filter a df with NA to get only individuals that appear more than one time in r - r

Question

I am using a national survey to run a regression: the survey is conducted every two years and some individual are repeatedly interviewed while others just one time.
Now I want to make the df a panel one (have only the individual that appears more than one time). The df is like this:
year nquest nord nordp sex age
2000 10 1 1 F 40
2000 10 2 2 M 43
2000 30 1 1 M 30
2002 10 1 1 F 42
2002 10 2 2 M 45
2002 10 3 NA F 15
2002 30 1 1 M 32
2004 10 1 1 F 44
2004 10 2 2 M 47
2004 10 3 3 F 17
2004 50 1 NA M 66
where nquest is the code number of the family, nord is the code number of the individual and nordp is the code number that the individual had in the previous survey; when a new individual is interviewed the value in nordp is "missing" (R automatically insert NA). For example the individual 3 of family 10 has nordp=NA in 2002 because it is the first time that she is interviewed, while in 2004 nordp is 3 (because 3 was the number that she had in 2002).
I can't use nord to filter the df because the composition of the family may change (for example in 2002 in family x the mother has nordp=2 (it means that in 2000 nord was 2) and nord=2 but the next year nord could be 1 (for example if she gets divorced) but nordp is still 2).
I tried to filter using this command:
df <- df %>%
group_by(nquest, nordp)
filter(n()>1)
but I don't get the right df because if for the same family there are more than one individual insert (NA) they will be considered as the same person since nordp is NA the first time.
How can I consider also the individual that appears for the first time in a certain year (nordp=NA)? I tried to a create a command using age (the age in t shoul be equal to (age (in t-2) + 2; for example in 2000 age is 20, in 2002 is 22) but it didn't worked.
Consider that the df is composed by thousand observations and I can't check manually.
The final df should be:
year nquest nordp sex age
2000 10 1 F 40
2000 10 2 M 43
2000 30 1 M 30
2002 10 1 F 42
2002 10 2 M 45
2002 10 3 F 15
2002 30 1 M 32
2004 10 1 F 44
2004 10 2 M 47
2004 10 3 F 17
As you can see there are only the individual that appears more than one time and nquest=10 nordp=30 appears three times; with my command it appears just two times because in the first year nordp was NA.

Answer 1

We wish to assign unique IDs to individuals, then filter by the count of unique IDs. The main idea is to chain together the nordp and nord values within each family over years. Here's an idea inspired by Identify groups of linked episodes which chain together. First, load the igraph package, via library(igraph). Then the following function assigns IDs for a given family.
assignID <- function(d) {
fields <- names(d) # store original column names
d$nordp[is.na(d$nordp)] <- seq_len(sum(is.na(d$nordp))) + 100
d$nordp_x <- (d$year-2) * 1000 + d$nordp
d$nord_x <- d$year * 1000 + d$nord
dd <- d[, c("nordp_x", "nord_x")]
gr.test <- graph.data.frame(dd)
links <- data.frame(org_id = unique(unlist(dd)),
id = clusters(gr.test)$membership)
d <- merge(d, links, by.x = "nord_x", by.y = "org_id", all.x = TRUE)
d$uid <- d$nquest * 100 + d$id
d[, c(fields, "uid")]
}
The function can "tell", for example, that
year nordp nord
2000 1 1
2002 1 2
2004 2 3
is the same individual, by chaining together the nordp and nord over the years, and assigns the same unique ID to all 3 rows. So, for example,
assignID(subset(df, nquest == 10))
# year nquest nord nordp sex age dob uid
# 1 2000 10 1 1 F 40 1960 1001
# 2 2000 10 2 2 M 43 1957 1002
# 3 2002 10 1 1 F 42 1960 1001
# 4 2002 10 2 2 M 45 1957 1002
# 5 2002 10 3 101 F 15 1987 1003
# 6 2004 10 1 1 F 44 1960 1001
# 7 2004 10 2 2 M 47 1957 1002
# 8 2004 10 3 3 F 17 1987 1003
gives us an additional column with the uid for each individual.
The remaining steps are straightforward. We split the dataframe by nquest, apply assignID to each subset, and rbind the output:
dd <- do.call(rbind, by(df, df$nquest, assignID))
Then we can just group by uid and filter by count:
dd %>% group_by(uid) %>% filter(n()>1)
# Source: local data frame [10 x 8]
# Groups: uid [4]
# year nquest nord nordp sex age dob uid
# <int> <int> <int> <dbl> <fctr> <int> <int> <dbl>
# 1 2000 10 1 1 F 40 1960 1001
# 2 2000 10 2 2 M 43 1957 1002
# 3 2002 10 1 1 F 42 1960 1001
# 4 2002 10 2 2 M 45 1957 1002
# 5 2002 10 3 101 F 15 1987 1003
# 6 2004 10 1 1 F 44 1960 1001
# 7 2004 10 2 2 M 47 1957 1002
# 8 2004 10 3 3 F 17 1987 1003
# 9 2000 30 1 1 M 30 1970 3001
# 10 2002 30 1 1 M 32 1970 3001