# Thread: Calculating values of a reference group

1. ## Calculating values of a reference group

Hi everyone,

Thanks for the great homework help!

I am supposed to calculate the estimated BMI for those at with 122 SBP, 134 SBP and 147 SBP (these are the 25th, 50th and 75th percentiles) and those who have poorly self-rated health (I have already done so for those with "excellent," based off of these regression equation and the attached output:

BMI = β0 + β1globrat + β2SBP + β3(globrat × SBP)

This is what I did for excellent, as I think the globrat is "1" for these (as it is dummy coded, I think...):

BMI = 42.315 – 21.420 -.0926(122) + .134(122) = 25.945
BMI = 42.315 – 21.420 -.0926(134) + .134(134) = 26.443
BMI = 42.315 – 21.420 -.0926(147) + .134(147) = 26.981

I am confused about what to sub in for "poor," which is the reference group--when I put in zeroes I get numbers that are way too high to be realistic. I guess I am generally confused about what numbers to use when you are calculating these sorts of values for a reference group in a logistic regression.

Any help would be much appreciated!

Thank you,

smashing

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