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From |
"Feiveson, Alan H. (JSC-SK311)" <alan.h.feiveson@nasa.gov> |

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

Subject |
st: RE: Interval regression with skewed data |

Date |
Mon, 9 Jan 2012 11:32:29 -0600 |

Gillian - I don't know what -intcens- does, but it is not too hard to write a maximum likelihood program for interval censored data from any assumed parametric distribution where the CDF is in a convenient form to evaluate in Stata - for example the Weibull, lognormal, normal, Gamma, Beta. Then you could let the parameters of the model depend either linearly or log-linearly or logit-linearly on your covariates. The contribution to the likelihood for non-censored observations is just the density evaluated at those observations. For censored observation known to lie in the interval(a, b) ,the contribution to the likelihood would be F(b) - F(a), where F(.) is the CDF of the assumed distribution. AL Feiveson -----Original Message----- From: owner-statalist@hsphsun2.harvard.edu [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Gillian.Frost@hsl.gov.uk Sent: Monday, January 09, 2012 9:57 AM To: statalist@hsphsun2.harvard.edu Subject: st: Interval regression with skewed data Hello all, I am struggling with an analysis and would like your insight. I think that I am looking at using interval regression but there are certain aspects of the data that are worrying me. First some background... A number of water samples have been taken from around the UK, and a microbiological examination of the water has been undertaken. Whenever a sample is sent to a lab, a whole suite of tests are done to count the number of colony forming units of various organisms. I therefore have a number of outcomes, whose units are the number of colony forming units per ml. The aim of this part of the analysis is to compare the organism levels found in different regions of the UK. Some observations are left censored (0-6% depending on the outcome) - ie <1 CFU/ml, or <10 CFU/ml - and some are right censored (0-59%) - ie. >3000 CFU/ml. The censoring point varies,and so I thought that I would have to use interval regression (Stata's -intreg-). However, the data are not Normally distributed (which is an assumption of interval regression), but are positively skewed with some outcomes having a high number of zero counts (one has 75% zeros!). In the book by J S Long (Regression models for categorical and limited dependent variables, 2007), there was a discussion about how accelerated failure time (AFT) models can be used to perform interval regression when the data are not Normally distributed, but there was no example of how to do this. Unfortunately I no longer have the book to provide you with the page reference. I have found a user written command -intcens-, which can perform interval-censored survival analysis and fits a number of different distributions, but I cannot find any documentation or examples of its use (apart from the help file). Does anyone have any examples of using AFT models to perform interval regression or examples of using -intcens-? Or do you think that there is a better way I could be handling the data? Any help would be hugely appreciated. Many thanks, Gillian ------------------------------------------------------------------------ ATTENTION: This message contains privileged and confidential information intended for the addressee(s) only. If this message was sent to you in error, you must not disseminate, copy or take any action in reliance on it and we request that you notify the sender immediately by return email. Opinions expressed in this message and any attachments are not necessarily those held by the Health and Safety Laboratory or any person connected with the organisation, save those by whom the opinions were expressed. Please note that any messages sent or received by the Health and Safety Laboratory email system may be monitored and stored in an information retrieval system. ------------------------------------------------------------------------ Think before you print - do you really need to print this email? ------------------------------------------------------------------------ ------------------------------------------------------------------------ Scanned by MailMarshal - Marshal's comprehensive email content security solution. Download a free evaluation of MailMarshal at www.marshal.com ------------------------------------------------------------------------ * * 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**:**st: Interval regression with skewed data***From:*Gillian.Frost@hsl.gov.uk

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