ST 610N --  FALL 2006
       HOMEWORK 4
       due Thursday, 19 October 2006

       In the file maunaloa.co2 are atmospheric carbon dioxide 
       measurements taken on a station on the Big Island of Hawaii.
       (This site is one of the oldest for this important data.)
       The data have the format
      
       Year Jan Feb ...... Dec Mean Fit 

       where 'Mean' should be the mean for that year, and 'fit'
       is more complicated and we don't want to deal with it.
       Notice the header and foot that also describe missing value
       format.

       a) Check the yearly means -- are the records consistent?
       
       b) Create a dataset with each observation corresponding to
       a month, with variables YEAR, MONTH (numeric) and CO2.
       Print out the first few and last few observations.
       
       c) Plot the yearly means by year.

       d) Create a chart or table that addresses the question, 
       'Within a year, which month most often has the highest value of CO2?'

       e) Create a dataset with each observation corresponding to
       a quarterly average, but where the quarters are given by
       Jan, Feb, Mar = 1; Apr, May, Jun = 2, etc.

       f) Using (e) plot the trends of the four quarters versus
       time (year).  (Put 4 curves on one plot and connect the dots.)