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# How to do slope comparison in SPSS?

Wed 23 Jan, 2019 07:33 am
I have data from density rearing. Organism was reared in several different densities and the number of surviving individuals was counted once a week for four weeks.

My aim is to spesify what would be suitable rearing density. I was asked to do a slope comparison; the highest density for which the slope does not differ from the flat line will be considered as the threshold density.

The question is: What test should I use and how to do this in SPSS?

If I have understood right, I should do some sort of univariate test. But how to organize the data? In the experiment there was several replicates and blocks. Should I first count mean of alive individuals for each density at certain measurement time? To what do I compare each slope in the test if it even is the correct test?
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engineer

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Wed 23 Jan, 2019 08:59 am
@Dumbstudent,
Not familiar with your particular stats package, but this link explains how to test the slope of a least squares fit lit for significance. This number is almost certainly generated by your stats package. The one I use shows the t value and the probability from that anytime you pull a least squares fit. The value of the t statistic that you find meaningful is up to you, but if you fit a line through four points, you have two degrees of freedom and
want a 95% confidence level, you need a t value of 4.3 or higher.
Quote:
Test statistic. The test statistic is a t statistic (t) defined by the following equation.
t = b1 / SE

where b1 is the slope of the sample regression line, and SE is the standard error of the slope.
P-value. The P-value is the probability of observing a sample statistic as extreme as the test statistic. Since the test statistic is a t statistic, use the t Distribution Calculator to assess the probability associated with the test statistic. Use the degrees of freedom computed above.
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