Dear Sir, can u tel me which test used for ANCOVA equivalent for non-parametric test

[sachi]

which test used for ANCOVA equivalent for non-parametric test

[Bruce Weaver]

Why do you want a non-parametric version of ANCOVA?  I ask because parametric models are often more robust and non-parametric tests less robust than people realize.

Having said that, here are some excerpts from Conover's book on nonparametric statistics that may be helpful.

"The recommended procedure in experimental designs for which no nonparametric test exists is to use the usual analysis of variance on the data and then to use the same procedure on the rank transformed data.  If the two procedures give nearly identical results the assumptions underlying the usual analysis of variance are likely to be reasonable and the regular parametric analysis valid.  When the two procedures give substantially different results, the experimenter may want to take a closer look at the data and to look especially for outliers...or very nonsymmetric distributions." (p. 419)

But note what was said in the paragraph immediately prior to that:

"The rank transformation works well in a two-way layout without interaction (see Iman, Hora and Conover, 1984 and Hora and Iman, 1988) where it compares well with the Friedman test, the Quade test, and the parametric F test. However, attempts to apply the rank transformation procedure to test for interaction have met with mixed results, showing that it has good robustness and power by Iman (1974b), Conover and Iman (1976), and Pavur and Nath (1986), but poor robustness and power by Blair, Sawilowsky, and Higgins (1987). A theoretical study by Thompson (1991) shows a flaw in the rank transformation test for interaction showing conclusively that it is not a valid procedure and shouldn't be used. However, Mansouri and Chang (1995) used normal scores instead of ranks and found no problems with the normal scores transformation test for interaction, so the conversion to normal scores might correct the flaw found by Thompson."

Conover W.J. (1999). "Practical Nonparametric Statistics" (3rd Ed.), Wiley.

HTH.
 

sachi wrote

which test used for ANCOVA equivalent for non-parametric test