Perhaps you should try some spelling variations:
"multiple population research : july 2006" » "multitude population research : july 2006", "multiple imputation research : july 2006", "multi population research : july 2006", "multiple population research : july 2008", "multiple population research : july 2007", "multiple population research : july 2009"
"multi opinion research : july 2013" » "multi action research : july 2013", "multi option research : july 2013"

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

• If you are trying to use Boolean operators, they must be ALL CAPS: (("multiple population research : july 2006") OR ("multi opinion research : july 2013"))~.
• Removing quotes may allow a broader search: (( multiple population research : july 2006 ) or ( multi opinion research : july 2013 ))~.
• Adding a wildcard symbol may retrieve word variants: (("multiple population research : july 2006") or ("multi opinion research : july 2013"))*.