[R-sig-eco] report out by t.test

[Jari Oksanen]

Except that t-test does not assume that *observations* are normally distributed, nor that variances are equal.

Avoid non-parametric tests: they assume too much of data properties.

For var.equal assumption in t.test, see ?t.test.

Cheers, Jari Oksanen
________________________________________
From: r-sig-ecology-bounces at r-project.org [r-sig-ecology-bounces at r-project.org] on behalf of Richard Boyce [boycer at nku.edu]
Sent: 24 March 2014 13:23
To: r-sig-ecology at r-project.org
Subject: Re: [R-sig-eco] report out by t.test

Mike,

There is no way that your data meet the assumptions of a t-test (normal distributions, equal variance). A nonparametric Mann-Whitney (aka Wilcoxon) test is much better suited to your data.

Here's what I got when I ran it:

Q<-c(13,0,10,2,0,0,1,0,0,1,5)
WD<-c(0,0,1,0,0,0,0,0,0,0,1)
wilcox.test(Q,WD)

Wilcoxon rank sum test with continuity correction

data:  Q and WD
W = 86.5, p-value = 0.05119
alternative hypothesis: true location shift is not equal to 0

Warning message:
In wilcox.test.default(Q, WD) : cannot compute exact p-value with ties

This has a p-value quite close to 0.05, giving some evidence that there's a difference between your groups. Note that this you have different null and alternative hypothesis: groups are the same vs. groups are different.

Rick Boyce

On Mar 24, 2014, at 7:00 AM, r-sig-ecology-request at r-project.org<mailto:r-sig-ecology-request at r-project.org> wrote:

Message: 1
Date: Sun, 23 Mar 2014 14:21:41 -0700
From: Michael Marsh <swamp at blarg.net<mailto:swamp at blarg.net>>
To: r-sig-ecology at r-project.org<mailto:r-sig-ecology at r-project.org>
Subject: [R-sig-eco] report out by t.test
Message-ID: <532F5065.7030102 at blarg.net<mailto:532F5065.7030102 at blarg.net>>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed

I test differences between frequency of hits of exotic annual forbs in
plots on  two sites, Q and WD.

Q<-c(13,0,10,2,0,0,1,0,0,1,5)
WD<-c(0,0,1,0,0,0,0,0,0,0,1)
t.test(Q,WD)

        Welch Two Sample t-test

data:  Q and WD
t = 1.9807, df = 10.158, p-value = 0.07533
alternative hypothesis: true difference in means is not equal to 0
95 percent confidence interval:
 -0.3342006  5.7887460
sample estimates:
mean of x mean of y
2.9090909 0.1818182

The p-value is greater than 0.05, thus does not reach the 95% confidence
level, yet the difference in means is reported as not equal to 0.
Am I encountering a one-sided versus two sided comparison that I don't
understand, or is ther another explanation?

Mike Marsh




================================
Richard L. Boyce, Ph.D.
Director, Environmental Science Program
Professor
Department of Biological Sciences, SC 150
Northern Kentucky University
Nunn Drive
Highland Heights, KY  41099  USA

859-572-1407 (tel.)
859-572-5639 (fax)
boycer at nku.edu<mailto:boycer at nku.edu>
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