# proportional weights Classic List Threaded 10 messages Open this post in threaded view
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## proportional weights

 Hello all, can help clarify something? According to R's lm() doc: > Non-NULL weights can be used to indicate that different observations > have different variances (with the values in weights being inversely > *proportional* to the variances); or equivalently, when the elements > of weights are positive integers w_i, that each response y_i is the > mean of w_i unit-weight observations (including the case that there > are w_i observations equal to y_i and the data have been summarized). Since the idea here is *proportion*, not equality, shouldn't the vectors of weights x, 2*x give the same result? And yet they don't, standard errors differs: > > summary(lm(c(1,2,3,1,2,3)~c(1,2.1,2.9,1.1,2,3),weight=rep(1,6)))$sigma >  0.07108323 > > summary(lm(c(1,2,3,1,2,3)~c(1,2.1,2.9,1.1,2,3),weight=rep(2,6)))$sigma >  0.1005269 So what if I know a-priori, observation A has variance 2 times bigger than observation B? Both weights=c(1,2) and weights=c(2,4) (and so on) represent very well this knowledge, but we get different regression (since sigma is different). Also, if we do the same thing with a glm() model, than we get a lot of other differences like in the deviance. ______________________________________________ [hidden email] mailing list https://stat.ethz.ch/mailman/listinfo/r-helpPLEASE do read the posting guide http://www.R-project.org/posting-guide.htmland provide commented, minimal, self-contained, reproducible code.
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## Re: proportional weights

 On 05/02/14 22:40, Marco Inacio wrote: > Hello all, can help clarify something? > > According to R's lm() doc: > >> Non-NULL weights can be used to indicate that different observations >> have different variances (with the values in weights being inversely >> *proportional* to the variances); or equivalently, when the elements >> of weights are positive integers w_i, that each response y_i is the >> mean of w_i unit-weight observations (including the case that there >> are w_i observations equal to y_i and the data have been summarized). > > Since the idea here is *proportion*, not equality, shouldn't the vectors > of weights x, 2*x give the same result? And yet they don't, standard > errors differs: > >>> summary(lm(c(1,2,3,1,2,3)~c(1,2.1,2.9,1.1,2,3),weight=rep(1,6)))$sigma >>  0.07108323 >>> summary(lm(c(1,2,3,1,2,3)~c(1,2.1,2.9,1.1,2,3),weight=rep(2,6)))$sigma >>  0.1005269 The weights are in fact case weights, i.e., a weight of 2 is the same as including the corresponding item twice. I agree that the documentation is no wonder of clarity in this respect. Btw, note that, in your example, (0.1005269 / 0.07108323)^2 = 2, your constant weight. Göran Broström > > > So what if I know a-priori, observation A has variance 2 times bigger > than observation B? Both weights=c(1,2) and weights=c(2,4) (and so on) > represent very well this knowledge, but we get different regression > (since sigma is different). > > > Also, if we do the same thing with a glm() model, than we get a lot of > other differences like in the deviance. > > ______________________________________________ > [hidden email] mailing list > https://stat.ethz.ch/mailman/listinfo/r-help> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html> and provide commented, minimal, self-contained, reproducible code. > ______________________________________________ [hidden email] mailing list https://stat.ethz.ch/mailman/listinfo/r-helpPLEASE do read the posting guide http://www.R-project.org/posting-guide.htmland provide commented, minimal, self-contained, reproducible code.
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## Re: proportional weights

 Dear Marco and Goran, Perhaps the documentation could be clearer, but it is after all a brief help page. Using weights of 2 to lm() is *not* equivalent to entering the observation twice. The weights are variance weights, not case weights. You can see this by looking at the whole summary() output for the models, not just the residual standard errors: ------------- snip --------- > summary(lm(c(1,2,3,1,2,3)~c(1,2.1,2.9,1.1,2,3),weight=rep(1,6))) Call: lm(formula = c(1, 2, 3, 1, 2, 3) ~ c(1, 2.1, 2.9, 1.1, 2, 3),     weights = rep(1, 6)) Residuals:        1        2        3        4        5        6  0.06477 -0.08728  0.07487 -0.03996  0.01746 -0.02986 Coefficients:                           Estimate Std. Error t value Pr(>|t|)     (Intercept)               -0.11208    0.08066   -1.39    0.237     c(1, 2.1, 2.9, 1.1, 2, 3)  1.04731    0.03732   28.07 9.59e-06 *** --- Signif. codes:  0 *** 0.001 ** 0.01 * 0.05 . 0.1   1 Residual standard error: 0.07108 on 4 degrees of freedom Multiple R-squared:  0.9949, Adjusted R-squared:  0.9937 F-statistic: 787.6 on 1 and 4 DF,  p-value: 9.59e-06 > summary(lm(c(1,2,3,1,2,3)~c(1,2.1,2.9,1.1,2,3),weight=rep(2,6))) Call: lm(formula = c(1, 2, 3, 1, 2, 3) ~ c(1, 2.1, 2.9, 1.1, 2, 3),     weights = rep(2, 6)) Residuals:        1        2        3        4        5        6  0.09160 -0.12343  0.10589 -0.05652  0.02469 -0.04223 Coefficients:                           Estimate Std. Error t value Pr(>|t|)     (Intercept)               -0.11208    0.08066   -1.39    0.237     c(1, 2.1, 2.9, 1.1, 2, 3)  1.04731    0.03732   28.07 9.59e-06 *** --- Signif. codes:  0 *** 0.001 ** 0.01 * 0.05 . 0.1   1 Residual standard error: 0.1005 on 4 degrees of freedom Multiple R-squared:  0.9949, Adjusted R-squared:  0.9937 F-statistic: 787.6 on 1 and 4 DF,  p-value: 9.59e-06 ------------- snip ------------- Notice that while the residual standard errors differ, the coefficients and their standard errors are identical. There are compensating changes in the residual variance and the weighted sum of squares and products matrix for X. In contrast, literally entering each observation twice reduces the coefficient standard errors by a factor of sqrt((6 - 2)/(12 - 2)), i.e., the square root of the relative residual df of the models: ------------- snip -------- > summary(lm(rep(c(1,2,3,1,2,3),2)~rep(c(1,2.1,2.9,1.1,2,3),2))) Call: lm(formula = rep(c(1, 2, 3, 1, 2, 3), 2) ~ rep(c(1, 2.1, 2.9,     1.1, 2, 3), 2)) Residuals:       Min        1Q    Median        3Q       Max -0.087276 -0.039963 -0.006201  0.064768  0.074874 Coefficients:                                   Estimate Std. Error t value Pr(>|t|) (Intercept)                       -0.11208    0.05101  -2.197   0.0527 rep(c(1, 2.1, 2.9, 1.1, 2, 3), 2)  1.04731    0.02360  44.374 8.12e-13                                       (Intercept)                       .   rep(c(1, 2.1, 2.9, 1.1, 2, 3), 2) *** --- Signif. codes:  0 *** 0.001 ** 0.01 * 0.05 . 0.1   1 Residual standard error: 0.06358 on 10 degrees of freedom Multiple R-squared:  0.9949, Adjusted R-squared:  0.9944 F-statistic:  1969 on 1 and 10 DF,  p-value: 8.122e-13 ---------- snip ------------- I hope this helps, John ------------------------------------------------ John Fox, Professor McMaster University Hamilton, Ontario, Canada http://socserv.mcmaster.ca/jfox/                On Thu, 6 Feb 2014 09:27:22 +0100  Göran Broström <[hidden email]> wrote: > On 05/02/14 22:40, Marco Inacio wrote: > > Hello all, can help clarify something? > > > > According to R's lm() doc: > > > >> Non-NULL weights can be used to indicate that different observations > >> have different variances (with the values in weights being inversely > >> *proportional* to the variances); or equivalently, when the elements > >> of weights are positive integers w_i, that each response y_i is the > >> mean of w_i unit-weight observations (including the case that there > >> are w_i observations equal to y_i and the data have been summarized). > > > > Since the idea here is *proportion*, not equality, shouldn't the vectors > > of weights x, 2*x give the same result? And yet they don't, standard > > errors differs: > > > >>> summary(lm(c(1,2,3,1,2,3)~c(1,2.1,2.9,1.1,2,3),weight=rep(1,6)))$sigma > >>  0.07108323 > >>> summary(lm(c(1,2,3,1,2,3)~c(1,2.1,2.9,1.1,2,3),weight=rep(2,6)))$sigma > >>  0.1005269 > > The weights are in fact case weights, i.e., a weight of 2 is the same as including the corresponding item twice. I agree that the documentation is no wonder of clarity in this respect. > > Btw, note that, in your example, (0.1005269 / 0.07108323)^2 = 2, your constant weight. > > Göran Broström > > > > > > So what if I know a-priori, observation A has variance 2 times bigger > > than observation B? Both weights=c(1,2) and weights=c(2,4) (and so on) > > represent very well this knowledge, but we get different regression > > (since sigma is different). > > > > > > Also, if we do the same thing with a glm() model, than we get a lot of > > other differences like in the deviance. > > > > ______________________________________________ > > [hidden email] mailing list > > https://stat.ethz.ch/mailman/listinfo/r-help> > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html> > and provide commented, minimal, self-contained, reproducible code. > > > > ______________________________________________ > [hidden email] mailing list > https://stat.ethz.ch/mailman/listinfo/r-help> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html> and provide commented, minimal, self-contained, reproducible code. ______________________________________________ [hidden email] mailing list https://stat.ethz.ch/mailman/listinfo/r-helpPLEASE do read the posting guide http://www.R-project.org/posting-guide.htmland provide commented, minimal, self-contained, reproducible code.
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## Re: proportional weights

 Dear John, thanks for the clarification! The lesson to be learned is that one should be aware of the fact that weights may mean different things in different functions, and sometimes different things in the same function (glm)! Göran On 02/06/2014 02:17 PM, John Fox wrote: > Dear Marco and Goran, > > Perhaps the documentation could be clearer, but it is after all a > brief help page. Using weights of 2 to lm() is *not* equivalent to > entering the observation twice. The weights are variance weights, not > case weights. > > You can see this by looking at the whole summary() output for the > models, not just the residual standard errors: > > ------------- snip --------- > >> summary(lm(c(1,2,3,1,2,3)~c(1,2.1,2.9,1.1,2,3),weight=rep(1,6))) > > Call: lm(formula = c(1, 2, 3, 1, 2, 3) ~ c(1, 2.1, 2.9, 1.1, 2, 3), > weights = rep(1, 6)) > > Residuals: 1        2        3        4        5        6 0.06477 > -0.08728  0.07487 -0.03996  0.01746 -0.02986 > > Coefficients: Estimate Std. Error t value Pr(>|t|) (Intercept) > -0.11208    0.08066   -1.39    0.237 c(1, 2.1, 2.9, 1.1, 2, 3) > 1.04731    0.03732   28.07 9.59e-06 *** --- Signif. codes:  0 ‘***’ > 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1 > > Residual standard error: 0.07108 on 4 degrees of freedom Multiple > R-squared:  0.9949, Adjusted R-squared:  0.9937 F-statistic: 787.6 on > 1 and 4 DF,  p-value: 9.59e-06 > >> summary(lm(c(1,2,3,1,2,3)~c(1,2.1,2.9,1.1,2,3),weight=rep(2,6))) > > Call: lm(formula = c(1, 2, 3, 1, 2, 3) ~ c(1, 2.1, 2.9, 1.1, 2, 3), > weights = rep(2, 6)) > > Residuals: 1        2        3        4        5        6 0.09160 > -0.12343  0.10589 -0.05652  0.02469 -0.04223 > > Coefficients: Estimate Std. Error t value Pr(>|t|) (Intercept) > -0.11208    0.08066   -1.39    0.237 c(1, 2.1, 2.9, 1.1, 2, 3) > 1.04731    0.03732   28.07 9.59e-06 *** --- Signif. codes:  0 ‘***’ > 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1 > > Residual standard error: 0.1005 on 4 degrees of freedom Multiple > R-squared:  0.9949, Adjusted R-squared:  0.9937 F-statistic: 787.6 on > 1 and 4 DF,  p-value: 9.59e-06 > > ------------- snip ------------- > > Notice that while the residual standard errors differ, the > coefficients and their standard errors are identical. There are > compensating changes in the residual variance and the weighted sum of > squares and products matrix for X. > > In contrast, literally entering each observation twice reduces the > coefficient standard errors by a factor of sqrt((6 - 2)/(12 - 2)), > i.e., the square root of the relative residual df of the models: > > ------------- snip -------- > >> summary(lm(rep(c(1,2,3,1,2,3),2)~rep(c(1,2.1,2.9,1.1,2,3),2))) > > Call: lm(formula = rep(c(1, 2, 3, 1, 2, 3), 2) ~ rep(c(1, 2.1, 2.9, > 1.1, 2, 3), 2)) > > Residuals: Min        1Q    Median        3Q       Max -0.087276 > -0.039963 -0.006201  0.064768  0.074874 > > Coefficients: Estimate Std. Error t value Pr(>|t|) (Intercept) > -0.11208    0.05101  -2.197   0.0527 rep(c(1, 2.1, 2.9, 1.1, 2, 3), > 2)  1.04731    0.02360  44.374 8.12e-13 > > (Intercept)                       . rep(c(1, 2.1, 2.9, 1.1, 2, 3), 2) > *** --- Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ > 1 > > Residual standard error: 0.06358 on 10 degrees of freedom Multiple > R-squared:  0.9949, Adjusted R-squared:  0.9944 F-statistic:  1969 on > 1 and 10 DF,  p-value: 8.122e-13 > > ---------- snip ------------- > > I hope this helps, > > John > > ------------------------------------------------ John Fox, Professor > McMaster University Hamilton, Ontario, Canada > http://socserv.mcmaster.ca/jfox/   On Thu, 6 Feb 2014 09:27:22 +0100 > Göran Broström <[hidden email]> wrote: >> On 05/02/14 22:40, Marco Inacio wrote: >>> Hello all, can help clarify something? >>> >>> According to R's lm() doc: >>> >>>> Non-NULL weights can be used to indicate that different >>>> observations have different variances (with the values in >>>> weights being inversely *proportional* to the variances); or >>>> equivalently, when the elements of weights are positive >>>> integers w_i, that each response y_i is the mean of w_i >>>> unit-weight observations (including the case that there are w_i >>>> observations equal to y_i and the data have been summarized). >>> >>> Since the idea here is *proportion*, not equality, shouldn't the >>> vectors of weights x, 2*x give the same result? And yet they >>> don't, standard errors differs: >>> >>>>> summary(lm(c(1,2,3,1,2,3)~c(1,2.1,2.9,1.1,2,3),weight=rep(1,6)))$sigma >>>> >>>>>  0.07108323 >>>>> summary(lm(c(1,2,3,1,2,3)~c(1,2.1,2.9,1.1,2,3),weight=rep(2,6)))$sigma >>>> >>>>>  0.1005269 >> >> The weights are in fact case weights, i.e., a weight of 2 is the >> same as including the corresponding item twice. I agree that the >> documentation is no wonder of clarity in this respect. >> >> Btw, note that, in your example, (0.1005269 / 0.07108323)^2 = 2, >> your constant weight. >> >> Göran Broström >>> >>> >>> So what if I know a-priori, observation A has variance 2 times >>> bigger than observation B? Both weights=c(1,2) and weights=c(2,4) >>> (and so on) represent very well this knowledge, but we get >>> different regression (since sigma is different). >>> >>> >>> Also, if we do the same thing with a glm() model, than we get a >>> lot of other differences like in the deviance. >>> >>> ______________________________________________ >>> [hidden email] mailing list >>> https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the >>> posting guide http://www.R-project.org/posting-guide.html and >>> provide commented, minimal, self-contained, reproducible code. >>> >> >> ______________________________________________ [hidden email] >> mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do >> read the posting guide http://www.R-project.org/posting-guide.html>> and provide commented, minimal, self-contained, reproducible code. > > > ______________________________________________ [hidden email] mailing list https://stat.ethz.ch/mailman/listinfo/r-helpPLEASE do read the posting guide http://www.R-project.org/posting-guide.htmland provide commented, minimal, self-contained, reproducible code.
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## Re: proportional weights

 In reply to this post by Fox, John Thanks for the answers. > Dear Marco and Goran, > > Perhaps the documentation could be clearer, but it is after all a brief help page. Using weights of 2 to lm() is *not* equivalent to entering the observation twice. The weights are variance weights, not case weights. > According to your post here:    http://tolstoy.newcastle.edu.au/R/e2/help/07/05/16311.html   there are 3 possible kinds of weights. The person in this one:    http://tolstoy.newcastle.edu.au/R/e2/help/07/06/18743.html   includes 2 others making a distinction between weights inverse proportional to variance and weight equal to inverse variance. (looking at other posts in the thread shows that other people also make confusions on this matter) So R's lm(), glm(), etc weights **are** the inverse of the variance of the observations, right? They'are not **proportional** to the inverse of variance because if this were true, then weight and 2*weight would archive the same results, right? I needed a method to use proportional weights on observations as I know their proportion of variance among each other. And it doesn't need to be a R function, just an explanation on how construct the likehood would be fine. If anybody know an article on the subject, would be of great help to. ______________________________________________ [hidden email] mailing list https://stat.ethz.ch/mailman/listinfo/r-helpPLEASE do read the posting guide http://www.R-project.org/posting-guide.htmland provide commented, minimal, self-contained, reproducible code.
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## Re: proportional weights

 Dear Marco, What I said in the 2007 r-help posting to which you refer is, "The weights used by lm() are (inverse-)'variance weights,' reflecting the variances of the errors, with observations that have low-variance errors therefore being accorded greater weight in the resulting WLS regression." ?lm says, "Non-NULL weights can be used to indicate that different observations have different variances (with the values in weights being inversely proportional to the variances)." If I understand your situation correctly, you know the error variances up to a constant of proportionality, in which case you can set the weights argument to lm() to the inverses of these values. As I showed you in the example I just posted, weight and 2*weight *do* produce the same coefficient estimates and standard errors, with the difference between the two absorbed by the residual standard error, which is the square-root of the estimated constant of proportionality. If this is insufficiently clear, I'm afraid that I'll have to defer to someone with greater powers of explanation. Best,  John > -----Original Message----- > From: [hidden email] [mailto:r-help-bounces@r- > project.org] On Behalf Of Marco Inacio > Sent: Thursday, February 06, 2014 9:06 AM > To: [hidden email] > Subject: Re: [R] proportional weights > > Thanks for the answers. > > > Dear Marco and Goran, > > > > Perhaps the documentation could be clearer, but it is after all a > brief help page. Using weights of 2 to lm() is *not* equivalent to > entering the observation twice. The weights are variance weights, not > case weights. > > > According to your post here: >    http://tolstoy.newcastle.edu.au/R/e2/help/07/05/16311.html>    there are 3 possible kinds of weights. > > The person in this one: >    http://tolstoy.newcastle.edu.au/R/e2/help/07/06/18743.html>    includes 2 others making a distinction between weights inverse > proportional to variance and weight equal to inverse variance. > > (looking at other posts in the thread shows that other people also make > confusions on this matter) > > So R's lm(), glm(), etc weights **are** the inverse of the variance of > the observations, right? > They'are not **proportional** to the inverse of variance because if > this > were true, then weight and 2*weight would archive the same results, > right? > > > I needed a method to use proportional weights on observations as I know > their proportion of variance among each other. > And it doesn't need to be a R function, just an explanation on how > construct the likehood would be fine. If anybody know an article on the > subject, would be of great help to. > > ______________________________________________ > [hidden email] mailing list > https://stat.ethz.ch/mailman/listinfo/r-help> PLEASE do read the posting guide http://www.R-project.org/posting-> guide.html > and provide commented, minimal, self-contained, reproducible code. ______________________________________________ [hidden email] mailing list https://stat.ethz.ch/mailman/listinfo/r-helpPLEASE do read the posting guide http://www.R-project.org/posting-guide.htmland provide commented, minimal, self-contained, reproducible code.
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## Re: proportional weights

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## Re: proportional weights

 > I think we can blame Tim Hesterberg for the confusion: > > He writes > > " > I'll add: > * inverse-variance weights, where var(y for observation) = 1/weight   (as opposed to just being inversely proportional to the weight) * > " > > And, although I'm not a native English speaker, I think there's a spurious comma in there. The intention was clearly to have this as a  4th type of weight which is a special case of inverse-variance weights, not as an elaboration on the definition of inv.var. weights. > > I.e., it is the difference between > > Motorists who are reckless drivers... > > and > > Motorists, who are reckless drivers... > > -pd In fact, that wasn't what caused the confusion as I have understood what he meant despite the problem with the comma. But I got the idea now, R uses weighted least squares and: "Var[\epsilon | X] = \Omega" (equal, not proportion) (https://en.wikipedia.org/wiki/Generalized_least_squares) (since WLS is just a special case of GLS) "The weights should, ideally, be equal to the reciprocal of the variance of the measurement." (https://en.wikipedia.org/wiki/Linear_least_squares_(mathematics)#Weighted_linear_least_squares) I guess I need to find another strategy to use proportional weights (weights know up to a constant, as John says). So, thank you much to you all, and sorry the inconvenience I caused. ______________________________________________ [hidden email] mailing list https://stat.ethz.ch/mailman/listinfo/r-helpPLEASE do read the posting guide http://www.R-project.org/posting-guide.htmland provide commented, minimal, self-contained, reproducible code.