> head(statistics)
     persnr year NOBS     value
1: 61961225 1993    1 0.5777598
2: 62037645 1993    1 0.5777598
3: 62181514 1993    1 0.5777598
4: 62499451 1993    1 0.5777598
5: 62649247 1993    1 0.5777598
6: 62744472 1993    1 0.5777598

dataTable = data.table(cast(statistics, persnr ~ year, value='totalWage'))
# y is the second year. Iterate over that
for (y in tail(unique(statistics[, jahr]), n=-1)):
  # get the first year
  x <- y - 1
  dataTable[!is.na(`x`) & !is.na(`y`), `y`-`x`]
}