I've just started learning r and have had trouble finding an (understandable) explanation of what the prop.table() function does. I found the following explanation and example:
prop.table: Express Table Entries as Fraction of Marginal Table
Examples
m <- matrix(1:4, 2) m prop.table(m, 1)
But, as a beginner, I do not understand what this explanation means. I've also attempted to discern its functionality from the result of the above example, but I haven't been able to make sense of it.
With reference to the example above, what does the prop.table() function do? Furthermore, what is a "marginal table"?
3 Answers
The values in each cell divided by the sum of the 4 cells:
prop.table(m) The value of each cell divided by the sum of the row cells:
prop.table(m, 1) The value of each cell divided by the sum of the column cells:
prop.table(m, 2) I think this can help
include all those things like prop.table(m), prop.table(m, 1), prop.table(m, 2)
m <- matrix(1:4, 2) > m [,1] [,2] [1,] 1 3 [2,] 2 4 > prop.table(m) #sum=1+2+3+4=10, 1/10=0.1, 2/10=0.2, 3/10=0.3,4/10=0.4 [,1] [,2] [1,] 0.1 0.3 [2,] 0.2 0.4 > prop.table(m,1) [,1] [,2] [1,] 0.2500000 0.7500000 #row1: sum=1+3=4, m(0,0)=1/4=0.25, m(0,1)=3/4=0.75 [2,] 0.3333333 0.6666667 #row2: sum=2+4=6, m(1,0)=2/6=0.33, m(1,1)=4/6=0.66 > prop.table(m,2) [,1] [,2] [1,] 0.3333333 0.4285714 #col1: sum=1+2=3, m(0,0)=1/3=0.33, m(1,0)=2/3=0.4285 [2,] 0.6666667 0.5714286 #col2: sum=3+4=7, m(0,1)=3/7=0.66, m(1,1)=4/7=0.57 > when m is the 2D matrix: (m,1) refers to a fraction of row marginal table (sum over each row), (m,2) refers to a fraction of column marginal table (sum over each column). In short, just a "% of total sum of row of column", if you dont want to care about the term marginal.
Example:
m with extra row and column margin
[,1] [,2] *** [1,] 1 4 5 [2,] 2 5 7 [3,] 3 6 9 *** 6 15 > prop.table(m,1) ` [,1] [,2] [1,] 0.2000000 0.8000000 [2,] 0.2857143 0.7142857 [3,] 0.3333333 0.6666667 > prop.table(m,2) [,1] [,2] [1,] 0.1666667 0.2666667 [2,] 0.3333333 0.3333333 [3,] 0.5000000 0.4000000 
