Hi all,

I have a question when I am running a fixed effect regression which is used to investigate how the dependent variable will be when the level of organizations changes over the year. Here is my command:

Code:
xtset year

xtreg coverage i.high i.year i.performance i.finance i.planning i.planning#i.year i.finance#i.year i.performance#i.year,r
**Performance/Planning/Finance are all the assurances elements that can examine the quality of the organization. Level A.B.C.D is the rating of each element. (A is the best rating, D is the worst)

The results are:
Array


The first part of the result seems reasonable. All the coefficients of level D are omitted for the purpose of baseline so that all other coefficients can be compared with the baseline level. For example, performance level B is 1.53% lower than performance level D in the dependent variable (immunization coverage rate). Here the planning level A is omitted. I think it is because of a high correlation between level A and level D, since the fixed effects take out all the variance at the group level, there is nothing left for level A to explain. If it is the case, it can be explained as level D organizations may have coverage as high as level A organizations.

Now for the second part: integration of the assurance levels and years. all the year 2005 are omitted for the baseline. For example, in the year 2006, the finance level 2 (level B) organization has a 3.96% higher coverage rate than the finance level B in the year 2005. Now my question is why the coefficients of level A * years are all omitted?
Array


Thank you so much!! Very looking forward to your reply!