XFILE= analyzed response file

If XFILE=filename is specified in the control file, a file is output which enables a detailed analysis of individual response anomalies.

 

XFILE=? opens a Browse window

 

This file contains 4 heading lines (unless HLINES=N or ROW1HEADING=N) followed by one line for each person-by-item response used in the estimation. Each line contains:

 

 

 

Fields can be selected interactively and the default field selection changed at XFILE= dialog box.

 

If CSV=Y, the values are separated by commas. When CSV=T, the commas are replaced by tab characters. For "non-numeric values in quotation marks", specify QUOTED=Y.

 

This file enables a detailed analysis of individual response anomalies. The response residual can be analyzed in three forms:

1) in response-level score units, from [(observed value - expected value)].

2) in logits, from [(observed value - expected value)/variance].

3) in standard units, [(observed value - expected value)/(square root of variance)].

 

Predicted person measure:  Imagine that this observation was the only observation made for the person ... this value is the measure we would predict for that person given the item measure.

Predicted item measure: Imagine that this observation is the only observation made for this item ... this value is the measure we would predict for that item given the person measure.

The formulas are the same as for a response string of more than 1 observation. For dichotomies, see www.rasch.org/rmt/rmt102t.htm and for polytomies www.rasch.org/rmt/rmt122q.htm

 

Example 1: I need the the S.D. of the response values for each item

Output the Xfile= to Excel. Sort by Item number. Then compute the S.D. for each item separately or use the Excel SUBTOTAL function:

1.On the Data tab, in the Outline group, click Subtotal. The Subtotal dialog box is displayed.

2.In the At each change in box, click the nested subtotal column, item number. ...

3.In the Use function box, click the summary function that you want to use to calculate the subtotals, StDevP. ...

4.Clear the Replace current subtotals check box.

5.Click OK

 

Example 2: You wish to compute differential item functioning, DIF, for a specific classification group of people:
If Table 30 is not suitable, here is a simple approximation:
Since one item does not have enough information to measure a person, for item bias we have to do it on the basis of a classification group of people.

From the XFILE,

add the "score residuals" (not standardized) for everyone in classification "A" on a particular item.

Add the "modeled variance" for everyone in the classification.

Divide the residual sum by the variance sum. This gives an estimate of the DIF for classification "A" relative to the grand mean measure.

Do the same for classification "B" on the same item.

To contrast classification "A" with classification "B" then

DIF size "AB" =DIF estimate for "A" - DIF estimate for "B"

A significance t-test is t =DIF size "AB" / square root ( 1/variance sum for classification A + 1/variance sum for classification B))

 

Example 3: You want to convert lucky guesses into missing data for some items.

Click on Output Files menu

Click on XFILE=

In the XFILE= Fields dialog box, type the Item numbers you want.

Click on OK

Output to Excel (Temporary file)

Sort the Excel file on Residual

The largest positive residuals are the lucky guesses

Rectangular- Copy the person entry numbers and the item entry numbers into your Winsteps control file:

EDFILE=*

person  item  .    ; the . is to indicate "make this response missing data"

person  item  .    ; the . is to indicate "make this response missing data"

person  item  .    ; the . is to indicate "make this response missing data"

*

 

Save your Winsteps control file.

In the next analysis these unexpected correct responses will be scored as missing. The raw scores will also change.