Ah ok, thanks. I understand now. The data are chemical concentrations but are reported back as negative values. I have a Lower limit of detection (LLD) of 1 for one element but a value of -2.9 gets reported back to me.
One last question, for a different element the LLD is reported as 10, anything <= 10 is given a value of -10. So, I now have: Data Censored -10 TRUE -10 TRUE 20 FALSE In order for this to work in the NADA package, it must be transformed to: Data Censored 10 TRUE 10 TRUE 20 FALSE Thanks On Thu, Mar 14, 2013 at 3:20 PM, Rich Shepard <rshep...@appl-ecosys.com>wrote: > On Thu, 14 Mar 2013, Shane Carey wrote: > > Thanks for your reply. My data frame contains the value and a true/false >> to indicate whether they are censored or not. >> >> So I have something like: >> >> Data Censored >> -1.2 TRUE >> -5.5 TRUE >> 5 FALSE >> > > These negative values are actual values so do these have to be made >> absolute? >> > > Shane, > > If your data represent chemical concentrations of some sort then, yes, > the data must all be positive. You cannot have a chemical concentration > less > than zero. If the negative values are actual, then what is the reporting > level? And what do they measure that is really a censored value? > > The concept of censored data, whether right or left censored, is that > there is no way of knowing the actual numeric value. In right-censored > survival analyses (e.g., medical trials) the death of an individual is > recorded. But, when the study ends for whatever reason, there are still > subjects alive and there's no way of knowing how long after the end of the > study they die. Ergo, their age at death is unknown or censored. > > With left censored data such as chemical constituent concentrations in > air, water, or some other medium, there is a concentration below which the > instruments cannot distinguish it from noise. All we know is that the > constituent is present but its concentration is somewhere between zero and > the detection/reporting limit. Therefore, having a number that is below > this > detection/reporting limit is meaningless, and it cannot be negative. That's > why it is flagged as being censored. The cenmle() function assumes these > conditions to be true. > > Please keep this thread on the mail list so others can participate and > learn from the conversation. > > > Rich > > ______________________________**________________ > R-help@r-project.org mailing list > https://stat.ethz.ch/mailman/**listinfo/r-help<https://stat.ethz.ch/mailman/listinfo/r-help> > PLEASE do read the posting guide http://www.R-project.org/** > posting-guide.html <http://www.R-project.org/posting-guide.html> > and provide commented, minimal, self-contained, reproducible code. > -- Shane [[alternative HTML version deleted]] ______________________________________________ R-help@r-project.org 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.
