# No variability calculation **URL:** \ **Category:** Modelling and Simulation\ **Created:** [June 24, 2014, 2:10pm UTC](https://forum.certara.com/t/no-variability-calculation/2747 "2014-06-24T14:10:29Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![boetschc](https://avatars.discourse-cdn.com/v4/letter/b/35a633/32.png) [@boetschc](https://forum.certara.com/u/boetschc)\ **Post date:** [June 24, 2014, 2:10pm UTC](https://forum.certara.com/t/no-variability-calculation/2747/1 "2014-06-24T14:10:29Z")
Hi, While running a popPK model with NLME, I do sometimes get the variability being calculated for the theta, sometimes not. Any idea as to why this happens? Using PHX 1.2, NLME 1.1 ---
**Author:** ![Samer\_Mouksassi](https://avatars.discourse-cdn.com/v4/letter/s/ce7236/32.png) [@Samer\_Mouksassi](https://forum.certara.com/u/Samer_Mouksassi)\ **Post date:** [June 25, 2014, 6:24pm UTC](https://forum.certara.com/t/no-variability-calculation/2747/2 "2014-06-25T18:24:32Z")
Variability can mean different things: residual error between subject variability uncertainty ( standard error on estimate) I assume you meant the standard error of estimates. Most of the time it is because the hessian cannot be inverted ( overparameterization , local minimum, ) try to fix a parameter or change your initial estimate. Samer ---
**Author:** ![SimonDavis](https://avatars.discourse-cdn.com/v4/letter/s/d9b06d/32.png) [@SimonDavis](https://forum.certara.com/u/SimonDavis)\ **Post date:** [June 26, 2014, 4:58am UTC](https://forum.certara.com/t/no-variability-calculation/2747/3 "2014-06-26T04:58:45Z")
Christoph, as Samer indicated, I sometimes find that ‘accepting all’ on the final estimates and then re-executing will give me the SE. Another thing to consider is the algorithm you have selected; the FOCE-ELS often gives better results for many problem types that the -LB variant. Simon