Actually, tying to a file has some drawbacks. Foremost of which is that you have to have a file to tie to. I often pipe in data from another program or read in the data from a database. A tied file approach would require the overhead of writing to disk and reading back. Also, some statistics (such as median) require the data to be in sorted order -- that means you'd have to sort the file in memory or sort and write to disk again which defeats the purpose. The approach I used gets around all of those limitations -- you don't need an intermediate file and you don't have to sort the original data points.