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Math Help - Calculate Intercept for Multiple Regression

  1. #1
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    Calculate Intercept for Multiple Regression

    I have been using a simple Logistic Regression method with Log odds and it works very well for me

    This is an example
    X yes no total Prob Odds Log Odds
    0 4 1 5 0.8 4 1.39
    1 5 2 7 0.71 2.5 0.92
    2 3 2 5 0.6 1.5 0.41
    3 1 2 3 0.33 0.5 -0.69

    y1weights x1weights 1slope intercept
    0.66 1.3 -0.65 1.50

    The intercept is calculated as y1weightsts1-x1weights*1slope
    the prediction follows the normal linear regression
    intercept+1slope*x


    I have had a idea (maybe slighty crazy) and perform some multiple regression

    here is the 2nd set
    X yes no total Prob Odds Log Odds
    0 2 6 8 0.25 0.33 -1.10
    1 3 9 12 0.25 0.33 -1.10
    2 2 11 13 0.15 0.18 -1.70
    3 1 15 16 0.06 0.07 -2.71

    y2weights x2weights 2slope
    -1.784954575 1.755102041 -0.595625389

    If this was normal multiple regression it would fall like this

    intercept+1slope*x1+2slope*x2

    But how do I calculate the intercept so it takes in the 2nd set that I am looking at
    I had thought
    (y1weights-x1weights*1slope)-(y2weights-x2weights*2slope)

    But I am not sure of if this is right?
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  2. #2
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    Re: Calculate Intercept for Multiple Regression

    Hey aivoryuk.

    Can you explain in non-mathematical terms what you are trying to do?
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  3. #3
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    Re: Calculate Intercept for Multiple Regression

    Hi Again

    Basically I have 2 sets of data which have the same Independent variable (X) but different dependendent variables (Y)

    I would like to combine the 2 sets to create one prediction model

    I have created seperate slopes and intercepts for each of the sets but I need to know how to calculate an overall intercept so I can use the normal regression formula
    intercept+1slope*x1+2slope*x2
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  4. #4
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    Re: Calculate Intercept for Multiple Regression

    That doesn't sound right: you have two different models for two sets of data: why don't you just merge all dependent variables to the same data set and do the regression with that data?
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  5. #5
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    Re: Calculate Intercept for Multiple Regression

    Quote Originally Posted by chiro View Post
    That doesn't sound right: you have two different models for two sets of data: why don't you just merge all dependent variables to the same data set and do the regression with that data?
    Well thats why I wsa asking, I will have another think about this and see if I can merge the dependent variables. Do you ahve any advice on the best way to do that?
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  6. #6
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    Re: Calculate Intercept for Multiple Regression

    Just merge the data and do a regression, and if they are disjoint sets of data, then consider either setting the values to zero, using some averaging technique, or using existing regression models to create fitted data for the other data set and merge the two data sets together and compare the results.

    But I should warn you that if both sets of data don't represent the same model, then the output you will get will largely be garbage and be useless (regression model that is).
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