st: dropping incomplete frequencies without 'missing' in panel data

[Abdul <bihiabdul@gmail.com>]

Dear List,

I have a panel dataset and I wanted to drop those observations where
the variable 'hhnr' is missing in some time periods. for example when
hhnr is 6, it is only reported in 2001 and not in subsequent years,
whereas when hhnr is 9, it is reported in all years from 2001 to 2007.

hhnr    year    volumew    valuew
6    2001    770    4585
9    2001    9374    91672
9    2002    13576    145520
9    2003    4720    44709
9    2004    3429    30254
9    2005    5589.25    43225.37
9    2006    4577.1    44175.56
9    2007    6079.05    74066.045
13    2001    1273    11399
13    2002    1705    15619
13    2003    1306    11060
13    2004    728    5823
13    2005    331.2    2930.16
13    2006    541.2    2900.175
13    2007    635.6    2910.115
14    2004    467    5893
14    2006    360    4548
14    2007    897.15    11333.995
14    2007    897.15    11333.995
18    2001    4308    37267
18    2002    1234    13419

If those observations where hhnr is not there were reported as missing
I could have used this FAQ:
http://www.stata.com/support/faqs/data/dropmiss.html
Unfortunately, they are just left out and I am unable to drop them
despite many attmpts. I would appreciate if you either help me with a
way to make them explicitly missing in this dataset or yet better if
you konw a way to drop them and keep only those 'hhnrs' with complete
frequency (reported in all years).

Thanks,
Abdul
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