Perhaps you should try some spelling variations:
"multiple operations research : july 2013" » "multiple options research : july 2013", "multiple imputations research : july 2013", "mobile operations research : july 2013"
"call options research : july 2009" » "call action research : july 2009", "call opinion research : july 2009", "call national research : july 2009", "call option research : july 2009", "real options research : july 2009", "call options research : july 2006", "call options research : july 2012", "call options research : july 2011"

Try widening your search to All Fields.

You may be able to get more results by adjusting your search query.

• Performing a fuzzy search may retrieve terms with similar spellings: (("call options research : july 2009") or ("multiple operations research : july 2013"))~.
• If you are trying to use Boolean operators, they must be ALL CAPS: (("call options research : july 2009") OR ("multiple operations research : july 2013")).
• Removing quotes may allow a broader search: (( call options research : july 2009 ) or ( multiple operations research : july 2013 )).
• Adding a wildcard symbol may retrieve word variants: (("call options research : july 2009") or ("multiple operations research : july 2013"))*.