I don't see what unbalanced panels have to do with generating a set of
dummies that add up to one. The dummy means _in the sample_ of an
unbalanced panel will sum up to 1.0, even though they do not represent
100% of a balanced panel:
. webuse grunfeld,clear
. tsset
panel variable: company, 1 to 10
time variable: year, 1935 to 1954
. drop in 14
(1 observation deleted)
. drop in 29
(1 observation deleted)
. drop in 101
(1 observation deleted)
. xtdes
company: 1, 2, ..., 10 n =
10
year: 1935, 1936, ..., 1954 T =
20
Delta(year) = 1; (1954-1935)+1 = 20
(company*year uniquely identifies each observation)
Distribution of T_i: min 5% 25% 50% 75% 95%
max
19 19 19 20 20 20
20
Freq. Percent Cum. | Pattern
---------------------------+----------------------
7 70.00 70.00 | 11111111111111111111
1 10.00 80.00 | 11.11111111111111111
1 10.00 90.00 | 111111111.1111111111
1 10.00 100.00 | 1111111111111.111111
---------------------------+----------------------
10 100.00 | XXXXXXXXXXXXXXXXXXXX
. tab year,gen(Y)
year | Freq. Percent Cum.
------------+-----------------------------------
1935 | 10 5.08 5.08
1936 | 10 5.08 10.15
1937 | 9 4.57 14.72
1938 | 10 5.08 19.80
1939 | 10 5.08 24.87
1940 | 10 5.08 29.95
1941 | 10 5.08 35.03
1942 | 10 5.08 40.10
1943 | 10 5.08 45.18
1944 | 9 4.57 49.75
1945 | 10 5.08 54.82
1946 | 10 5.08 59.90
1947 | 10 5.08 64.97
1948 | 9 4.57 69.54
1949 | 10 5.08 74.62
1950 | 10 5.08 79.70
1951 | 10 5.08 84.77
1952 | 10 5.08 89.85
1953 | 10 5.08 94.92
1954 | 10 5.08 100.00
------------+-----------------------------------
Total | 197 100.00
. egen rr=rsum(Y*)
. su rr
Variable | Obs Mean Std. Dev. Min Max
-------------+--------------------------------------------------------
rr | 197 1 0 1 1
Kit Baum, Boston College Economics
http://ideas.repec.org/e/pba1.html
On Jun 9, 2005, at 2:33 AM, Michael wrote:
If your panels are unbalanced, then xtreg may not need to drop any of
the
time dummies because they will not always sum to 1 and so will not be
perfectly collinear.
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