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From |
Phil Clayton <philclayton@internode.on.net> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
Re: st: Is it valid to use the individual ratios (i.e. Xi/Yi) in the dependent or independent part of a regression model? |

Date |
Sun, 27 May 2012 23:50:48 +1000 |

On 27/05/2012, at 9:35 PM, David Hoaglin wrote: > As an explanatory variable, ACR is one function > of urinary albumin and urinary creatinine; but you could reasonably > consider other functions, such as the linear combination of urinary > albumin and urinary creatinine that arises from using those two as > explanatory variables or the nonlinear function in which the > explanatory variables in that part of the model are urinary albumin, > urinary creatinine, and their product What we're interested in, biologically, is the 24-h urinary albumin excretion. The reason the albumin is divided by creatinine is that the ACR is used to estimate the 24-h urinary albumin excretion from a single urine specimen rather than asking someone to collect their urine for a day. The urinary concentration can vary several fold (eg if you're dehydrated it goes up) which changes the albumin concentration in that specimen - but it changes the creatinine concentration by a similar amount, and we know the normal 24-h excretion of creatinine, so we divide the albumin by the creatinine to estimate the 24-h albumin excretion. So biologically ACR should be a ratio and must be nonnegative. You could include it in a regression model as a surrogate for the 24-h urine albumin excretion, but would need to careful how to model it as it generally has a nonlinear effect. For example it is commonly modelled as a categorical variable - normal, microalbuminuria, macroalbuminuria. Phil * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

**Follow-Ups**:

**References**:**st: Is it valid to use the individual ratios (i.e. Xi/Yi) in the dependent or independent part of a regression model?***From:*guhjy@kmu.edu.tw

**Re: st: Is it valid to use the individual ratios (i.e. Xi/Yi) in the dependent or independent part of a regression model?***From:*Nick Cox <njcoxstata@gmail.com>

**Re: st: Is it valid to use the individual ratios (i.e. Xi/Yi) in the dependent or independent part of a regression model?***From:*Steve Samuels <sjsamuels@gmail.com>

*From:*guhjy@kmu.edu.tw

*From:*Tirthankar Chakravarty <tirthankar.chakravarty@gmail.com>

*From:*guhjy@kmu.edu.tw

*From:*Nick Cox <njcoxstata@gmail.com>

*From:*guhjy@kmu.edu.tw

*From:*David Hoaglin <dchoaglin@gmail.com>

*From:*guhjy@kmu.edu.tw

*From:*David Hoaglin <dchoaglin@gmail.com>

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