Since July my MSc student is having trouble with her binary logistic regression. SHe is now picking this back up so she can submit her thesis. Maybe someone here can help?

She is assessing the use of particular characters to infer sex. Her variables are ordinal (the scores for the development of said features) and her outcome binary is male or female. The analyses work well for the regression itself but we seem unable to predict the sex of an "unknown". I would expect to write a prediction equation to determine sex with a cut off of 0.5 (which is what we set in SPSS). The problem is that the prediction never works. In the Discriminant Analysis this works well but not in the Binary logistic regression - which is what her data requires her to do rather than a DF. What are we doing wrong?

So she creating the dummy variables (parameter coding in the log regr window).

I would expect the prediction equation to be = log(p/1-p) = b0 + b1*x1 + b2*x2 + b3*x3 + b3*x3+b4*x4

Her categories become dummy variables (e.g. gonial angle varies from 1-4 so she gets 4 variables this way gonial angle (1), gonial angle (2) etc)

Below is the equation she worked out this way – then she replaces the variable name with a score of 1 or 0 depending on the ordinal variable.

sex= 75.8 + (-57.296*(gonialangle1)) + (-37.333*(gonialangle2)) + (-18.504*(externaloccipitalprotuberance1)) + (-55.213*(externaloccipitalprotuberance2)) + (38.256*(externaloccipitalprotuberance3)) + (-57.519*(nuchalmarkeringen1)) + (-1.946*(nuchalmarkeringen2))

she replaces the variable name with a score of 1 or 0 depending on the ordinal variable.

Here is is worked out with an actual indiviual's scores:

SK001sex = 75.8 + (-57.296*1) + (-37.333*0) + (-18.504*1) + (-55.213*0) + (38.256*0) + (-57.519*1) + (-1.946*0)

SK001sex = 75.8 + (-57.296) + (-18.504) + (-57.519)

SK001sex = 75.8 -57.296 -18.504 -57.519

SK001sex = -57.519

We never seem to get to a value between 0 and 1. Do we need to do something with the log? Are we not doing this correctly?

Can someone please help us with this?

All the best,

Isabelle

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Isabelle De Groote

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