[Rasch] Differential Item Functioning: Help Needed

Mike Linacre rmt at rasch.org
Thu Feb 7 00:25:44 EST 2013

```Svend and folks:

The nomogram is the plot of a standard Student's t-statistic with two
groups of different group sizes but equal variances. The plot would look
the same for comparing the heights of a group of boys and a group of
girls. How many boys and girls do we need if the differences in their
average heights (=DIF size) are xx centimeters? The exact algebra of the
t-test is at the Wikipedia reference. For the height plot, we would
choose a reasonable variance for the children's heights. In the same
way, the DIF nomogram is based on a (hopefully) reasonable variance for
the Reference and Focal groups.

OK?

Mike L.

On 2/6/2013 4:58 AM, Svend Kreiner wrote:
> Sorry Mike, i don't get it. So you calculate a t-test. Exactly how? Please give us the formula rather than the words.
>
> And,yes, I did misread the nomogram. So you need two times 1500 persons to discover DIF equal to 0.25. To me, this sounds almost like a test without power, so there must be something that I misunderstand.
>
> Svend

```