Am I interpreting THETA(1) incorrectly?

From: Eleveld, DJ d.j.eleveld@anest.umcg.nl Subject: Am I interpreting THETA(1) incorrectly? Date: Thu, March 24, 2005 9:04 am Hello NONMEM users, I am a NONMEM beginner and am trying some prelimiary tests and have found an (at least to me) confusing result. Possibly a more experienced NONMEM user can explain the situation. We have parameters that are log-normally distributed using V1=THETA(1)*EXP(ETA(1)) in the $PK section. From this notation it seems to me that the estimated value for THETA(1) should be the geometric mean value of the individual V1 values. However when I do POSTHOC to get the individual V1 values, the geometric mean of these values is different than the estimated typical V1 value (i.e. THETA(1)). Can someone explain why this occurs? as the estimated geometric mean of the parameter population values? As an aside the data I am fitting comes from a monte-carlo simulation and the geometric-mean of the POSTHOC individual parameter values is closer to the 'real' values than the THETA values. Thank you, Doug