Re: Post-hoc comparisons

[Paul R Swank]

At 08:43 AM 3/2/01 -0600, Esa M. Rantanen wrote:
>Dear All:
>
>I have a question concerning pairwise comparisons between four treatment
>conditions. My experience is mostly with ANOVA, and (I think!) I can
>understand the reasoning for the use of multiple comparison procedures
>(e.g., Duncan's, Tukey's, or LSD) instead of individual t-tests between
>conditions.

You should forget Duncan's or Neuman-Keuls procedures as beeing too liberal.

>
>I assume the case is the same with my current problem: I have a single
>-factor experiment with four levels of the factor (treatment conditions)
>and a discrete dependent measure (pass/fail), resulting in a 2 x 4
>contingency table. I have used a Chi-Sq. analysis to determine if there is
>a statisitcally significant difference between the (treatment) groups (all
>4!), and indeed there is. I assume, however, that I cannot simply do
>pairwise comparisons between the groups using Chi-Sq. and 2 x 2 matrices
>without inflating the probability of Type 1 error, (1-alpha)^4 in this
>case. As far as I know, there are no equivalents to Duncan's or Tukey's
>tests for the type of data (binary) I have to deal with.
>
>I would appreciate if anyone would confirm my reasoning above and offer any
>advice on how to proceed with the analysis of pairwise differences in the
>case of categorical (dichotomous) data. References to relevant literature
>would also be welcome!

You are correct. Unless you have a priori, non-redundant hypotheses where contrasts would be feasible, you should correct the probability of you pairwise chi-squares using Bonferroni or Sidak corrections. If you do all pairwise comparisons there would be 6. Thus, the comparisonwise error rate would be .0084 by Bonferroni and .0085 by Sidak.


>
>Best,
>
>Esa
>____________________________
>Esa M. Rantanen, Ph.D.
>Assistant Professor
>University of Illinois at Urbana-Champaign
>Institute of Aviation, Aviation Human Factors Division
>Aviation Research Laboratory, Q5, MC-394
>One Airport Road, Willard Airport
>Savoy, IL 61874
>Tel. 217-244-8657 (ARL)
>Tel. 217-244-7397 (Psych.)
>Tel. 217-373-8276 (Home)
>Fax 217-244-8647
>e-mail: [EMAIL PROTECTED]
>url: http://www.aviation.uiuc.edu
>____________________________
>
>
>
>
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------------------------------------
Paul R. Swank, PhD.
Professor & Advanced Quantitative Methodologist
UT-Houston School of Nursing
Center for Nursing Research
Phone (713)500-2031
Fax (713) 500-2033

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