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From |
Steven Samuels <sjhsamuels@earthlink.net> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
Re: st: Increasing variance of dependent variable, logit, inter-rater agreement |

Date |
Sat, 28 Feb 2009 19:45:56 -0500 |

-- Anupit,

Best wishes, Steve On Feb 27, 2009, at 8:28 PM, Supnithadnaporn, Anupit wrote:

<> Dear Steven,I appreciate your reply to my post. I am sorry if my explanation istoo long.Thank you, AnupitPlease give more detail about what is being assessed. Is there a gold standard, measured or latent, for what these technologies are trying to agree upon?The subject of my study is the in-used vehicles. In some areas ofthe US,there is a regulation that requires a vehicle to be tested for itsemission.In the past, this instrument measured the real tailpipe emission.The testis typically performed at the commercial inspection station. If theamount of emission surpasses the threshold standard, the vehiclefails.The owner of failing vehicle has to repair his/her vehicle until itmeetsthe standard level otherwise he/she cannot renew the vehicleregistration.However, this tailpipe-test technology has been replaced by the newone calledOBD II test. This test no longer measures the tailpipe emission.Instead,it gives the fail result if there is an error codes relating to theemissioncontrol part of the vehicle.Despite the different technologies measuring different things, theyshare the common goal of the regulation. That is to identify thehigh-polluting vehicles.* What is the first technology that measures characteristics and arrives at a pass-fail? How does it make this decision? Was age one of these characteristics?So, the first technology is the OBD II that detects the error codesand yieldthe pass-fail result which is the *nominal level*. Having certainerror codesmeans that the vehicle is likely to emit high level of pollutionbeyond thestandards. As the vehicle become older, it is likely to pollute more.Moreover, the OBD II which is the computer unit of the vehicle islikely tomalfunction. If the OBD II is malfunction, it can give either thefalse-passor false-fail result.* How was the cut point y2b arrived at?Fortunately, the regulator also has set up several unobtrusivemonitoringstations on road. Basically, this technology uses the remote-sensing device(RSD) to measure the real tailpipe emission from numerous vehiclesrunningpass by. This is the second technology in my analysis. It measuresthe realtailpipe emission which is the *interval level*. And the thresholdis basedon the EPA regulation set for particular type of vehicle make,model year,and weight - *the cut point of y2b*.* You say that the variability of y2a increases with age. Is the level of y2a related to age?Correct. As a vehicle is getting older, its emission level islikely to behigh due to deterioration. Moreover, its emission can vary vastlydifferentfrom one measurement (by RSD) to the other. This is what I amtrying totake into account in my analysis My data is a pooled-cross section time series of 4 years.My unit of analysis is a matched pair of a vehicle tested by OBD IIandmeasured by RSD on road in the same year-testing cycle.My hypothesis is that the OBD-RSD agreement is greater for theolder vehiclefleets. My sample size ~ 80,000 observations. Of the total, 72% is classified as 'agree' For 28% of 'disagree' group, around 90% is the Fail-RSD, Pass-OBD.During the early analysis, I split the vehicles into different agegroupsfrom 3-9 years. I obtain Kappa for each group and compare them.However,I run into problem of Kappa when the prevalence (the disagree casesforeach age-group) is small. Cicchetti DV, Feinstein AR. High agreement but low Kappa: II Resolving the paradoxes. J Clin Epidemiol 1990; 43:551-8 * * 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/

* * 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/

**References**:**Re: st: Increasing variance of dependent variable, logit, inter-rater agreement***From:*"Supnithadnaporn, Anupit" <gtg065t@mail.gatech.edu>

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