Hello, I am trying to figure out how to conduct a t-test on a specific contrast for my data. I have four factors in my data and would like to conduct a t-test on the average of the data from the first two factors against the average of the data on the second two factor (i.e. is the average of the first two different from the average of the second two). Is there a quick way to do this? I found the contrast function, but wasn't sure how to apply it. Thank you, Kim
I hope you get a reply from someone who knows more about this than
I. However, in the spirit that a quick hack is sometimes better than an
elegant answer later, consider the following:
DF <- data.frame(a=rep(letters[1:4], 2), y=1:8)
> DF$b <- ((DF$a %in% letters[1:2])-(DF$a%in% letters[3:4]))
> fit <- lm(y~b+a, DF, singular.ok=T)
> summary(fit)
Call:
lm(formula = y ~ b + a, data = DF, singular.ok = T)
Residuals:
1 2 3 4 5 6 7 8
-2 -2 -2 -2 2 2 2 2
Coefficients: (1 not defined because of singularities)
Estimate Std. Error t value Pr(>|t|)
(Intercept) 4.500 1.414 3.182 0.0335 *
b -1.500 1.414 -1.061 0.3486
ab 1.000 2.828 0.354 0.7415
ac -1.000 2.828 -0.354 0.7415
ad NA NA NA NA
---
Signif. codes: 0 `***' 0.001 `**' 0.01 `*' 0.05 `.' 0.1 ` '
1
Residual standard error: 2.828 on 4 degrees of freedom
Multiple R-Squared: 0.2381, Adjusted R-squared: -0.3333
F-statistic: 0.4167 on 3 and 4 DF, p-value: 0.751
This will give you what you want ONLY if you have the same number
of observations in the two subsets to be compared. If not, then you can
redefine DF$b to produce what you want.
hope this helps. spencer graves
Kimberly Ann Fernandes wrote:
>Hello,
>
>I am trying to figure out how to conduct a t-test on a specific contrast
>for my data. I have four factors in my data and would like to conduct a
>t-test on the average of the data from the first two factors against the
>average of the data on the second two factor (i.e. is the average of the
>first two different from the average of the second two). Is there a
>quick way to do this? I found the contrast function, but wasn't sure
>how to apply it.
>
>Thank you,
>Kim
>
>______________________________________________
>R-help at stat.math.ethz.ch mailing list
>https://www.stat.math.ethz.ch/mailman/listinfo/r-help
>PLEASE do read the posting guide!
http://www.R-project.org/posting-guide.html
>
>
Try the fit.contrast() and estimable() functions from my gregmisc package, or the contrasts() function in Frank Harrell's Design package. -Greg> -----Original Message----- > From: r-help-bounces at stat.math.ethz.ch > [mailto:r-help-bounces at stat.math.ethz.ch]On Behalf Of Kimberly Ann > Fernandes > Sent: Monday, May 31, 2004 1:10 PM > To: r-help at stat.math.ethz.ch > Subject: [R] Contrasts > > > Hello, > > I am trying to figure out how to conduct a t-test on a > specific contrast > for my data. I have four factors in my data and would like > to conduct a > t-test on the average of the data from the first two factors > against the > average of the data on the second two factor (i.e. is the > average of the > first two different from the average of the second two). Is there a > quick way to do this? I found the contrast function, but wasn't sure > how to apply it. > > Thank you, > Kim > > ______________________________________________ > R-help at stat.math.ethz.ch mailing list > https://www.stat.math.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide! > http://www.R-project.org/posting-guide.html >LEGAL NOTICE\ Unless expressly stated otherwise, this messag...{{dropped}}