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From |
"Joseph Coveney" <jcoveney@bigplanet.com> |

To |
"Statalist" <statalist@hsphsun2.harvard.edu> |

Subject |
Re: st: longitudinal data with non-detectable measurements |

Date |
Mon, 12 May 2008 01:49:36 +0900 |

Leny Mathew wrote: I'm trying to model data on change in certain hormones over a period of time. The data on some of these have a number of values that are set (currently) at a non-zero minimum detectable value (MDL). Currently, I'm using a GLM with a gamma family along with robust estimation of SE to model this data. The model works reasonably fine on the data but I would like to improve this by properly accounting for the information on the non-detectables. I'm interested in the change over time of these hormones and so change from the MDL to a higher value is useful information. Apart from the GLM, I've looked at the tobit models and zero-inflated models. From what I understand, the tobit model assumes that the data that is not censored follows a normal distribution and this not the case in my data. Zero-inflated models seem to be used for count rather than continuous data. Does anybody in the list have experience on modeling continuous data with zero-inflation or data with a MDL issue? -------------------------------------------------------------------------------- Take a look at the user-written module -intcens- by Jamie Griffin. It is available from SSC (-ssc describe intcens-). -intcens- will handle left-censored assay data (below limit of detection or limit of quantitation) with a variety of distributions, including gamma. It doesn't specifically handle panel (longitudinal) data, but it has a -cluster()- option, which might be adequate for your needs, especially if you're using -glm- for your analyses, anyway. Joseph Coveney clear * set more off set seed0 `=date("2008-05-11", "YMD")' rndgam 1000 2 6 summarize xg, meanonly display in smcl as text r(mean) glm xg , family(gamma) nolog display in smcl as text 1 / _b[_cons] display in smcl as text 1 / e(phi) // Approx. estimate display in smcl as text e(phi) / _b[_cons] gammafit xg, nolog clonevar a = xg clonevar b = xg intcens a b, distribution(gam) nolog matrix define B = e(b) display in smcl as text exp(B[1,2]) display in smcl as text exp(B[1,1] - B[1,2]) quietly centile xg, centile(5) quietly replace a = .b if a < r(c_1) label define LOQ .b "< LOQ" label values a LOQ intcens a b, distribution(gam) nolog exit * * For searches and help try: * http://www.stata.com/support/faqs/res/findit.html * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

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