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Setting/fixing/specifying Y-intercept in Proc GLM

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Setting/fixing/specifying Y-intercept in Proc GLM

Hi all

I was wondering how/if it was possible to specify the Y-intercept in Proc GLM?

I have been looking everywhere for a solution.

I need to set the intercept at 100.

I have a very simple model:

PROC GLM data; Class line; Model BW1= Log_CFU_Total*LINE / solution; run;

Any ideas/tips would be very much appreciated as I am totally stuck!

Thanks!

i


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‎10-26-2012 08:34 AM
Respected Advisor
Posts: 2,655

Re: Setting/fixing/specifying Y-intercept in Proc GLM

If you run the GLM model I suggested, get the slopes out and plug them into an equation that looks like %BW = 100 - log_cfu_total * slope estimate for LINE (based on the NOINT model) what do you get?  All of these lines should converge on 100.  They might not look like the best fit to the data, so plotting the residuals of the constructed equation would be very informative.

Since there is a log transform of the independent variable made me think of PROC NLIN as a valid alternative.  Again you would need to code the LINE variable (0,1 type coding), but then a model statement that looked like:

model BW1=100 - (beta_1*Log_CFU_Total*LINE_1 + beta_2*Log_CFU_Total*LINE_2 + ... + beta_N*Log_CFU*LINE_N);

should give you what you need (I hope).  You could get this back on the original (non-log transformed scale) if you wanted, but I would really worry about heterogeneous variances then.  In any case, NLIN doesn't require a non-linear function, and it is a good place to fit "unusual" looking equations.

Steve Denham

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Respected Advisor
Posts: 2,655

Re: Setting/fixing/specifying Y-intercept in Proc GLM

What would happen if you subtract 100 from every value of BW_slaughter to get the variable BW_slaughter_minus_100 and fit:

PROC GLM data; Class line; Model BW_slaughter_minus_100 = Log_CFU_Total*LINE / solution noint; run;

This is untested, but it's about the only way I see to stay in PROC GLM.

If you can code up LINE as a set of dummy variables, then PROC REG has a RESTRICT statement, where you can set intercept=100.  I get comparable results from both methods, but you should check to see which best fits your needs.

Steve Denham


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Posts: 3

Re: Setting/fixing/specifying Y-intercept in Proc GLM

Hi Steve

Thanks for your prompt response!

I have tried both methods, which sadly do not work with my data.

To explain, my BW variable is %Body weight. So, this must start at 100%, and decrease as CFU increases.

Despite trying these methods, the lines still do not cross the Y-axis at the same point.

Is there anything else you could recommend?

Thanks again!

Solution
‎10-26-2012 08:34 AM
Respected Advisor
Posts: 2,655

Re: Setting/fixing/specifying Y-intercept in Proc GLM

If you run the GLM model I suggested, get the slopes out and plug them into an equation that looks like %BW = 100 - log_cfu_total * slope estimate for LINE (based on the NOINT model) what do you get?  All of these lines should converge on 100.  They might not look like the best fit to the data, so plotting the residuals of the constructed equation would be very informative.

Since there is a log transform of the independent variable made me think of PROC NLIN as a valid alternative.  Again you would need to code the LINE variable (0,1 type coding), but then a model statement that looked like:

model BW1=100 - (beta_1*Log_CFU_Total*LINE_1 + beta_2*Log_CFU_Total*LINE_2 + ... + beta_N*Log_CFU*LINE_N);

should give you what you need (I hope).  You could get this back on the original (non-log transformed scale) if you wanted, but I would really worry about heterogeneous variances then.  In any case, NLIN doesn't require a non-linear function, and it is a good place to fit "unusual" looking equations.

Steve Denham

New Contributor
Posts: 3

Re: Setting/fixing/specifying Y-intercept in Proc GLM

Got it!! After a little bit of axes altering!

Thank you very much for your advice!

Graham

Regular Learner
Posts: 1

Re: Setting/fixing/specifying Y-intercept in Proc GLM

I am running with the PROC GLM model, I need to force the global intercept (constant value) (only vary the independent variables) into the model with the by variables. Does anyone know the SAS code to perform this operation?
Respected Advisor
Posts: 2,655

Re: Setting/fixing/specifying Y-intercept in Proc GLM

If you have a known fixed value for the intercept, you can subtract that number from every Y value, and then fit a model with the NOINT option.  

 

Steve Denhm

Respected Advisor
Posts: 2,655

Re: Setting/fixing/specifying Y-intercept in Proc GLM

Also, appending a question to a thread that has been marked "Answered" will often result in people by-passing your question.

 

Steve Denham

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