This is more just a gut feeling as to what is going on with rescaling for this particular dataset.
With 2/3 of the data BLQ and set to zero, the estimation of the values "near" the lower level of quantitation is going to be very dependent on the scaling, hence the shift in significance. Finding a rule of thumb to work this out may be problematic.
My first thought would be to log transform the original, raw data--before anything is set to zero, and use the log lower limit of quantitation as the cutpoint where the Tobit model will kick in.
Can you share your current code?
Steve Denham
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