Many thanks - this was very helpful! Regards, Kay Am 28.01.2013 13:19 schrieb "Achim Zeileis" <achim.zeil...@uibk.ac.at>:
> On Sun, 27 Jan 2013, Kay Cichini wrote: > > That said, >> >> wilcox_test(x ~ factor(y), distribution = "exact") >>> >> >> or the same with oneway_test, i.e would be ok? >> > > Yep, exactly. > > And you could also look at chisq_test(factor(x > 0) ~ factor(y), > distribtuion = approximate(9999)) or something like that. Or of course > Fisher's exact test. > > Best, > Z > > >> 2013/1/27 Achim Zeileis <achim.zeil...@uibk.ac.at> >> >> On Sun, 27 Jan 2013, Kay Cichini wrote: >>> >>> Thanks for the reply! >>> >>>> >>>> Still, aren't there issues with 2-sample test vs y and excess zeroes >>>> (->many ties), like for Mann-Whitney-U tests? >>>> >>>> >>> If you use the (approximate) exact distribution, that is no problem. >>> >>> The problem with the Wilcoxon/Mann-Whitney test and ties is only that the >>> simple recursion formula for computing the exact distribution only works >>> without ties. Thus, it's not the exact distribution that is wrong but >>> only >>> the standard algorithm for evaluating it. >>> >>> Best, >>> Z >>> >>> Kind regards, >>> >>>> Kay >>>> >>>> >>>> 2013/1/26 Achim Zeileis <achim.zeil...@uibk.ac.at> >>>> >>>> On Fri, 25 Jan 2013, Kay Cichini wrote: >>>> >>>>> >>>>> Hello, >>>>> >>>>> >>>>>> I'm searching for a test that applies to a dataset (N=36) with a >>>>>> continuous zero-inflated dependent variable >>>>>> >>>>>> >>>>>> In a regression setup, one can use a regression model with a response >>>>> censored at zero. survreg() in survival fits such models, tobit() in >>>>> AER >>>>> is >>>>> a convenience interface for this special case. >>>>> >>>>> If the effects of a regressor can be different for the probability of a >>>>> zero and the mean of the non-zero observations, then a two-part model >>>>> can >>>>> be used. E.g. a probit fit (via glm) plus a truncated regression (via >>>>> truncreg in the package of the same name). >>>>> >>>>> However: >>>>> >>>>> >>>>> and only one nominal grouping variable with 2 levels (balanced). >>>>> >>>>> >>>>>> >>>>>> In that case I would probably use no regression model but two-sample >>>>> permutation tests, e.g. via the "coin" package. >>>>> >>>>> >>>>> In fact there are 4 response variables of this kind which I plan to >>>>> test >>>>> >>>>> seperately - the amount of zeroes ranges from 75 to 97%.. >>>>>> >>>>>> >>>>>> That means you have between one (!) and nine non-zero observations. >>>>> In >>>>> the >>>>> former case, it will be hard to model anything. And even in the latter >>>>> case >>>>> it will be hard to investigate the probability of zero and the mean of >>>>> the >>>>> non-zero observations separately. >>>>> >>>>> I would start out with a simple two-way table of (y > 0) vs group and >>>>> conduct Fisher's exact test. >>>>> >>>>> And then you might try also your favorite two sample test of y vs >>>>> group, >>>>> preferably using the approximate exact distribution. >>>>> >>>>> Hope that helps, >>>>> Z >>>>> >>>>> I searched the web and found several modelling approaches but have the >>>>> >>>>> feeling that they are overly complex for my very simple dataset. >>>>>> >>>>>> Thanks in advance for any help! >>>>>> Kay >>>>>> >>>>>> -- >>>>>> >>>>>> Kay Cichini, MSc Biol >>>>>> >>>>>> Grubenweg 22, 6071 Aldrans >>>>>> >>>>>> Tel.: 0650 9359101 >>>>>> >>>>>> E-Mail: kay.cich...@gmail.com >>>>>> >>>>>> Web: www.theBioBucket.blogspot.co.******at<http://www.theBioBucket.** >>>>>> ** >>>>>> blogspot.co.at >>>>>> 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<http://www.R-project.org/**posting-guide.html<http://www.R-project.org/posting-guide.html> >>>> > >>>> and provide commented, minimal, self-contained, reproducible code. >>>> >>>> >>>> >> >> -- >> >> Kay Cichini, MSc Biol >> >> Grubenweg 22, 6071 Aldrans >> >> Tel.: 0650 9359101 >> >> E-Mail: kay.cich...@gmail.com >> >> Web: >> www.theBioBucket.blogspot.co.**at<http://www.theBioBucket.blogspot.co.at> >> <http://www.thebiobucket.**blogspot.co.at/<http://www.thebiobucket.blogspot.co.at/> >> ><http://www.**theBioBucket.blogspot.co.at<http://www.theBioBucket.blogspot.co.at> >> > >> -- >> >> [[alternative HTML version deleted]] >> >> ______________________________**________________ >> R-help@r-project.org mailing list >> https://stat.ethz.ch/mailman/**listinfo/r-help<https://stat.ethz.ch/mailman/listinfo/r-help> >> PLEASE do read the posting guide http://www.R-project.org/** >> posting-guide.html <http://www.R-project.org/posting-guide.html> >> and provide commented, minimal, self-contained, reproducible code. >> >> [[alternative HTML version deleted]] ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
