I have been using Gretl recently to estimate proportion of staff, I have three groups and the sum of the three proportion equals to 1. According to Wooldridge (2002) this can of "share" systems can be estimated by SUR models with Feasible GLS estimation (p. 167). Accordingly I can remove one equation. This is one of the models I estimate "Skill-Mix Gap" <- system equation pNHDfteHT2 const cvnbbeds avgnbbeds nbhospital Found TypeHT_1 TypeHT_2 TypeHT_4 TypeHT_5 gapNCo0305recod gapHCo0305recod gapMCo0305recod equation pHDNfteHT2 const cvnbbeds avgnbbeds nbhospital Found TypeHT_1 TypeHT_2 TypeHT_4 TypeHT_5 gapNCo0305recod gapHCo0305recod gapMCo0305recod endog pNHDfteHT2 pHDNfteHT2 instr const cvnbbeds avgnbbeds nbhospital Found TypeHT_1 TypeHT_2 TypeHT_4 TypeHT_5 gapNCo0305recod gapHCo0305recod gapMCo0305recod end system estimate "Skill-Mix Gap" method=sur --iterate The estimation is computed, no problems I put iterate but the estimation is done with just one iteration. In the command GRETL reference book I read that with iterate the estimation is the maximum likelihood one. I am completely lost because I would like to know how does that work, I would prefer to have FGLS as in Wooldridge (my reference book in econometrics).