Jay Tanzman wrote: > I just got chewed out by my boss for modelling the means of some 7-point > semantic differential scales. The scales were part of a written, > self-administered questionnaire, and were laid out like this: > > Not stressful 1__ 2__ 3__ 4__ 5__ 6__ 7__ Very stressful > > So, why or why not is it kosher to model the means of scales like this? > > -Jay
1) Why do you think the scale is interval data, and not ordinal or categorical? If interval, the increments between the levels are more or less equal. If ordinal we know they are sequential, but have no idea how far apart each pair is. Categorical means there is no relationship between them - 4 is not greater than 3 - it's only different. Some people use a response of 4 to mean 'no response' as well as 'no opinion' and 'neutral opinion.' sorry, these are not intervals. 2) Is it possible for a respondent to come back with 2.5? If so, they think it is interval data, regardless of your opinion. Would you throw out a response of 2.5, or would you enter it in your dataset as 2.5? If the latter, you think it is interval, also. 3) What makes you think the scale is linear (equal intervals)? It ain't - since respondents can't go below 1 or above 7 . Well, maybe 0 and 8, but the point is the same. If you must, make a transformation (arc-sine for starters) to make it more 'linear' and more likely to contain Normal dist. data. 4) Why might the respondents use the same increments that you think exist, or the same as other respondents? If there is some way you can 'anchor' at end points or mid point, you will get much more informative data. I mean, what is 'very stressful' to you? To me? to anyone? Perhaps you are evaluating how people respond to specific scenarios with their impression of anticipated stress. In which case, the strength of 'very' is at issue, and perhaps you can argue that it is what you are measuring. (remember the old maps: there be dragons). When I sit down with a client to work out an experimental design for a project, one might call this highly stressful. I am in full control of the alternatives and options, so to me it is great fun, and very invigorating. the situation is far from 'Not stressful' - it is not opposite of 'stressful.' I know my muscles have been stressed, because it is also very tiring. so what might be 'stressful'? Is that worked out with your respondents beforehand? 5) In cases where I have been able to anchor firmly, and in some where I haven't, I find that treating the scale as incremental data work just fine, thank you. As soon as you compute an average of responses on this scale, you have done just that. If you restrict yourself to categorical analysis for frequencies between categories, you have avoided that assumption. And you have far less to say about the data, as well. Cheers, Jay -- Jay Warner Principal Scientist Warner Consulting, Inc. 4444 North Green Bay Road Racine, WI 53404-1216 USA Ph: (262) 634-9100 FAX: (262) 681-1133 email: [EMAIL PROTECTED] web: http://www.a2q.com The A2Q Method (tm) -- What do you want to improve today? ================================================================= Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at http://jse.stat.ncsu.edu/ =================================================================
