Analysis of Steady State Data

More information would be helpful. How many records in the data set? How many individuals? What is the maximum number of observations per individual? What is the maximum number of data records per individual? Does this happen with other problems on the VAX, or just this one data set? When this same problem is run on the Sun, is the same minimum reached? Are the final value of the objective function and final parameter estimates the same (to the no. of significant digits printed)? My guess is that someone modified NONMEM source on the VAX and made an error. The changes might have been to raise the maximum number of observations per individual from its default value 50 (e.g., not all instances of "PARAMETER NO=50" were changed), or to insert some additional WRITE statements (e.g., in ZXMIN1) to obtain intermediate output from the estimation step. The NONMEM source on the VAX should be carefully compared with the distributed code.