ST 445 -- SPRING 2008 
     HOMEWORK #6 
   DUE MONDAY, 07 APRIL 2008 

   EXERCISE 

      In the apr08 directory are three files containing the
      NC prison population for each of the years 2005, 2006,
      and 2007.  Your task is to read in these files, put
      them together to get the whole series from 01 Jan 2005
      through 31 Dec 2007, and to find some information.
      (Among the issues are goofy format, missing values (more 
      below), ordering of obs by year, month, day.)

      The files are
        prisn05.csv
        prisn06.csv
        prisn07.csv
      which, as you may surmise, are comma-delimited files.
      The creator of these files (actually a web-app) committed
      a crime by leaving some empty cells in moving from a
      spreadsheet to a .csv file.  You have learned about
      reading comma-delimited files using the dlm=',' option in
      the infile statement; here the trick is a different option
      called dsd, which 1) sets the delimiter to a comma, 2) treats
      two consecutive delimiters as a missing value, and 3) removes
      quotation marks from values (common for character values in 
      spreadsheets).

      To check your numbers, the correponding .html files are also
      included in the apr08 directory.

      Other information to be found:
      a) maximum population (and when)
      b) minimum population (and when)
      c) biggest one day increase in population (and when)
      d) biggest one day decrease in population (and when)

      And, lastly, plot the data.

      (In looking at these data several years ago, it appeared as
      if about a thousand prisoners took a Christmas vacation.)

      A function that may be useful is mdy(month,day,year) which
      give the SAS date for given values of month, day, and year.
      If no such day occurs, such as 31 June 1984, then the function
      returns a missing value.