On Wed, 25 Jun 2003, John Mercer wrote: > [EMAIL PROTECTED] (Karl L. Wuensch) wrote: > > > Dunnett's test ... each of several means is contrasted with a single > > reference mean. ... > > So would this be the proper choice if I have a single experimental > group and three control groups?
Depends partly on why you have three control groups. If they were thought to be equivalent, or exchangeable, it would probably be efficient to pool them into a single group. If they are not (e.g., if each control group is "controlling" for a different thing), will you be interested in differences between the several controls? (If so, this would argue against Dunnett's test, since not all the comparisons of interest are of that kind. But if you are indifferent to whatever differences may exist between control groups, and pairwise comparisons involving your one experimental group are your ONLY interest, Dunnett's would seem appropriate.) > I am in this situation, with very large standard deviations caused > by uncontrollable factors. . . Does that mean your control groups aren't controlling for much of anything? You don't describe the patterns of means OR s.d.s in your several groups. If the control groups have large s.d.s but not the experimental group, and if the control group means appear more similar than the (E-C) differences, I'd be inclined either to pool the control groups or to construct a contrast of the form (3*E - C1 - C2 - C3), and use the Scheffe method (since the choice of contrasts would have arisen from consulting the results, hence a thoroughly post hoc method of making comparisons is required). Of course, if your s.d.s are REALLY large, you probably can't tell anything anyway... HTH. -- DFB. ----------------------------------------------------------------------- Donald F. Burrill [EMAIL PROTECTED] 56 Sebbins Pond Drive, Bedford, NH 03110 (603) 626-0816 . . ================================================================= Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at: . http://jse.stat.ncsu.edu/ . =================================================================
