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Multiple regression with dummy variables

Tohar

How can I select intercept in multiple regression?

I want two lines to cross the Y axis in the (0,100) point.

Thanks

Tohar

Accepted answers

All comments

Mark_Bailey

I assume that you selected Analyze > Fit Model to start the multiple regression. A check box is at the center of the bottom of the launch dialog. Select it to remove the intercept from the model

hogi

This post describes how to use the Fit Y-by-X/Bivariate platform with by() .... to collect the slopes and intercepts:

Creating-columns-from-Linear-Fit-Constants-slope-intercept

Dan_Obermiller

Perhaps more details are needed as I am the third to respond and I have a different take on your question. You said you wanted the lines to go through (0, 100). That is not the same as having the y-intercept of zero. You could use the nonlinear platform to build a model with a fixed y-intercept of 100 and the rest of the linear model to estimate the parameters with that restriction.

Provide a bit more clarity of your situation so that we can better assist you.

hogi

Oh, thanks, I miss-read this sentence.
In the Fit Y - by - X / Bivariate Platform there is also an option to set the intercept to a fixed value:
Constrain intercept to

Tohar

I'm trying to do a multivariable linear regression. A continuous variable explains -X and a continuous variable is explained -Y and another dummy factor is categorical 0 or 1. I know that the lines should cut the Y-axis at 100 and I want to define this in the model.

Similar to the solution in the Fit Y by X platform that "hogi" suggested but I have two lines- two groups.

Thank you very much

MRB3855

@Tohar , so it sounds like you have an ANCOVA model (one categorical factor C, and one continuous X) and you want to regress Y on these. Define Y100 = Y-100. then use Y100 as your response. then the only effect in your model will be C*X, with no intercept. See pic below. Also, in the red triangle pull down menu deselect "Center Polynomials".

Run this and get the following: And remember Y100 is Y-100. So, when Y100=0, Y=100. You can then get creative with Save Prediction Formula/Graph Builder/etc to get the Y axis back to the raw scale (shift it back up by 100).

Tohar

Thank you very much

I am not about the result I got; the lines cross the Y axis in (0,0) - see attached.

My goal is to check if there is a difference in rate (ie slope) between the two treatment groups. It is not necessary to check whether there is a difference in the intercept, I want to set it both in the model and in the presentation graph at (0,100).

Many thanks!!!!!

Tohar

Multiple Regression with Dummy Variables.pdf

Mark_Bailey

Did you try the Group By command in the platform menu (red triangle) before using Fit Special and specifying a Y-intercept = 100?