[Rasch] development and validation of instrument
Timothy W. Pelton
tpelton at uvic.ca
Sun Jun 15 14:53:11 EST 2008
I sure appreciate your willingness and ability to provide clarification to the rest of us in the community with less understanding. Your responses are very clear and helpful - especially to me when you bring up ideas that would otherwise atrophy and fill in blanks that would otherwise remain empty. You are doing a great service.
Thanks, and keep well.
> on 14/06/08 12:45 PM, Mike Linacre (RMT) at rmt at rasch.org wrote:
Thank you for more questions, Juanito.
We develop a 50-item instrument anchored on a theory with 4 constructs. We piloted the instrument to 203 persons, then we subjected to Rasch analysis. After our Rasch analysis we found that the 50 items are unidimentional and the 5-point likert scale categories are logically functional, but while retaining the 4 constructs, five items misfit the model. Thus, we decided to remove the 5 items and come up with a 45 item instrument. Our questions to Rasch expert are:
1. Shall we gather again another data (say n=200) to validate the instrument?
Reply: In your original analysis of the 50-item instrument, you discovered that 5 items were defective or off-dimensional in some way. So you want to omit them from the revised 45-item instrument. Fine!
Do you think that the responses to those 5 items have skewed the responses of your 203 persons to the other 45 items in any way? For instance, perhaps the 5 items were the first 5 items on the instrument and so could have caused the sample to misunderstand how they were supposed to respond to the remaining 45 items. If they would have distorted the responses to the other 45 items, then you need to repeat the data collection with a new sample on the 45 item instrument. If not, then the current sample's data is valid for the 45 items. There is no need to repeat the data collection.
2. How do Rasch validation is done? Do we just repeat the rasch analysis?
Reply: Yes, you would repeat the Rasch analysis but now with only the 45 items. But remember that you have already excluded the 5 worst items, so the sensitivity of the Rasch analysis to problems in the remaining 45 items has increased. You will now find another group of "worst items". Don't automatically reject them to make a 40-item instrument, but examine the new set of worst items closely. Is their misfit really bad enough to distort the person measures?
3. Is our sample big enough for Rasch analysis?
Reply: This depends on your purpose for this instrument, and how you are modeling the items. You have 45 items and each has a 5-point Likert scale. So the possible range of raw scores is 0 to 225 (or 45 to 270). So there are more possible raw scores (226) than your sample size (203). Consequently it is likely that you have few of your sample with very high or very low scores. If your instrument is intended to probe the full range of person performance, then you may need to collect data from more high or low performers to verify the operational range of the instrument. But if your sample of 203 persons matches the intended use of the instrument in all important respects, then your sample size is probably large enough, see http://www.rasch.org/rmt/rmt74m.htm - There are additional concerns if you are using the "Partial Credit" model.
Editor, Rasch Measurement Transactions
rmt at rasch.org www.rasch.org/rmt/ <http://www.rasch.org/rmt/> Latest RMT: 21:4 Spring 2008
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