Thread: Calculating predicted probabilities from a logistic regression model SPSS

1. Calculating predicted probabilities from a logistic regression model SPSS

Hello everyone,
I am trying to calculate individual predicted probabilities from a logistic regression model with SPSS (to describe how a individual probability could be calculated from my model in an article). I was looking for a formula how the predicted probabilities in SPSS are calculated (automatically when you check "save predicted probabilities"). I've found this video https://www.youtube.com/watch?v=6jj4muvWDgs , which explains quite clearly how the probabilities are calculated in SPSS: 1st step - predicted_logit calculation; 2nd step - predicted probability calculation. This approach works for me as well if I go this way, but in other sources, i.e. here http://www.pmean.com/13/predicted.html the probabilities are calculated differently and yield different individual probabilities.

Therefore my question: is the approach described in the first video correct, and if it is, is it possible to combine the two steps into one relatively simple equation to use in an article?

2. Re: Calculating predicted probabilities from a logistic regression model SPSS

hi,
without looking at the links, the equation you look for is:

ln(p/(1-p))=a0+a1*x

Expressing p is then just a bit of manipulation.

regards

3. Re: Calculating predicted probabilities from a logistic regression model SPSS

Thanks! p stands for probability, a0 for the coefficient on the constant term, a1 for the coefficient(s) on the independent variable(s) and x for the independent variable(s)?

4. Re: Calculating predicted probabilities from a logistic regression model SPSS

hi,
Yes. You can see the full equation in the output, in case of more variables, interactions etc... it is the same idea

regards

5. Re: Calculating predicted probabilities from a logistic regression model SPSS

Hello,
I happened to write the formula to get the predicted probability from a logistic regression model in an article of mine.
Feel free to give a look at this link:
http://journals.plos.org/plosone/art...l.pone.0091510

Hope this helps
Gm

6. Re: Calculating predicted probabilities from a logistic regression model SPSS

Originally Posted by gianmarco
Hello,
I happened to write the formula to get the predicted probability from a logistic regression model in an article of mine.
Feel free to give a look at this link:
http://journals.plos.org/plosone/art...l.pone.0091510

Hope this helps
Gm
Thank you! Could you pinpoint which formula in your article you have in mind?

7. Re: Calculating predicted probabilities from a logistic regression model SPSS

Hello,
the formula is right after the first paragraph in the "Materials and Methods" section.
I have found this YT video that can also help you in calculating the predicted probability:

cheers
gm

8. The Following User Says Thank You to gianmarco For This Useful Post:

kranas (11-02-2016)

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