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 Bill wrote:

> ... my preference is to plot the observations against
> time and use the actual Hc line from the beginning and end to establish a
> slope to fit the points to and identify outliers.

Where does this slope comes from? It sounds as though you are taking
the first and last observations and establishing a line between them?
If so, why would these sights have any authority to be used to compare
others with? Why would this slope have any relationship with the
actual slope, apart from when the sights are without fault, the nature
of the thing we are trying to find?

Or do you mean that you establish the body's actual apparent rise or
fall and use this slope to compare the pattern of sights with - this
is the method I understand and use.

> Far better than linear regression IMHO.

Using the actual slope should be more useful than using the pattern of
sights to create a line of best fit to suit that data. With few sights
even one outlier can significantly skew the results of linear
regression. Using the actual slope enables such an outlier to be
identified and discarded.

> On the other hand, when I had horrible horizons on Lake Michigan but guessed
> at the hidden horizon line, then averaged and posted the results (very close
> to Hc) you quipped it was luck, one could not trust results with a sigma
> that large. 

Such a 'horrible horizon' could lead to a systematic error. This would
not be resolved via methods such as comparison of slope with sights,
but could be amenable to correction via methods to resolve systematic
error.

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