# [R] conditionally weighted mean with NAs

David Winsemius dwinsemius at comcast.net
Sun Apr 3 04:59:20 CEST 2011

```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

```