Negative binomial and time trend

[Mario Scintille]

Dear all,

I am working on regressing patents on a series of variable using Stata. To this end I am using both xtpoisson and xtnbreg. In the process of the analysis of the results, I am now considering if I should add time dummies or a time trend. On this regard, I have two questions:

1) What is the best approach to understand if time dummies or a time trend should be inserted?
2) What is the most effective procedure to insert them in the xt regression? Below there is my attempt

*** time dummies
xtpoisson f1.patents regulation competition $control i.year, fe
xtnbreg f1.patents regulation competition $control i.year, fe


***** Time trend
gen ttrend=0
forvalues i=1/10 { /*if these are the years for the sample*/
replace ttrend=`i' if year==1999+`i'
}
*
xtpoisson f1.patents regulation competition $control ttrend, fe
xtnbreg f1.patents regulation competition $control ttrend, fe

Thanks a lot for any suggestions you might have.

Best Regards