[R] imputation of sub-threshold values

[Thomas Lumley]

On Mon, 15 Mar 2004, Jonathan Williams wrote:

> Is there a good way in R to impute values which exist,
> but are less than the detection level for an assay?


If there were a good way to do it, it would probably be implementable or
implemented in R.


If you can persuade the people measuring the values to give you the
numbers (assuming they are just below `limit of detection' rather
than genuine non-detects) you will reduce the need for imputation. This is
often the most powerful technique -- analytical chemists are trained not
to give out numbers less than some multiple of the measurement error, but
they can often be persuaded that statisticians can be trusted with these
numbers.

If you think your data are close to one of the parametric survival models
in the survival package, you can analyse the data as left-censored,
without imputation. You could impute from the fitted model, if you need
imputations.

Otherwise you may be stuck with some ad hoc imputation with a sensitivity
analysis to see how the results depend on the imputed value.

	-thomas

Thomas Lumley			Assoc. Professor, Biostatistics
tlumley at u.washington.edu	University of Washington, Seattle