Using PARSCALE via Stata and Dan’s spreadsheet Laura Gibbons, PhD

What does PARSCALE do? Estimates the unidimensional latent trait from the test items.

Useful output files: SCO RUNNAME.SCO (where RUNNAME is the name you assign the job) Score files have –an estimate of theta (the latent trait) for each person (line of data). –the standard error of that estimate. Remember that not all subjects are estimated with equal precision.

Useful output files: PAR RUNNAME.PAR Parameter files give the difficulty and discrimination parameters for each item.

Not so useful output files: RUNNAME.FIT (PARSCALE fit statistics are generally too conservative) Internal to PARSCALE; we never use: –code_runname.PH0 –code_runname.PH1 –code_runname.PH2 –code_runname.PH3 –pscal*

Why Stata? You can use PARSCALE alone, but the input files are cumbersome. Our user interfaces make it much simpler: –runparscale (Calculates theta and merges thetas and SE back into your data set. Among other things) –prepar (Just prepares the input file and data set for PARSCALE.)

Using Stata Now we’ll switch to Stata. I’ve saved many of the commands in.do files for you. Best to open Stata from the Start menu. (explain the 4 windows, including saving a log file)

Stata Do files Open a.do file from the envelope that’s right below the ‘USER’ pull down. Open “Preliminaries for PARSCALE workshop 2007.do”

runparscale, prepar Open “making thetas in PARSCALE.do”

PARSCALE after prepar File: open Run: run all Run: plot PARSCALE numbers your items in order Show ‘ICC and Info’ and ‘Total Info’ To see another type of plot, click on ‘main menu’

You try it: Repeat all this for some other set of items from the 3MS, or for the whole 3MS.

Dan’s amazing spreadsheet To see the effect on the information curve/sem when you include or exclude items

Inputting the parameter file From within Excel, File->Open (all files) Open GLOBAL.PAR Click on: –Delimited –Starting at row 6 –Next –Space –Next –Finish

IRT Scale Summary.xls Enable macros Copy the Excel cells from GLOBAL.PAR into cell A3 in the Parameters sheet. Go to Row 184 and click In the TIC-ICC sheet, put a 1 above all items you want to include. Change to 0 to remove an item. Note that you will have n-1 columns if an item has n response categories

And viola! Go to GM to GU 14 and below to see the curves! Add or subtract items as desired.