Aerial survey methods are regularly used to estimate the number of snow geese
in their summer range areas west of Hudson Bay in Canada.  To obtain estimates
small aircraft fly over the range, and when a flock of geese is
spotted, an experienced person estimates the number of
geese in the flock.  To investigate the reliability of this method of
counting, an experiment was conducted in which an airplane carrying
two observers flew over n = 45 flocks, and each obserer made an
independent estimate of the number of  birds in each flock.  Also,
a photograph of the flock was taken so that an exact count of the number of
birds in teh flock could be made.  The resulting data are given in the
dataset "snowgees.dat".  (Cooka nd Jacobsen, 1978).

The variables are:
  photo --  count based on photo
  obs1  --  observer 1's count
  obs2  --  observer 2's count