can you please try to create a BASIC_WORDLIST with a stemmer preference set to english?
http://docs.oracle.com/cd/B19306_01/text.102/b14218/cdatadic.htm#i1008929
OK, I have researched it in more depth for you, and I don't think you'll like he reuslts... I used the following schema:
execute ctxsys.ctx_ddl.create_preference('my_multi','MULTI_COLUMN_DATASTORE');
execute ctxsys.ctx_ddl.set_attribute('my_multi','columns','a,b,c');
execute ctxsys.ctx_ddl.create_preference('my_lexer','BASIC_LEXER');
execute ctxsys.ctx_ddl.set_attribute('my_lexer','index_stems','1');
execute ctxsys.ctx_ddl.create_preference('my_wordlist','BASIC_WORDLIST');
execute ctxsys.ctx_ddl.set_attribute('my_wordlist','stemmer','ENGLISH');
create index tt on test(a) indextype is ctxsys.context parameters('DATASTORE my_multi WORDLIST my_wordlist LEXER my_lexer SYNC(on commit)');
insert into test values ('Superannuation Contributions Splitting','test','test');
insert into test values ('commit','Superannuation Contributions Splitting','test');
insert into test values ('commit','test','Superannuation Contributions Splitting');
commit;
So after creating the schema - I went to the DR$INDEX_NAME$I table, and looked at the data there. it contains : B,C,commit,contribution, contributions,split,splitting,superannuation,test. It means that these are the list of "roots" you can search for.
Since "contribute" is not here, you cannot "get to it".
But, if you'll search for '$contribution' you will find it, or even for "$committing" since the root of it, commit, is in the list.
Bottom line - Oracle does not know that contribute and contribution are related. This is either a bug in the engine or a linguistic property (you can never know).
BUT - I can assure you the stemming works - since the search of '$contribution' and '$split' will return the results...