How to calculate time-weighted average and create lags - r

Question

I have searched the forum, but found nothing that could answer or provide hint on how to do what I wish to on the forum.
I have yearly measurement of exposure data from which I wish to calculate individual level annual average based on entry of each individual into the study. For each row the one year exposure assignment should include data from the preceding 12 months starting from the last month before joining the study.
As an example the first person in the sample data joined the study on Feb 7, 2002. His exposure will include a contribution of January 2002 (annual average is 18) and February to December 2001 (annual average is 19). The time weighted average for this person would be (1/12*18) + (11/12*19). The two year average exposure for the same person would extend back from January 2002 to February 2000.
Similarly, for last person who joined the study in December 2004 will include contribution on 11 months in 2004 and one month in 2003 and his annual average exposure will be (11/12*5 ) derived form 2004 and (1/12*6) which comes from the annual average of 2003.
How can I calculate the 1, 2 and 5 year average exposure going back from the date of entry into study? How can I use lags in the manner taht I hve described?
Sample data is accessed from this link
https://drive.google.com/file/d/0B_4NdfcEvU7La1ZCd2EtbEdaeGs/view?usp=sharing

Answer 1

This is not an elegant answer. But, I would like to leave what I tried. I first arranged the data frame. I wanted to identify which year will be the key year for each subject. So, I created id. variable comes from the column names (e.g., pol_2000) in your original data set. entryYear comes from entry in your data. entryMonth comes from entry as well. check was created in order to identify which year is the base year for each participant. In my next step, I extracted six rows for each participant using getMyRows in the SOfun package. In the next step, I used lapply and did math as you described in your question. For the calculation for two/five year average, I divided the total values by year (2 or 5). I was not sure how the final output would look like. So I decided to use the base year for each subject and added three columns to it.
library(stringi)
library(SOfun)
devtools::install_github("hadley/tidyr")
library(tidyr)
library(dplyr)
### Big thanks to BondedDust for this function
### http://stackoverflow.com/questions/6987478/convert-a-month-abbreviation-to-a-numeric-month-in-r
mo2Num <- function(x) match(tolower(x), tolower(month.abb))
### Arrange the data frame.
ana <- foo %>%
mutate(id = 1:n()) %>%
melt(id.vars = c("id","entry")) %>%
arrange(id) %>%
mutate(variable = as.numeric(gsub("^.*_", "", variable)),
entryYear = as.numeric(stri_extract_last(entry, regex = "\\d+")),
entryMonth = mo2Num(substr(entry, 3,5)) - 1,
check = ifelse(variable == entryYear, "Y", "N"))
### Find a base year for each subject and get some parts of data for each participant.
indx <- which(ana$check == "Y")
bob <- getMyRows(ana, pattern = indx, -5:0)
### Get one-year average
cathy <- lapply(bob, function(x){
x$one <- ((x[6,6] / 12) * x[6,4]) + (((12-x[5,6])/12) * x[5,4])
x
})
one <- unnest(lapply(cathy, `[`, i = 6, j = 8))
### Get two-year average
cathy <- lapply(bob, function(x){
x$two <- (((x[6,6] / 12) * x[6,4]) + x[5,4] + (((12-x[4,6])/12) * x[4,4])) / 2
x
})
two <- unnest(lapply(cathy, `[`, i = 6, j =8))
### Get five-year average
cathy <- lapply(bob, function(x){
x$five <- (((x[6,6] / 12) * x[6,4]) + x[5,4] + x[4,4] + x[3,4] + x[2,4] + (((12-x[2,6])/12) * x[1,4])) / 5
x
})
five <- unnest(lapply(cathy, `[`, i =6 , j =8))
### Combine the results with the key observations
final <- cbind(ana[which(ana$check == "Y"),], one, two, five)
colnames(final) <- c(names(ana), "one", "two", "five")
# id entry variable value entryYear entryMonth check one two five
#6 1 07feb2002 2002 18 2002 1 Y 18.916667 18.500000 18.766667
#14 2 06jun2002 2002 16 2002 5 Y 16.583333 16.791667 17.150000
#23 3 16apr2003 2003 14 2003 3 Y 15.500000 15.750000 16.050000
#31 4 26may2003 2003 16 2003 4 Y 16.666667 17.166667 17.400000
#39 5 11jun2003 2003 13 2003 5 Y 13.583333 14.083333 14.233333
#48 6 20feb2004 2004 3 2004 1 Y 3.000000 3.458333 3.783333
#56 7 25jul2004 2004 2 2004 6 Y 2.000000 2.250000 2.700000
#64 8 19aug2004 2004 4 2004 7 Y 4.000000 4.208333 4.683333
#72 9 19dec2004 2004 5 2004 11 Y 5.083333 5.458333 4.800000