Ellen Hertz wrote:
>
> Mike,
> Yes, you are correct. A purist might say that you didn't actually prove
> that the slopes are the same, only that you failed to demonstrate a
> significant difference between them (because non-significant parameters can
> become significant with more data). However, your interpretation is correct
> and, also, including an interaction term to examine its statistical
> significance is the best approach.
>
Careful!
I think you have to take the purist's view - with most data sets I could
get a non-significant interaction even if the slopes are different, just
by removing some of the data. If the data it plentiful, then the
interpretation may be reasonable (even if still not strivtly correct).
The interpretation you're advocating is logically dodgy - your
conclusion could depend as much on the number of data points you have as
on the difference between the slopes.
If you want to argue that two slopes are the same, then it's better to
look at the confidence limits, and see if they only cover a range that
is practically insignificant, then you can say that any difference is
too small to worry about.
Bob
--
Bob O'Hara
Metapopulation Research Group
Division of Population Biology
Department of Ecology and Systematics
PO Box 17 (Arkadiankatu 7)
FIN-00014 University of Helsinki
Finland
tel: +358 9 191 28782 fax: +358 9 191 28701
email: [EMAIL PROTECTED]
To induce catatonia, visit:
http://www.helsinki.fi/science/metapop/
It is being said of a certain poet, that though he tortures the English
language, he has still never yet succeeded in forcing it to reveal his
meaning
- Beachcomber
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