Basic summary statistics by group

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Description

Report basic summary statistics by a grouping variable. Useful if the grouping variable is some experimental variable and data are to be aggregated for plotting. Partly a wrapper for by and describe

Usage

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describeBy(x, group=NULL,mat=FALSE,type=3,digits=15,...)
describe.by(x, group=NULL,mat=FALSE,type=3,...)  # deprecated

Arguments

x

a data.frame or matrix. See note for statsBy.

group

a grouping variable or a list of grouping variables

mat

provide a matrix output rather than a list

type

Which type of skew and kurtosis should be found

digits

When giving matrix output, how many digits should be reported?

...

parameters to be passed to describe

Details

To get descriptive statistics for several different grouping variables, make sure that group is a list. In the case of matrix output with multiple grouping variables, the grouping variable values are added to the output.

The type parameter specifies which version of skew and kurtosis should be found. See describe for more details.

An alternative function (statsBy) returns a list of means, n, and standard deviations for each group. This is particularly useful if finding weighted correlations of group means using cor.wt. More importantly, it does a proper within and between group decomposition of the correlation.

Value

A data.frame of the relevant statistics broken down by group:
item name
item number
number of valid cases
mean
standard deviation
median
mad: median absolute deviation (from the median)
minimum
maximum
skew
standard error

Author(s)

William Revelle

See Also

describe, statsBy, densityBy and violinBy as well as error.bars and error.bars.by for other graphical displays.

Examples

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data(sat.act)
describeBy(sat.act,sat.act$gender) #just one grouping variable	
#describeBy(sat.act,list(sat.act$gender,sat.act$education))  #two grouping variables
des.mat <- describeBy(sat.act$age,sat.act$education,mat=TRUE) #matrix (data.frame) output 
des.mat <- describeBy(sat.act$age,list(sat.act$education,sat.act$gender),
             mat=TRUE,digits=2)  #matrix output

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