

Best wishes
Eveline