Colleagues
I have fit an exposure response model using NONMEM — the optimal model is a
segmented two-part regression with Cp on the x-axis and response on the y-axis.
The two regression lines intercept at the cutpoint.
The parameters are:
slope of the left regression
cutpoint between regressions
“intercept” — y value at the cutpoint
slope of the right regression (fixed at zero; models in which the value
was estimated yielded similar values for the objective function)
I have been asked to calculate the confidence interval for the response at
various Cp values.
Above the cutpoint, this seems straightforward:
a. if NONMEM yielded standard errors, the only relevant parameter is
the y value at the cutpoint and its standard error
b. if NONMEM did not yield standard errors, the confidence interval
could come from either likelihood profiles or bootstrap
My concern is calculating at Cp values below the cutpoint, for which both slope
and intercept come into play. Any thoughts as to how to do this in the
presence or absence of NONMEM standard errors?
The reason that I mention with / without presence of SE’s is that this model
was fit to two different datasets, one of which yielded SE’s, the other not.
Any thoughts on this would be appreciated.
Dennis
Dennis Fisher MD
P < (The "P Less Than" Company)
Phone: 1-866-PLessThan (1-866-753-7784)
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