Perhaps you should try some spelling variations:
"multiple imputations research : july 2013" » "multiple imputation research : july 2013", "multiple implications research : july 2013", "multiple mutation research : july 2013", "multiple reputation research : july 2013"
"call options research : july 2009" » "call actions research : july 2009", "call optics research : july 2009", "call operations research : july 2009", "call option research : july 2009", "real options research : july 2009", "call options research : july 2008", "call options research : july 2007", "call options research : july 2019"

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 imputations research : july 2013"))~.
• If you are trying to use Boolean operators, they must be ALL CAPS: (("call options research : july 2009") OR ("multiple imputations research : july 2013")).
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• Adding a wildcard symbol may retrieve word variants: (("call options research : july 2009") or ("multiple imputations research : july 2013"))*.