error.bars.by: Plot means and confidence intervals for multiple groups

Description

One of the many functions in R to plot means and confidence intervals. Meant mainly for demonstration purposes for showing the probabilty of replication from multiple samples. Can also be combined with such functions as boxplot to summarize distributions. Means and standard errors for each group are calculated using describe.by.

Usage

error.bars.by(x, group,by.var = FALSE,x.cat=TRUE, ylab = "NULL", xlab = "NULL",main=NULL, ylim = NULL, alpha = .05, labels = NULL, pos = NULL, arrow.len = 0.05, add = FALSE,bars=FALSE, ...)

Arguments

A data frame or matrix

group

A grouping variable

by.var

A different line for each group (default) or each variable

x.cat

Is the grouping variable categorical (TRUE) or continuous (FALSE

ylab

y label

xlab

x label

main

title for figure

ylim

if specified, the limits for the plot, otherwise based upon the data

alpha

alpha level of confidence interval. Default is 1- alpha =95% confidence interval

labels

X axis label

pos

where to place text: below, left, above, right

arrow.len

How long should the top of the error bars be?

add

add=FALSE, new plot, add=TRUE, just points and error bars

bars

Draw a barplot with error bars rather than a simple plot of the means

...

other parameters to pass to the plot function, e.g., typ="b" to draw lines, lty="dashed" to draw dashed lines

Value

Graphic output showing the means + x% confidence intervals for each group. For ci=1.96, and normal data, this will be the 95% confidence region. For ci=1, the 68% confidence region.

Details

Drawing the mean +/- a confidence interval is a frequently used function when reporting experimental results. By default, the confidence interval is 1.96 standard errors.

This function is a wrapper for error.bars and allows groups to be organized either as the x axis or as separate lines.

If desired, a barplot with error bars can be shown. Many find this type of plot to be uninformative (e.g., http://biostat.mc.vanderbilt.edu/DynamitePlots )

Examples

Run this code

data(sat.act)
error.bars.by(sat.act,sat.act$gender)
error.bars.by(sat.act[5:6],sat.act$gender,bars=TRUE,labels=c("male","female"))  #draw a barplot

error.bars.by(sat.act[5:6],sat.act$education,bars=TRUE) 

error.bars.by(sat.act[5:6],sat.act$education,TRUE)  #plot SAT V and SAT Q by education