Dear Masteras, thank you for the advice! I am still a beginner in Statistics, I don't understand well what bootstrapping means. I will study about it.

I use SPSS 17. I ran Analyze>Regression>Ordinal, combining 2 and 3 (putting the only 3 observation as 2). Would this be the right way to do it? I am sorry if it's a very dumb problem, but I get an output that I don't know how to interpret and to draw the line in the graph, since it doesn't show me the coefficients of the formula to predict the logit transformation of the probabilities of having an alien plant in the different levels of biomass.

The output for logistic regression gave me the Z and p values and the a and b values for the equation, and I could draw the graph that you saw previously. Why doesn't the ordinal regression give me these values?

In attachment I send the output that I get from SPSS 17 by running the logistic regression and the ordinal regression. So you can know what I am talking about.

Your help is very precious, thank you from deep heart.