Calculate the Median of Already Grouped Data

Calculates the median of already grouped data given the interval ranges and the frequencies of each group.

GroupedMedian(frequencies, intervals, sep = NULL, trim = NULL)

Arguments

frequencies	A vector of frequencies.
intervals	A 2-row matrix with the same number of columns as the length of frequencies, with the first row being the lower class boundary, and the second row being the upper class boundary. Alternatively, intervals may be a column in your data.frame, and you may specify sep (and possibly, trim) to have the GroupedMedian function automatically create the required matrix for you.
sep	Optional. If the intervals are represented by a character vector with a character separating the interval ranges.
trim	Characters to trim from the vector before splitting. For example, if you are doing this on the output of cut (where, for some reason, you no longer have access to the original data), you can use the pre-set trim pattern "cut".

Value

A single numeric value representing the grouped median.

References

http://stackoverflow.com/a/18931054/1270695

Author

Ananda Mahto

Examples

mydf <- structure(list(salary = c("1500-1600", "1600-1700", "1700-1800",
        "1800-1900", "1900-2000", "2000-2100", "2100-2200", "2200-2300",
        "2300-2400", "2400-2500"), number = c(110L, 180L, 320L, 460L,
        850L, 250L, 130L, 70L, 20L, 10L)), .Names = c("salary", "number"),
        class = "data.frame", row.names = c(NA, -10L))
mydf
#>       salary number
#> 1  1500-1600    110
#> 2  1600-1700    180
#> 3  1700-1800    320
#> 4  1800-1900    460
#> 5  1900-2000    850
#> 6  2000-2100    250
#> 7  2100-2200    130
#> 8  2200-2300     70
#> 9  2300-2400     20
#> 10 2400-2500     10

GroupedMedian(frequencies = mydf$number, intervals = mydf$salary, sep = "-")
#> [1] 1915.294

## Example with intervals manually specified
X <- rbind(c(1500, 1600, 1700, 1800, 1900, 2000, 2100, 2200, 2300, 2400),
           c(1600, 1700, 1800, 1900, 2000, 2100, 2200, 2300, 2400, 2500))

GroupedMedian(mydf$number, X)
#> [1] 1915.294

set.seed(1)
x <- sample(100, 100, replace = TRUE)
y <- data.frame(table(cut(x, 10)))

GroupedMedian(y$Freq, y$Var1, sep = ",", trim = "cut")
#> [1] 47.2