On Apr 2, 2011, at 10:20 PM, Joshua Wiley wrote:
Dear List,
I am trying to take the weighted average of two numbers (visual acuity
measures from the left and right eye). For each row, the lowest value
should get a weight of .75, and the highest a weight of .25. My
problem is, if one value is missing (NA), the remaining one should get
a weight of 1 (i.e., just return the nonmissing value), if both are
missing, NA should be returned. Below is some example data and the
code I tried to write (Desired is what I actually want). Any thoughts
or comments would be welcome.
Thanks,
Josh
VA <- cbind(OS = c(.2, 0, 1, -.1, NA, 3, NA),
OD = c(.3, -.1, .2, -.1, NA, 0, .1),
Desired = c(0.225, -0.075, 0.4, -0.1, NA, 0.75, 0.1))
## What I tried
weight.combine <- function(left, right) {
out.r <- right
out.l <- left
right <- ifelse(out.r <= out.l, out.r * .75, out.r * .25)
left <- ifelse(out.r > out.l, out.l * .75, out.l * .25)
rowSums(cbind(left, right), na.rm = TRUE)
}
VA <- cbind(VA, 0.75*pmin(VA[, "OS"], VA[, "OD"], na.rm=TRUE) +
0.25*pmax(VA[, "OS"], VA[, "OD"], na.rm=TRUE) )
> VA
OS OD Desired
[1,] 0.2 0.3 0.225 0.225
[2,] 0.0 -0.1 -0.075 -0.075
[3,] 1.0 0.2 0.400 0.400
[4,] -0.1 -0.1 -0.100 -0.100
[5,] NA NA NA NA
[6,] 3.0 0.0 0.750 0.750
[7,] NA 0.1 0.100 0.100
## This "works", except it does not handle NAs properly
weight.combine(VA[, "OS"], VA[, "OD"])
--
Joshua Wiley
David Winsemius, MD
West Hartford, CT
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