[R] Understanding cenros Error

[Rich Shepard]

On Wed, 11 Jul 2012, MacQueen, Don wrote:

> An my "easy" but not very useful answer is that this particular subset
> probably violates some assumption of the cenros() model. I myself would
> start with simple inspections of the data, such as
>
>  with( subset(chem, param=='Ag'),  table(ceneq1) )
>  with( subset(chem, param=='Ag'), qqnorm(quant) )
>  with( subset(chem, param=='Ag'), range(quant) )

Don,

   Good points. I'll do these.

> Do you have any zeros in quant?  (see ?cenros)

   Many, many. My client contact is trying to determine whether they are true
zeros or represent missing data. Data starts in 1994 and sometimes there are
blanks (definitely missing data) and sometimes zeros ... even on the same
spreadsheet row. Sigh. So we need to figure out just what they represent;
perhaps the original analytical lab reports can be found and examined.

   I don't recall reading in Dennis' book about too many zeros so I'll check
the R help. There are also errors trying cenboxplot() on the data frame.

   Only 9 of 37 chemicals produce results with cenros(), including 2 with
censored values and 1 with > 80% of concentrations censored. The remaining
28 all have censored values and/or many zeros.

   When I have more information, presumably positive results, I'll post here
so the thread can be complete for posterity.

Thanks again,

Rich