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
-----------------------------------------
B.-Markus Clarin aka Mango
-----------------------------------------
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-boun...@r-project.org
[mailto:r-sig-ecology-boun...@r-project.org] On Behalf Of Michael Marsh
Sent: Sunday, March 23, 2014 2:22 PM
To: r-sig-ecology@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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