[R] bars with sd

Matteo Vidali vidali at med.unipmn.it
Fri May 21 11:03:43 CEST 2004


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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