[Rasch] Item and Person Measures in survey questionnaire

Schulz, Matt mschulz at pacificmetrics.com
Tue Apr 19 00:05:17 EST 2011


Good question!  It depends on what you are measuring, plus there is a very subtle point to keep in mind.  If your rating scale is something like "How satisfied are you with....".  You are *not* measuring "satisfaction," per se but rather, persons' propensity to be satisfied, or "satisfiability" with the  particular  class of items you are having them rate.   The response to each item tells you how satisfied the person was with that item-and that is data representing a person's satisfaction.  But the data is not the measure.

This is not so hard to understand when applied to mathematics performance. You can give a person a easy test or a hard test.  A person's success rate, or 'performance' (the data) will be higher on the easy test than on the hard one.  But their mathematics "ability" is the same on both.   It's the same with a satisfaction questionnaire.  You can apply the questionnaire to items that are in general not satisfying to people (vegetables), or to items that are (candy).   A kid will be very satisfied with candy, but not vegetables.  But their underlying propensity to be satisfied is the same.  You might qualify this trait as propensity to be satisfied with food.  Or even more specifically, "with candy" or "with vegetables"  if you think the trait is not unidimensional across such different classes of food.

In a questionnaire I once worked on, the items were colleges services (e.g., counseling, health care, etc.) and we called the trait, "propensity to be satisfied with college services!"   I suppose, loosely speaking, we could have said we were measuring "satisfaction."  But it's important to understand the difference between the data and the measure.

Good luck,

Matt Schulz

From: rasch-bounces at acer.edu.au [mailto:rasch-bounces at acer.edu.au] On Behalf Of Juanito Talili
Sent: Monday, April 18, 2011 12:33 AM
To: rasch at acer.edu.au
Subject: [Rasch] Item and Person Measures in survey questionnaire

Dear co-members,

In a right/wrong response format, usually achievement test, the item measures represent the item difficulty while the person measures represent the person ability.

What would be the interpretation of the item and person measures for a case of a survey questionnaire that uses a four-point ordinal scale response format ( strongly disagree to strongly agree). I undertand that the measures are called logits (am I correct?) but I dont know how the item and person measures are interpreted.

Thank you in advance.


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