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 <rshepard@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
>
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>



-- 
Shane

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