error.bars.by {psych}R Documentation

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

x 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

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 )

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.

See Also

See Also as error.crosses, error.bars

Examples


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


[Package psych version 1.0-67 Index]