DIF contrast Interpretation June 19th, 2013, 10:22am

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kulin
 Posted: June 7th, 2012, 12:39pm
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 Dear forum members,I used WINSTEPS for purposes of estimating DIF contrasts for items from an achievement test (CONTEXT: I compared two groups of students; mean ability of students from group A was much lower than the mean ability of students from group B).QUESTION: For item X, the obtained DIF contrast amounted to 1 logit (and favored students from group A).Can we interpret the described finding as follows:If we compare equally able students from group A and group B, then the odds of solving correctly the item X is 2.7 times higher for students from group A than for correspondent students from group B.OR (the same):The odds of solving correctly item X, is 2.7 times higher for students from group A than for students from group B (provided that we control for ability).I would highly appreciate any help.Kulin
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Mike.Linacre
 Posted: June 7th, 2012, 10:38pm
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 Yes, that looks correct, Kulin.Most audiences have trouble with this type of arithmetic, so it may be easier to say something like:if a student from group B has a 50% probability of success, then an equally able student from group A will have a 73% probability fo success.
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kulin
 Posted: June 8th, 2012, 5:12am
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 Thank you very much, Mike!
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solenelcenit
 Posted: July 9th, 2012, 10:16am
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 Hi Mike... I'm trying to do the same than Kulin, however in my case I'm trying to compare differences between item location after item-linking. How can I write in terms of probability a difference of 0.44 logits, when group A item location :0.72 logits and groupB i loc: 0.28. Is... Group A found 61% more complexity compared to the 61% of the groupB.I reached this by: 0.72-0.28=0.44 (or 61%prob)Thanks!!Luis
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Mike.Linacre
 Posted: July 9th, 2012, 10:04pm
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 Luis, imagine a person of overall ability 0.28 logits in each group. Then, assuming the items are dichotomies, the person in group B has a 50% chance of success (0.28 ability - 0.28 difficulty = 0 logits). The person in group A has a 39% chance of success (0.28 ability - 0.72 difficulty = -0.44 logits).You may want to choose a person with ability at the overall average of everyone as the reference person, instead of 0.28 logits.
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