The one most often used function in the analysis of statistical data is the creation of tables. This edition of the R-Sessions describes the use of several functions to do some nifty cross-tabulations. And more.

TAPPLY

The function TAPPLY can be used to perform calculations on table-marginals. Different functions can be used, such as MEAN, SUM, VAR, SD, LENGTH (for frequency-tables). For example:

x <- c(0,1,2,3,4,5,6,7,8,9)
y <- c(1,1,1,1,1,1,2,2,2,2)
tapply(x,y,mean)
tapply(x,y,sum)
tapply(x,y,var)
tapply(x,y,length)

> x <- c(0,1,2,3,4,5,6,7,8,9)
> y <- c(1,1,1,1,1,1,2,2,2,2)
> tapply(x,y,mean)
  1     2
2.5   7.5
> tapply(x,y,sum)
 1  2
15 30
> tapply(x,y,var)
       1        2
3.500000 1.666667
> tapply(x,y,length)
1 2
6 4
>

FTABLE

More elaborate frequency tables can be created with the FTABLE-function. For example:

x <- c(0,1,2,3,4,5,6,7,8,9)
y <- c(1,1,1,1,1,1,2,2,2,2)
z <- c(1,1,1,2,2,2,2,2,1,1)
ftable(x,y,z)

> x <- c(0,1,2,3,4,5,6,7,8,9)
> y <- c(1,1,1,1,1,1,2,2,2,2)
> z <- c(1,1,1,2,2,2,2,2,1,1)
> ftable(x,y,z)
    z 1 2
x y
0 1   1 0
  2   0 0
1 1   1 0
  2   0 0
2 1   1 0
  2   0 0
3 1   0 1
  2   0 0
4 1   0 1
  2   0 0
5 1   0 1
  2   0 0
6 1   0 0
  2   0 1
7 1   0 0
  2   0 1
8 1   0 0
  2   1 0
9 1   0 0
  2   1 0

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