recode a factor by "combining" levels

This makes it easy to put levels together and create a new factor variable. If a factor variable is currently coded with levels c("Male","Female","Man", "M"), and the user needs to combine the redundant levels for males, this is the function to use! This is a surprisingly difficult problem in R.

Usage

combineLevels(fac, levs, newLabel = "combinedLevels")

Arguments

fac

An R factor variable, either ordered or not.

levs

The levels to be combined. Users may specify either a numerical vector of level values, such as c(1,2,3), to combine the first three elements of level(fac), or they may specify level names. This can be done as a character vector of *correctly spelled* factor values, such as c("Yes","Maybe","Always") or it may be provided as a subset of the output from levels, such as levels(fac)[1:3].

newLabel

A character string that represents the label of the new level to be created when levs values are combined.

Value

A new factor variable, with unused levels removed.

Details

If the factor is an ordinal factor, then levels may be combined only if they are adjacent. A factor with levels c("Lo","Med","Hi","Extreme") allows us to combine responses "Lo" and "Med", while it will NOT allow us to combine "Lo" with "Hi".

A non-ordered factor can be reorganized to combine any values, no matter what positions they occupy in the levels vector.

Author

Paul E. Johnson pauljohn@ku.edu

Examples

x <- c("M","A","B","C","A","B","A","M")
x <- factor(x)
levels(x)
#> [1] "A" "B" "C" "M"
x2a <- combineLevels(x, levs = c("M","A"), newLabel = "M_or_A")
#> The original levels A B C M 
#> have been replaced by B C M_or_A 
addmargins(table(x2a, x, exclude=NULL))
#>         x
#> x2a      A B C M Sum
#>   B      0 2 0 0   2
#>   C      0 0 1 0   1
#>   M_or_A 3 0 0 2   5
#>   Sum    3 2 1 2   8
x2b <- combineLevels(x, c(1,4), "M_or_A")
#> The original levels A B C M 
#> have been replaced by B C M_or_A 
addmargins(table(x2b, x, exclude=NULL))
#>         x
#> x2b      A B C M Sum
#>   B      0 2 0 0   2
#>   C      0 0 1 0   1
#>   M_or_A 3 0 0 2   5
#>   Sum    3 2 1 2   8
x3 <- combineLevels(x, levs = c("M","A","C"), newLabel = "MAC")
#> The original levels A B C M 
#> have been replaced by B MAC 
addmargins(table(x3, x, exclude=NULL))
#>      x
#> x3    A B C M Sum
#>   B   0 2 0 0   2
#>   MAC 3 0 1 2   6
#>   Sum 3 2 1 2   8
## Now an ordinal factor
z <- c("M","A","B","C","A","B","A","M")
z <- ordered(z)
levels(z)
#> [1] "A" "B" "C" "M"
table(z, exclude=NULL)
#> z
#> A B C M 
#> 3 2 1 2 
z2a <-  combineLevels(z, levs = c(1,2), "Good")
#> The original levels A B C M 
#> have been replaced by Good C M 
addmargins(table(z2a, z, exclude = NULL))
#>       z
#> z2a    A B C M Sum
#>   Good 3 2 0 0   5
#>   C    0 0 1 0   1
#>   M    0 0 0 2   2
#>   Sum  3 2 1 2   8
z2b <- combineLevels(z, levs = c("A","B"), "AorB")
#> The original levels A B C M 
#> have been replaced by AorB C M 
addmargins(table(z2b, z, exclude = NULL))
#>       z
#> z2b    A B C M Sum
#>   AorB 3 2 0 0   5
#>   C    0 0 1 0   1
#>   M    0 0 0 2   2
#>   Sum  3 2 1 2   8