[EMAIL PROTECTED] (F. Goldhammer) wrote in message news:<[EMAIL PROTECTED]>... > > - Does "internal consistency" as the measure of reliability > > make much sense for a time series? How tightly linked are > > the measures? If there is a high lagged correlation, then > > any 'internal consistency' is foolish to think about. Before > > you worry about that ratio, in fact, I think the nature of the > > time-series needs to be worked out. > > > > Is there an external criterion for this quotient, or is it > > intended to predict itself-in-the-future, and nothing more? > > > > Maybe a real-life example would impress me otherwise, > > but what comes to mind here, in the abstract, is that > > a) test-retest reliability would be more convincing; and > > b) even more than the usual cross-sectional report, > > your conclusions will be heavily tied to the character > > of the 'sample'; however well the quotient works, that > > depends on various variances remaining similar. > > Thank you for your answers and comments on my problem. I'd like to go > into detail. > The 18 Items are measurements of concentration performance: > item1=concentration performance in the 1st testminute ... item18= > concentration performance in the 18th testminute. > The quotient [Mean(item7, ... item 18)/Mean(item1, ... item6)] is a > simple measure of the enduring concentration performance of a person. > I'd like to know a way to calculate the reliability („true variance") > of this measure. I measured N=102 subjects just for once, so I can‘t > calculate a retest-reliability. > The lagged autocorrelation of the 18 occassions (18 testminutes): > lag0 1 > lag1 0,95 > lag2 0,89 > lag3 0,76 > lag4 0,58 > lag5 0,31 > Hope this helps you to give me further answers. Many thanks in advance > Frank Goldhammer
i guess i still like the idea of an internal consistency measure. if the performance of individuals correlates to a similar degree regardless of which two items (times) are selected, then their performance is "reliable." so i envision your test minutes like test items. if performance waxes or wanes over time, then the correlations for pairs of items will be different, cronbach's alpha will drop, the performance will not be internally consistent. it seems to me that test-retest reliability - even if you could calculate it - is answering a question about stability, and this is not what you are seeking. so i would calculate the alpha coefficient and report that as a measure of the consistency of concentration over time. JJD . . ================================================================= Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at: . http://jse.stat.ncsu.edu/ . =================================================================
