According to my understanding about the difference-in-differences (DID) model with fixed effects, there are two specifications

  1. y=a0 + a1*TREAT + a2*POST + a3*TREAT_POST + e
  2. y=a0 + a1*TREAT_POST + timeFE + individualFE + e

Here, the dependent variable is a count variable and TREAT is an indicator variable that represents multiple groups of individuals affected by an event/treatment. The treatment affects groups of individuals at different calendar years, so yrsfromtreatment is a dummy variable for year pre- and post-treatment.

TREAT represent 1 for treated individuals (regardless of group) and 0 for matched control individuals. POST represent 1 for all years after the treatment is introduced. TREAT_POST=1 for the treated individuals in the post-treatment period.

So, Model 2 is better if there are possible omitted time-invariant and time-specific variables. And TREAT and POST indicator variables should usually be dropped in Model 2.

However, I find that I can actually run a fixed-effect negative binomial regression with calendar year (i.year) and treatment year (i.yrsfromtreatment) dummies in Stata:

xtset panelid yrsfromtreatment
xtnbreg y i.TREAT##i.POST i.yrsfromtreatment i.year, fe robust

Stata reports positive signs and significance in TREAT and POST.

How do I explain the coefficients on TREAT and POST when fixed effects are included? Is this model incorrect?


1 Answer 1


I'm a bit confused. Shouldn't your regression be collinear?

Your command is: xtnbreg y i.TREAT##i.POST i.yrsfromtreatment i.year, fe robust


  • $I_{i,t,\tau}$ be a variable indicating whether individual $i$ is $\tau$ years from treatment at time $t$.
  • $\mathrm{Treat}_i$ is an indicator as to whether $i$ at any point receives treatment.
  • $\mathrm{Post}_{it}$ is an indicator as to whether $i$ has received treatment at time $t$ or in prior periods.

It looks that command is running the regression: $$ y_{it} = b \mathrm{Treat}_{i}\mathrm{Post}_{it}+ \sum_{\tau=0}^\bar{\tau} c_\tau I_{i,t,\tau }+ u_i + v_t + \epsilon_{it} $$

What confuses me is that shouldn't this be collinear? If $\tau$ takes discrete values $0$, $1$, $2$, etc... then shouldn't

$$ \mathrm{Treat}_{i}\mathrm{Post}_{it} = \sum_{\tau = 0}^\bar{\tau} I_{i,t,\tau}$$ That is, if individual $i$ had treatment prior to or in year $t$ (i.e. $\mathrm{Treat}_{i}\mathrm{Post}_{it} = 1$), then they were either treated this year (i.e. $I_{i,t,0}$ = 1), or they were treated last year (i.e. $I_{i,t,1} = 1$), or treated the year before that ($I_{i,t,2} = 1$), etc....

  • $\begingroup$ Yes, I think the regression with panelid and year fixed effects needs to be: xtnbreg y i.TREAT##i.POST i.year, fe robust $\endgroup$
    – EconoQ
    Mar 25, 2017 at 17:08

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