Question

I have a table called tableOne in R like this:

idNum        binaryVariable        salePrice
2               1                    55.56
4               0                    88.33
15              0                     4.45
87              1                    35.77
...            ...                    ...

I’d like to take the values produced from: summary(tableOne$salePrice) to create four quartiles by salePrice. I’d then like to create a column tableOne$quartile with which quartile each rows salePrice is in. It would look like:

idNum        binaryVariable            salePrice      quartile
    2               1                    55.56            3
    4               0                    88.33            4
    15              0                     4.45            1
    87              1                    35.77            2 
    ...            ...                    ...            ...  

Any suggestions?

Answer 1

This should do it:

tableOne <- within(tableOne, quartile <- as.integer(cut(salesPrice, quantile(salesPrice, probs=0:4/4), include.lowest=TRUE)))

…Some details:

The within function is great for calculating new columns. You don’t have to refer to columns as
tableOne$salesPrice etc.

tableOne <- within(tableOne, quartile <- <<<some expression>>>)

The quantile function calculates the quantiles (or in your case, quartiles). 0:4/4 evaluates to c(0, 0.25, 0.50, 0.75, 1).

Finally the cut function splits your data into those quartiles. But you get a factor with weird names, so as.integer turns it into groups 1,2,3,4.

Try ?within etc to learn more about the functions mentioned here…