This example, and comments so far received via the list, appear to show the need for data analysis to be grounded in an understanding of the application, a point made years ago in a paper that has greatly influenced my own thinking:
WG Hunter "Six statistical tales". The Statistician. June 1981.
Other people have pointed to the *mathematical* desirability of retaining the intercept in the estimation procedure.
My question is whether x=0 y=0 is a valid or useful assumption. Where does the pesticide come from? If it is being leached, then zero flow would imply zero pesticide movement at that point, but as the flow decreases, perhaps the transport time increases? Does the transport mechanism break down at flows above zero - maybe because the pesticide decomposes, chelates or whatever? At the other end, if flow *increases*, presumably the pesticide gets more and more dilute, ie y decreases. My uninformed view is therefore that the relationship might be some quadratic, or might be linear *over the x range of interest*, but with negative coefficient and almost certainly not consistent with x=0 y=0.
As always, plot the graph and consider the implications (interpretation) as well as the sums.
Allan
-----Original Message-----
From: Environmental Statistics [mailto:[log in to unmask]]On Behalf Of Sion Roberts
Sent: 17 March 2006 18:38
To: [log in to unmask]
Subject: [ENVSTAT] Regression through the origin
Dear List,
I have a question regarding interpretations of equivalent R2 for regression
through the origin.
I have two variables, river discharge (x) and pesticide concentration (y), for
which I wish to explore the relationship between concentration and discharge,
with a view to regressing y on x. It is my assumption that any regression
should be forced through the origin on the basis that zero discharge (m3/s)
corresponds to zero concentration.
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