[R] Predicting random effects in GAM using mgcv

[Meagan Dunphy-Daly]

Hi all, 

I am interested in modeling total fish catch using gam in mgcv to model simple random effects for individual vessels (that make repeated trips over time in the fishery). I have 98 subjects, so I thought I would use gam instead of gamm to model the random effects. My model is:

modelGOM <- gam(TotalFish ~ factor(SetYear) + factor(SetMonth) + factor(TimePeriod) +  s(SST) + s(VesselID, bs = "re", by = dum) + s(Distance, by = TimePeriod) + offset(log(HooksSet)), data = GOM, family = tw(), method = "REML")

I have coded the random effect with bs = "re" and by = dum (which should allow me to predict with the vessel effects at their predicted values or zero), where "dum" is a vector of 1.

The model runs, but I am having problems predicting.

grid.bin.GOM_A_Swordfish <- data.frame("Distance"=seq(min(GOM$Distance),max(GOM$Distance),length = 100),#range of distances over which to predict
                                 "SetYear" = '2006',
				 "SetMonth" = '6',
                                 "TimePeriod" = 'A',
                                 "SST" = mean(GOM$SST),
                                 "VesselID" = 'MEDIHO', #I picked one of the levels of the random effect, Vessel MEDIHO
                                 "dum" = '0', #to predict without vessel effect
                                 "HooksSet" = mean(GOM$HooksSet))

pred_GOM_A_Swordfish <- predict(modelGOM, grid.bin.GOM_A_Swordfish, type = "response", se = T)

The error that I'm getting is:
Error in Predict.matrix.tprs.smooth(object, dk$data) : 
  NA/NaN/Inf in foreign function call (arg 1)
In addition: Warning message:
In Ops.factor(xx, object$shift[i]) : - not meaningful for factors

I think this is being called because VesselID is a factor, but I'm using it a smooth for the random effects. Can you provide any advice on how to predict this model without the VesselID term (but still include it in fitting)?

Thanks!
Meagan