# Re: [R] bars with sd

From: Spencer Graves (spencer.graves@pdf.com)
Date: Sat 22 May 2004 - 01:15:45 EST

Message-id: <40AE1D21.6080406@pdf.com>

> DF <- data.frame(mean=c(1.25, 2.65, 3.45), sd=c(.35, .65, .5),
+ row.names=letters[1:3])
> plot(1:3, DF\$mean, ylim=range(DF\$mean-2*DF\$sd, DF\$mean+2*DF\$sd))
> segments(1:3, DF\$mean-2*DF\$sd, 1:3, DF\$mean+2*DF\$sd)

hope this helps. spencer graves

Matteo Vidali wrote:

>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?
>Matteo
>
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>

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