st: Interval regression: Categories with 100% censoring

[Gillian.Frost@hsl.gov.uk]

Hello all,

I would appreciate some advice regrading the output from -intreg-.

A number of water samples have been collected, and a microbiological 
examination undertaken to assess the number of colony forming units per 
100ml (CFU).  Some observations are right censored, some are left 
censored, and the censoring point is not always the same.  I have 
therefore been using interval regression (-intreg-) to look for regional 
differences in the organism levels.

Based on previous advice from Statalist (thank you!), my dependent 
variable is log10 colony forming units and my independent variable is the 
categorical variable of region.  Some samples were collected from the same 
location.  My command is as follows:

intreg depvar1 depvar2 i.region, vce(cluster location)

This seems to be working quite nicely and the results seem sensible. 
However, when I have a region where all observations are, say, right 
censored, then the predicted log10 CFU for this region is substantially 
higher than the other regions (statistically significantly so).  But its 
value does not seem sensible - for example, all regions generally predict 
around 3-4 CFU, whereas the region with all censored values has a 
predicted value of around 7 CFU!

I am guessing that this is happening because all of the observations are 
right censored and so the model doesn't really have sufficient information 
to estimate a reliable coefficient.  But I cannot find this written 
anywhere.

Do you think it is justifiable to say that "results can be unreliable when 
all observations are censored, and so regions where this happens were 
excluded from the analysis", or something along those lines?

I would be grateful for any advice.

Many thanks,

Gillian











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