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| PRIOR |
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MEANING: PRIOR subroutine
CONTEXT: User-supplied subroutine; replaces a NONMEM dummy routine
USAGE:
SUBROUTINE PRIOR (ICALL,CNT)
INTEGER ICALL
(If double precision is to be used):
DOUBLE PRECISION CNT
(If single precision is to be used):
REAL CNT
DISCUSSION:
The user-written PRIOR subroutine allows a penalty function based on a
frequency prior to be specified and added to the -2log likelihood
function (Gisleskog et al, JPP, 2002, p. 473-505). This serves as a
constraint on the estimates of THETA, OMEGA, and SIGMA and thus as a
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way for stable estimates to be obtained with insufficient data.
When the Simulation Step is implemented, THETA and OMEGA, and SIGMA
may also be simulated (along with etas and epsilons, should any such
appear in the model and have nonzero-variances) in the PRIOR subrou-
tine (See NMPR16). However, this need not be done (simply execute a
return in PRIOR at ICALL=4), in which case the values for THETA,
OMEGA, and SIGMA used in the simulation are the values given in the
NONMEM control stream or input Model Specification File.
Input argument:
ICALL
Similar to ICALL for PRED subroutine.
Possible values: 0, 1, 2, and 4.
In a multiple problem run, PRIOR is called with ICALL=1 only with
the first problem.
Output argument:
CNT
CNT is the penalty term.
Other Inputs:
Other inputs are available to PRIOR in NONMEM read-only commons.
(See NONMEM_commons).
NONMEM Utility Routines:
PRIOR may call NONMEM utility routines, NWPRI and TNPRI.
(See nwpri, tnpri). E.g.,
SUBROUTINE PRIOR (ICALL,CNT)
DOUBLE PRECISION CNT
NTHETA=3
NETA=3
NEPS=1
NTHP=2
NETP=2
NEPP=1
PLEV=0
CALL NWPRI (NTHETA,NETA,NEPS,NTHP,NETP,NEPP,NPEXP,ITYP,PLEV,
1 NSAM,ISS,CNT)
RETURN
END
NWPRI and TNPRI compute particular types of penalty functions. NWPRI
computes a function based on a frequency prior that has a multivariate
normal form for THETA, and also, in the case of population data, an
inverse Wishart form for OMEGA (independent from the normal for
THETA). When used during a Simulation Step, it produces a random
value of THETA and a random value of OMEGA from the frequency prior.
The prior information is entered manually into the control stream.
This prior is especially useful for a subjective specification of a
penalty function. (See nwpri, tnpri).
Model parameters may be constrained in various ways. Those parameters
whose values are not fixed are transformed to new parameters whose
values are not at all constrained a priori (the "unconstrained parame-
ters", or UCP). TNPRI computes a penalty function based on a fre-
quency prior that has a multivariate normal form for all UCP. When
used during a Simulation Step, it produces a random value of the vec-
tor of all model parameters (whose values are not fixed). The prior
information is found in an input model specification file, and it has
been automatically stored there as part of a NONMEM analysis of a
prior data set. (See $msfi, model specification file). This prior is
especially useful for an empirical specification of a penalty func-
tion. (See TNPRI).
REFERENCES: Guide III Section VII.3.5.4
REFERENCES: Guide V Section 12.4.16
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