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Re: Statistical problem


jeff houlahan <[log in to unmask]>


Academic forum on fisheries ecology and related topics <[log in to unmask]>


Wed, 18 Sep 1996 08:01:42 -0500





text/plain (1 lines)

Thomas, keeping in mind that I am no statistician, I think you have the
right idea. An ANCOVA is what you want to use. I use the SYSTAT stats
package and have had to compare slopes and intercepts of length weight
regressions. Using this software you set your data up in 3 columns,
length, weight and and if your comparing regressions between years a
categorical variable for the year.

L W Year

8 9 1
8 5 1
6 5 1
4 3 1
8 7 2
3 6 2
5 5 2
7 9 2

You then select the general linear model from your pulldown options. Put
weight in the dependent variable box, length in the independent variable
box, and year in the categorical variable box. There is also a box to
select interactions which you must do in this case, so select the
interaction between length and year. Your output will give you the
significance of the overall length-weight relationship, followed by a
p-value for the liklihood that the intercepts are different, followed by
the liklihood that the slopes are different. This is all on SYSTAT so I'm
not sure how that translates th Statistika. Hope this helps and good luck.

Jeff Houlahan
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