st: Data manipulation query

["Anirudh V. S. Ruhil" <ruhil@ohio.edu>]

I have two data manipulation questions:

(a) Lets says I have a dataset of students' scores for 2 subjects when 
tested in the 4th grade, and their scores on similar tests taken in the 6th 
grade. Some students are lost between the 4th and 6th grade but some show 
up in both grades. The data structure is as follows ...

ID 	grade 	subject	 	score
A1	04 		English	 	271
A1	04		Mathematics	190
A1	06		English	 	260
A1	06		Mathematics	214
A2	04		English		.
A2	04 		Mathematics	165
A2	06		English		187
A2	06		Mathematics	193
A3	04		English		.
A3	04		Mathematics	.
A3	06		English		216
A3	06		Mathematics	265

How would I create a subset of these data such that the subset only 
contains records for students with non-missing scores in a given subject on 
BOTH the 4th and the 6th grade tests?

(b) In the same dataset, let us assume there are multiple records for some 
students on a single grade and subject. For example,

A4	04		English		191
A4	04		English		219

Of these multiple records, how can I select entries with the HIGHER SCORE? 
(i.e., the lower scores have to be discarded).

I'm sure there is a quickfire way to solve both so I'd be grateful if 
someone points me in the right direction in STATAspeak (9.1).

By the way, the N is about 3 million, though the number of unique student 
IDs is about 250,000.

thanks in advance

Ani

Anirudh V. S. Ruhil, Ph.D.
Sr. Research Associate
Voinovich Center for Leadership and Public Affairs
Ohio University
Building 21, The Ridges
Athens, OH 45701-2979
Tel: 740.597.1949 | Fax: 740.597.3057
*
*   For searches and help try:
*   http://www.stata.com/support/faqs/res/findit.html
*   http://www.stata.com/support/statalist/faq
*   http://www.ats.ucla.edu/stat/stata/