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Thank you very much for your help and advice. Best regards, Leonor >I mostly disagree with David here. In particular, his proposal to >collapse the Likert scales just throws away information in an >arbitrary manner. >I don't think his advice is even consistent. If it's OK to treat means >of Bernoulli distributions as valid arguments for a t test, why is not >OK to treat means of Likert scales as if they were? >It's true that the reference case for a t test is two paired normal >distributions, and Likert scales can not be normal if only because >they are _not_ continuous, but there is always a judgment call on >whether summaries of the data will in practice work similarly. >A fair question is what exactly kind of advice is Leonor seeking? The >question presumably isn't really whether it is possible -- clearly it >is possible -- but perhaps somewhere between "Is it correct?" and "Is >it a good idea?" >Leonor's question appears to have the flavour of "I gather that this >is wrong. but is there a sense in which this is allowed?" The long >answer has to be that Leonor should tell us much more about the data >and the problem in hand if a good answer is to be given. If means make >sense as summary statistics, then comparing means with a t test is >likely to work well, but watch out. >David is clearly right in alluding to a purist literature in which you >are told as a matter of doctrine that ordinal data shouldn't be >summarised by means and so mean-based tests are also invalid. When >acting as academics, the same people work with grade-point averages >just like anybody else, at least in my experience. >There is also a pragmatist literature which points out that despite >all that, the sinful practice usually works well. Compare the t-test >with e.g. a Mann-Whitney-Wilcoxon test and it's very likely that the >P-values and z- or t-statistics will point to the same substantive >conclusion and indicate just about the same quantitative effect. It's >also likely that doing both tests will be needed because some reviewer >has been indoctrinated against t-tests here, and especially if anyone >is working with a rigid threshold (e.g. a 5% significance level). >Also, the behaviour of t-tests in cases like this can always be >examined by simulation, so no-one need be limited by textbook dogma >(or wickedness). >Nick On Fri, Aug 31, 2012 at 12:13 AM, David Radwin <dradwin@mprinc.com> wrote: > Leonor, > >No, you can't correctly calculate the mean of an ordinal-level measure >like the Likert scale you describe, although plenty of people do it > anyway. > > But you can use -ttest- with these data if you first collapse each > variable to a dichotomous (dummy) variable, because the mean of a > dichotomous variable is identical to the proportion where the value is 1. > As a guess, you might set the highest two values to 1, the lowest two > values to 0, and the middle value to missing to calculate the proportion > agreeing or somewhat agreeing. > > David > -- > David Radwin > Senior Research Associate > MPR Associates, Inc. > 2150 Shattuck Ave., Suite 800 > Berkeley, CA 94704 > Phone: 510-849-4942 > Fax: 510-849-0794 > > www.mprinc.com > > >> -----Original Message----- >> From: owner-statalist@hsphsun2.harvard.edu [mailto:owner- >> statalist@hsphsun2.harvard.edu] On Behalf Of Leonor Saravia >> Sent: Thursday, August 30, 2012 3:23 PM >> To: statalist@hsphsun2.harvard.edu >> Subject: st: Mean test in a Likert Scale >> >> Hello, >> >> I'm working with a survey that presents 26 questions and each of them >> has as possible answer a 5 point Likert scale from Desagree (1) to >> Agree (5). This survey was applyed for a treatment and a control >> group. >> >> As far as I know, it is possible to analyze the information given only >> by the proportions of each answer; for instance, 25% agrees, 50% >> desagree, or so. >> >> I have two questions that maybe one of you have had before: >> >> a) Is it possible to calculate the mean score of a sample (treatment >> or control group) - adding the individual answers - when one is >> working with a Likert scale? >> >> b) If it is possible to calculate a mean score of a sample when using >> a Likert scale, to compare the answers of the treatment versus the >> control group, is it well done if I use the 'ttest' command? >>Thank you in advance. >>Best regards, >>Leonor * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

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