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

[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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