Index doesn't appear to be working.

I have a table with a column titled 'passfail' that only contains either a P
for pass or an F for fail.  The table name is 'one'.  I created the index on
the table with:

CREATE INDEX one_passfail_idx ON one USING btree (passfail);

I then do:

VACUUM ANALYZE one;

Then I do an explain on this query:

SELECT * FROM one where passfail = 'P';

and it tells me:

Seq Scan on one (cost=0.00..263.02 rows=5613 width=56)

Shouldn't it tell me

Index Scan using one_passfail_idx on one?

Why isn't it using the index?  This query returns about 5,600 of 10,000
records.  It is faster to just do a seq scan because it returns such a high
percentage or records?  Thanks, I appreciate the help!

John Oakes