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

[B.-Markus Clarin]

I'd like to add that to me Q and WD don't look normally distributed. 
Isn't that a prerequisite for using the t.test to test for diffrences 
between the means? Please correct me if I am wrong.

Cheers,
Mango

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B.-Markus Clarin aka Mango


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Never run for the bus.
Never skip tea.

Am 24.03.2014 04:41, schrieb Baldwin, Jim -FS:
> The sentence "alternative hypothesis: true difference in means is not equal to 0" is stating what the alternative hypothesis is and not that your particular difference in means is significantly different from zero.  That sentence would appear (when you have a two-tailed test) no matter what the P-value might be.
>
> Try
>
> t.test(Q,WD)
> t.test(Q,WD,alternative="two.sided")   # Default
> t.test(Q,WD,alternative="less")
> t.test(Q,WD,alternative="greater")
>
>
> Jim
>
> Jim Baldwin
> USDA Forest Service
>
> -----Original Message-----
> From: r-sig-ecology-bounces at r-project.org [mailto:r-sig-ecology-bounces at r-project.org] On Behalf Of Michael Marsh
> Sent: Sunday, March 23, 2014 2:22 PM
> To: r-sig-ecology at r-project.org
> Subject: [R-sig-eco] report out by t.test
>
> 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
>
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