# Problem with Kruskal–Wallis test

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## Problem with Kruskal–Wallis test

 Dear all, I am a beginner with R (and also with the statistics) for which I hope to be clear. I should do this non-parametric test on data I extracted from maps. In practice I have a column that represents the landscape Dynamics of a certain time period (there are 3 dynamics, each of them marked by the number 1, 2 or 3) and the other column with the values of a topographic variable (for example the slope) . In all, there are more than 90,000 pairs of values. Going to do the test in R, for all the dynamics and for all the variables, I get out of the values of chi-square elevated (even in the order of thousands) and a p-value always <2.2e-16 .... why? Where can the error be? in the script or in the test approach? Thanks in advance         [[alternative HTML version deleted]] ______________________________________________ [hidden email] mailing list -- To UNSUBSCRIBE and more, see https://stat.ethz.ch/mailman/listinfo/r-helpPLEASE do read the posting guide http://www.R-project.org/posting-guide.htmland provide commented, minimal, self-contained, reproducible code.
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## Fwd: Problem with Kruskal–Wallis test

 Dear all, I am a beginner with R (and also with the statistics) for which I hope to be clear. I should do this non-parametric test on data I extracted from maps. In practice I have a column that represents the landscape Dynamics of a certain time period (there are 3 dynamics, each of them marked by the number 1, 2 or 3) and the other column with the values of a topographic variable (for example the slope) . In all, there are more than 90,000 pairs of values. Going to do the test in R, for all the dynamics and for all the variables, I get out of the values of chi-square elevated (even in the order of thousands) and a p-value always <2.2e-16 .... why? Where can the error be? in the script or in the test approach? Thanks in advance         [[alternative HTML version deleted]] ______________________________________________ [hidden email] mailing list -- To UNSUBSCRIBE and more, see https://stat.ethz.ch/mailman/listinfo/r-helpPLEASE do read the posting guide http://www.R-project.org/posting-guide.htmland provide commented, minimal, self-contained, reproducible code.
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## Re: Problem with Kruskal–Wallis test

 In reply to this post by mngon Dear Giuseppe If I understand you correctly you have a very large sample size so it is not surprising that you get very small p-values. Eevn a scientifically uninteresting difference can become statistically significant with large samples. You probably need to define a metric for meaningful differences between groups and calculate a confidence interval for it. Michael On 21/12/2018 15:37, Giuseppe Cillis wrote: > Dear all, > I am a beginner with R (and also with the statistics) for which I hope to > be clear. > I should do this non-parametric test on data I extracted from maps. > In practice I have a column that represents the landscape Dynamics of a > certain time period (there are 3 dynamics, each of them marked by the > number 1, 2 or 3) and the other column with the values of a topographic > variable (for example the slope) . In all, there are more than 90,000 pairs > of values. > Going to do the test in R, for all the dynamics and for all the variables, > I get out of the values of chi-square elevated (even in the order of > thousands) and a p-value always <2.2e-16 .... why? Where can the error be? in > the script or in the test approach? > Thanks in advance > > [[alternative HTML version deleted]] > > ______________________________________________ > [hidden email] mailing list -- To UNSUBSCRIBE and more, see > 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. > -- Michael http://www.dewey.myzen.co.uk/home.html______________________________________________ [hidden email] mailing list -- To UNSUBSCRIBE and more, see https://stat.ethz.ch/mailman/listinfo/r-helpPLEASE do read the posting guide http://www.R-project.org/posting-guide.htmland provide commented, minimal, self-contained, reproducible code.
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## Re: Problem with Kruskal–Wallis test

 Dear Michael, Thanks for your answer. So, I'm not an expert in R and statistics, how can I create this interval of confidence of groups? Thanks Gc Il giorno sab 22 dic 2018, 13:34 Michael Dewey <[hidden email]> ha scritto: > Dear Giuseppe > > If I understand you correctly you have a very large sample size so it is > not surprising that you get very small p-values. Eevn a scientifically > uninteresting difference can become statistically significant with large > samples. You probably need to define a metric for meaningful differences > between groups and calculate a confidence interval for it. > > Michael > > On 21/12/2018 15:37, Giuseppe Cillis wrote: > > Dear all, > > I am a beginner with R (and also with the statistics) for which I hope to > > be clear. > > I should do this non-parametric test on data I extracted from maps. > > In practice I have a column that represents the landscape Dynamics of a > > certain time period (there are 3 dynamics, each of them marked by the > > number 1, 2 or 3) and the other column with the values of a topographic > > variable (for example the slope) . In all, there are more than 90,000 > pairs > > of values. > > Going to do the test in R, for all the dynamics and for all the > variables, > > I get out of the values of chi-square elevated (even in the order of > > thousands) and a p-value always <2.2e-16 .... why? Where can the error > be? in > > the script or in the test approach? > > Thanks in advance > > > >       [[alternative HTML version deleted]] > > > > ______________________________________________ > > [hidden email] mailing list -- To UNSUBSCRIBE and more, see > > 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. > > > > -- > Michael > http://www.dewey.myzen.co.uk/home.html>         [[alternative HTML version deleted]] ______________________________________________ [hidden email] mailing list -- To UNSUBSCRIBE and more, see https://stat.ethz.ch/mailman/listinfo/r-helpPLEASE do read the posting guide http://www.R-project.org/posting-guide.htmland provide commented, minimal, self-contained, reproducible code.
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## Re: Problem with Kruskal–Wallis test

 I think you need to talk to someone who uses your sort of geographic data to find out what an appropriate metric for comparing your variables is. Only then will you know what might be a suitable way forward. Michael On 22/12/2018 17:27, Giuseppe Cillis wrote: > Dear Michael, > Thanks for your answer. > So, I'm not an expert in R and statistics, how can I create this > interval of confidence of groups? > Thanks > Gc > > Il giorno sab 22 dic 2018, 13:34 Michael Dewey <[hidden email] > > ha scritto: > >     Dear Giuseppe > >     If I understand you correctly you have a very large sample size so >     it is >     not surprising that you get very small p-values. Eevn a scientifically >     uninteresting difference can become statistically significant with >     large >     samples. You probably need to define a metric for meaningful >     differences >     between groups and calculate a confidence interval for it. > >     Michael > >     On 21/12/2018 15:37, Giuseppe Cillis wrote: >      > Dear all, >      > I am a beginner with R (and also with the statistics) for which I >     hope to >      > be clear. >      > I should do this non-parametric test on data I extracted from maps. >      > In practice I have a column that represents the landscape >     Dynamics of a >      > certain time period (there are 3 dynamics, each of them marked by the >      > number 1, 2 or 3) and the other column with the values of a >     topographic >      > variable (for example the slope) . In all, there are more than >     90,000 pairs >      > of values. >      > Going to do the test in R, for all the dynamics and for all the >     variables, >      > I get out of the values of chi-square elevated (even in the order of >      > thousands) and a p-value always <2.2e-16 .... why? Where can the >     error be? in >      > the script or in the test approach? >      > Thanks in advance >      > >      >       [[alternative HTML version deleted]] >      > >      > ______________________________________________ >      > [hidden email] mailing list >     -- To UNSUBSCRIBE and more, see >      > 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. >      > > >     -- >     Michael >     http://www.dewey.myzen.co.uk/home.html> -- Michael http://www.dewey.myzen.co.uk/home.html______________________________________________ [hidden email] mailing list -- To UNSUBSCRIBE and more, see https://stat.ethz.ch/mailman/listinfo/r-helpPLEASE do read the posting guide http://www.R-project.org/posting-guide.htmland provide commented, minimal, self-contained, reproducible code.