[R] bars with sd

[Robert W. Baer, Ph.D.]

You want to read the help ?segments.  Try the following to get the barplots:

ymean=c(1.25,2.65,3.45)
ysd=c(0.35,0.65,0.50)
xpos=barplot(ymean,ylim=c(0,max(ymean)+max(ysd)),col='yellow')
segments(xpos,ymean-ysd,xpos,ymean+ysd)

HTH,
Rob Baer

----- Original Message ----- 
From: "Matteo Vidali" <vidali at med.unipmn.it>
To: <R-help at stat.math.ethz.ch>
Sent: Friday, May 21, 2004 4:03 AM
Subject: [R] bars with sd


> I need some help for a curious question of a friend of mine.
> She usually does some experiments (3-5 repeats for each exp) and then she
> calculates mean and standard deviation.
> In microsoft excel she writes something like the following
>
> sample  mean   sd
> a       1.25   0.35
> b       2.65   0.65
> c       3.45   0.50
>
> She can do a vertical barplot graph just giving mean value and specifying
> the standard deviation value for each bar in the graph.
>
> My friend wants to know if it is possible to do the same with R. She has
> tried also with Open Office and seen that in that program you can plot
mean
> values as bars but it is not possible to specify a different sd for each
> bar 'cause the program, if you ask to put the standard deviation,
calculates
> a common sd using the mean values.
> I was not able to give her an answer for the barplot function in R. I have
> just said to her that it is likely due to the fact that using a barplot
> graph to plot mean and standard deviation values it is non a good
practice,
> since the height (or the area) of bars should represent frequency and that
> putting a standard deviation in that way it is at least quite confusing
for
> the reader to interpret the graph. Some scientific journal revisors
> discourage this practice. She should instead use a boxplot or just
plotting
> the different repetition values for each sample as points and add a linea
> indicating the mean.
> But if this is correct why in some programs like open office you can plot
> bars with a common standard deviation???? when will you need this last
type
> of graph?
>
> any suggestion? any help?
> thanks in advance
> Matteo
>
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