[R] bars with sd

[Spencer Graves]

 > 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?
>thanks in advance
>Matteo
>
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