Re: convergent validity
Donald Burrill writes: On Thu, 29 Mar 2001, H.Goudriaan wrote in part: - my questionnaire items are measured on 5- and 7-point Likert scales, so they're not measured on an interval level Non sequitur. and consequently not (bivariate) normally distributed; Real data hardly ever is. Do you need it to be? Usually the question of interest is whether it's close enough to be an adequate approximation for guv'mint work. Ok, I understand and agree. But isn't it a bit naive to think that a group of variables with 5 categories may result in a good factor analysis (or whatever other parametric analyses)? = Instructions for joining and leaving this list and remarks about the problem of INAPPROPRIATE MESSAGES are available at http://jse.stat.ncsu.edu/ =
Re: convergent validity
On Fri, 30 Mar 2001 [EMAIL PROTECTED] wrote: Donald Burrill writes: On Thu, 29 Mar 2001, H.Goudriaan wrote in part: - my questionnaire items are measured on 5- and 7-point Likert scales, and consequently not (bivariate) normally distributed; Real data hardly ever is. Do you need it to be? Usually the question of interest is whether it's close enough to be an adequate approximation for guv'mint work. Ok, I understand and agree. But isn't it a bit naive to think that a group of variables with 5 categories may result in a good factor analysis (or whatever other parametric analyses)? I frankly don't see the relevance of naivete to the question at hand. It isn't, one gathers, as though you had any choice in the matter: either in the number of points on each item scale (since this is all, as you told Dennis, an existing scale) nor in the bivariate distribution of the two constructs in which (one gathers) you are interested. (And you haven't said why you think you want these two constructs to be bivariate normal -- rather than, say, linearly related and unimodal. Nor, for that matter, have you indicated whether you have examined the bivariate distribution in question and actually found it to depart worrisomely from a reasonable distribution.) You also replied to Dennis that you have 16 items, 11 of which are alleged to measure one construct and 5 measure another. That sounds to me like two variables, one with a potential range of 11 to 55 and the other with a potential range of 5 to 25 (for the 5-point scales; where you have 7-point scales the potential range will be somewhat wider). I should think that your interest would then lie in the validity of these two variables, not in the individual items that contribute to them; unless you want to do an item analysis of one kind or another. You write also, "with 5 categories". If you insist that the item responses must be treated as _categories_, rather than ordered points on a scale, then you ought, one would think, to be applying the methods of dual scaling (also known as correspondence analysis). Or, if you allow that the responses are ordered, use the variation of dual scaling that applies to ordered categories. (All this for dealing with data at the item level, of course.) You haven't explicitly said (that I recall), but you seem to be unwilling to treat the item responses as of approximately interval scale. Why not? Do you have evidence that the scale intervals are grossly unequal? (That seems to me unlikely.) Or are the distributions of responses for some items so peculiar as to generate serious doubt about the intervals? (If so, you might wish to convert any such item to a series of 0/1 categories -- which brings us back to dual scaling.) -- DFB. Donald F. Burrill [EMAIL PROTECTED] 348 Hyde Hall, Plymouth State College, [EMAIL PROTECTED] MSC #29, Plymouth, NH 03264 603-535-2597 184 Nashua Road, Bedford, NH 03110 603-471-7128 = Instructions for joining and leaving this list and remarks about the problem of INAPPROPRIATE MESSAGES are available at http://jse.stat.ncsu.edu/ =
convergent validity
Hi Statisticians, First of all, sorry for posting my question in 3 groups, but I'm a bit of a newby here and I can't find out what the difference is (where can I read the Charters, or whatever it's called?). I would love to have some help on the following: I have 2 questionnaires assessing (physical and emotional) health of heart patients. The 1st measures present state and it's assessed before treatment and a couple of months after treatment, so that difference scores can be calculated. The 2nd questionnaire is assessed after treatment only, and asks respondents how much they have changed on every aspect (same aspects as the first questionnaire) since just before treatment. Respondents received both questionnaires. Now I would like to investigate the convergent validity of the two domains assessed with both questionnaire versions. Is there a standard, straightforward way of doing this? Someone advised me to do a factoranalysis (PCA) (on the baseline items, the serially measured change scores and the retrosepctively assessed change scores) and then compare the factorloadings (I assume after rotation? (Varimax?)). I haven't got a good feeling about this method for two reasons: - my questionnaire items are measured on 5- and 7-point Likert scales, so they're not measured on an interval level and consequently not (bivariate) normally distributed; - I have no idea how to compare the factorloadings. Could I calculate confidence intervals for the loadings? (If yes: how?) Thanks in advance for any help or references. Heike = Instructions for joining and leaving this list and remarks about the problem of INAPPROPRIATE MESSAGES are available at http://jse.stat.ncsu.edu/ =
Re: convergent validity
On Thu, 29 Mar 2001, H.Goudriaan wrote in part: - my questionnaire items are measured on 5- and 7-point Likert scales, so they're not measured on an interval level Non sequitur. and consequently not (bivariate) normally distributed; Real data hardly ever is. Do you need it to be? Usually the question of interest is whether it's close enough to be an adequate approximation for guv'mint work. -- DFB. Donald F. Burrill [EMAIL PROTECTED] 348 Hyde Hall, Plymouth State College, [EMAIL PROTECTED] MSC #29, Plymouth, NH 03264 603-535-2597 184 Nashua Road, Bedford, NH 03110 603-471-7128 = Instructions for joining and leaving this list and remarks about the problem of INAPPROPRIATE MESSAGES are available at http://jse.stat.ncsu.edu/ =
