# Re: [R] t value two.sided and one.sided

From: Marc Schwartz <marc_schwartz_at_comcast.net>
Date: Thu, 10 May 2007 19:11:32 -0500

On Fri, 2007-05-11 at 00:14 +0100, gatemaze_at_gmail.com wrote:
> Hi,
>
> on a
> > summary(lm(y~x))
> are the computed t-values for two.sided or one.sided. By looking on some
> tables they seem like they are for two.sided.

Yep.

> Is it possible to have them
> for one.sided? If this makes sense...

Yep.

Using the example model 'lm.D9' from ?lm:

summ.lm <- summary(lm.D9)

> summ.lm

Call:
lm(formula = weight ~ group)

Residuals:

Min 1Q Median 3Q Max -1.0710 -0.4938 0.0685 0.2462 1.3690

Coefficients:

```            Estimate Std. Error t value Pr(>|t|)
(Intercept)   5.0320     0.2202  22.850 9.55e-15 ***
groupTrt     -0.3710     0.3114  -1.191    0.249
```
```---
Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1

Residual standard error: 0.6964 on 18 degrees of freedom
Multiple R-Squared: 0.07308,    Adjusted R-squared: 0.02158
F-statistic: 1.419 on 1 and 18 DF,  p-value: 0.249

Now get the one sided values:

> pt(coef(summ.lm)[, 3], summ.lm\$df, lower = FALSE)

(Intercept)     groupTrt
4.773564e-15 8.754884e-01

> pt(coef(summ.lm)[, 3], summ.lm\$df, lower = TRUE)

(Intercept)    groupTrt
1.0000000   0.1245116

See ?summary.lm, ?coef and ?pt

HTH,

Marc Schwartz

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