Perhaps you should try some spelling variations:
"improving co performance series." » "improving co performance studies.", "improving co performance size.", "improving ceo performance series.", "improving do performance series.", "improving core performance series.", "improving eco performance series.", "improving crop performance series.", "improving ict performance series."
"improving poor performance risks." » "improving poor performance risk.", "improving sector performance risks.", "improving power performance risks.", "improving food performance risks.", "improving peer performance risks."

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: (("improving poor performance risks.") OR ("improving co performance series."))~.
• Removing quotes may allow a broader search: (( improving poor performance risks. ) or ( improving co performance series. ))~.
• Adding a wildcard symbol may retrieve word variants: (("improving poor performance risks.") or ("improving co performance series."))*.