# Calculating predicted probabilities from a logistic regression model SPSS

#### kranas

##### New Member
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?

#### rogojel

##### TS Contributor
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

#### kranas

##### New Member
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)?

#### rogojel

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

regards

#### gianmarco

##### TS Contributor
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: