collinearity

[Guest]

Hello
I got the below output, I can not understand why stata omitted y1987 and 1988.
Thanks

. xtreg gr_rate d.(y1980-y1988) d.ez
note: D.y1987 omitted because of collinearity
note: D.y1988 omitted because of collinearity

Random-effects GLS regression Number of obs = 176
Group variable: city Number of groups = 22

R-sq: within = 0.6372 Obs per group: min = 8
between = 0.0094 avg = 8.0
overall = 0.6230 max = 8

Wald chi2(8) = 275.97
corr(u_i, X) = 0 (assumed) Prob > chi2 = 0.0000

------------------------------------------------------------------------------
gr_rate | Coef. Std. Err. z P>|z| [95% Conf. Interval]
-------------+----------------------------------------------------------------
y1980 |
D1. | -1.517053 .3465387 -4.38 0.000 -2.196256 -.8378496
|
y1981 |
D1. | -1.5 .3006369 -4.99 0.000 -2.089238 -.9107629
|
y1982 |
D1. | -.7041884 .2547937 -2.76 0.006 -1.203575 -.2048019
|
y1983 |
D1. | -.720255 .2090476 -3.45 0.001 -1.129981 -.3105292
|
y1984 |
D1. | -.7203407 .1602024 -4.50 0.000 -1.034332 -.4063498
|
y1985 |
D1. | -.3802072 .1128333 -3.37 0.001 -.6013565 -.1590579
|
y1986 |
D1. | -.0710007 .0651444 -1.09 0.276 -.1986813 .0566799
|
y1987 |
D1. | 0 (omitted)
|
y1988 |
D1. | 0 (omitted)
|
ez |
D1. | -.1818775 .0781862 -2.33 0.020 -.3351196 -.0286355
|
_cons | -.3386845 .046064 -7.35 0.000 -.4289683 -.2484007
-------------+----------------------------------------------------------------

[Phil Bromiley]

You'll increase your chances of a helpful answer by following the FAQ on asking questions - provide Stata code in code delimiters, readable Stata output, and sample data using dataex.

Clearly, there are not enough separate observations that differ in 1987 and 1988. I'd start by generating the variables using generate, then look at the values for those years. What happens if you look at the observations used in the xtreg for those years?