[R] ANOVA/statistics question
drmh
douglasrmholmes at googlemail.com
Sun Apr 26 16:49:27 CEST 2009
Hello again,
In my situation, I have three variables: pretest, posttest, and cohesion.
I want to work out the correlation between postest and cohesion.
I looked at multiple sets of data and created ANOVA tables of them. However,
as pretest and postest are sometimes correlated (with a statistical
significance < 0.05), it is necessary to discount the effect of pretest to
work out the real correlation of posttest and coherence.. I need a system
for working out the strength of the correlation between posttest and
coherence, when does actually occur.
According to my understanding level refers the amount or magnitude of
experimental units. Pretest, posttest are scores - range from any value from
0 to 1. Cohesion can be any value.
What exactly would
cor(y[pre == 1], x[pre == 1])
cor(y[pre == 2], x[pre == 2])
give me?
I understand that my lack of understanding may be exasperating, but it is
not for want of effort - I have put in hours trying to understand this
stuff.
Thanks,
Douglas Holmes
Tal Galili wrote:
>
> Hi Douglas.
> So you want to check for correlation or regression ?
>
> how many levels does "pre" have ?
> you could subset the variables you want to check correlation on, by the
> pre
> levels.
>
> for example:
> Let's say pre has two levels: 1 and 2. then you can do:
> cor(y[pre == 1], x[pre == 1])
> cor(y[pre == 2], x[pre == 2])
>
>
> Also, If you want to go for regression, you can go to something like:
> summary(lm(y ~ x + pre))
> But I suggest getting much more understanding of what linear regression is
> before using it's methods...
>
> Good luck,
> Tal
>
>
>
> On Sat, Apr 25, 2009 at 1:26 PM, drmh
> <douglasrmholmes at googlemail.com>wrote:
>
>>
>> Hi, thanks for your prompt reply
>>
>> In my situation, the dependent variable is "post-test" and the
>> independent
>> variables are "pre" and "coh".
>> Howw would I find the correlation between coh and post with the effect of
>> "pre" regressed using your commands?
>>
>>
>>
>> Tal Galili wrote:
>> >
>> > Hi Douglas
>> > I would go for a different command then aov.
>> > something like:
>> > ?cor
>> > or
>> > ?cor.test
>> > To also get the p value of the correlation.
>> >
>> > Cheers,
>> > Tal
>> >
>> >
>> >
>> > On Sat, Apr 25, 2009 at 8:27 AM, drmh
>> > <douglasrmholmes at googlemail.com>wrote:
>> >
>> >>
>> >> (Have searched for this already)
>> >>
>> >> Hi,
>> >>
>> >> How do you find the strength of correlation between two variables
>> using
>> >> an
>> >> ANOVA table? "Pr(>F)" gives the statistical significance of the
>> >> association, but not the strength of the correlation.
>> >>
>> >> See data (from R) below
>> >>
>> >> Readable:
>> >> "Df" "Sum Sq" "Mean Sq" "F
>> >> value"
>> >> "Pr(>F)"
>> >> "pre" 1 0.00593 0.00593936
>> >> 0.7450563 0.401636958677004
>> >> "coh" 1 0.04311 0.04311302
>> >> 5.4082639
>> >> 0.0344751749542619
>> >> "Residuals" 15 0.11957 0.00797169 NA
>> >> NA
>> >>
>> >> Original:
>> >> "Df" "Sum Sq" "Mean Sq" "F value" "Pr(>F)"
>> >> "pre" 1 0.0059393604629317 0.0059393604629317 0.745056336657567
>> >> 0.401636958677004
>> >> "coh" 1 0.0431130207164516 0.0431130207164516 5.40826398359156
>> >> 0.0344751749542619
>> >> "Residuals" 15 0.119575396598395 0.00797169310655964 NA NA
>> >>
>> >> Any help would be greatly appreciated,
>> >> Douglas Holmes
>> >> --
>> >> View this message in context:
>> >>
>> http://www.nabble.com/ANOVA-statistics-question-tp23231563p23231563.html
>> >> Sent from the R help mailing list archive at Nabble.com.
>> >>
>> >> ______________________________________________
>> >> R-help at r-project.org mailing list
>> >> https://stat.ethz.ch/mailman/listinfo/r-help
>> >> PLEASE do read the posting guide
>> >> http://www.R-project.org/posting-guide.html
>> >> and provide commented, minimal, self-contained, reproducible code.
>> >>
>> >
>> >
>> >
>> > --
>> > ----------------------------------------------
>> >
>> >
>> > My contact information:
>> > Tal Galili
>> > Phone number: 972-50-3373767
>> > FaceBook: Tal Galili
>> > My Blogs:
>> > http://www.r-statistics.com/
>> > http://www.talgalili.com
>> > http://www.biostatistics.co.il
>> >
>> > [[alternative HTML version deleted]]
>> >
>> > ______________________________________________
>> > R-help at r-project.org mailing list
>> > https://stat.ethz.ch/mailman/listinfo/r-help
>> > PLEASE do read the posting guide
>> > http://www.R-project.org/posting-guide.html
>> > and provide commented, minimal, self-contained, reproducible code.
>> >
>> >
>>
>> --
>> View this message in context:
>> http://www.nabble.com/ANOVA-statistics-question-tp23231563p23234421.html
>> Sent from the R help mailing list archive at Nabble.com.
>>
>> ______________________________________________
>> R-help at r-project.org mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-help
>> PLEASE do read the posting guide
>> http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>>
>
>
>
> --
> ----------------------------------------------
>
>
> My contact information:
> Tal Galili
> Phone number: 972-50-3373767
> FaceBook: Tal Galili
> My Blogs:
> http://www.r-statistics.com/
> http://www.talgalili.com
> http://www.biostatistics.co.il
>
> [[alternative HTML version deleted]]
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>
>
--
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