Perhaps you should try some spelling variations:
"improving co performance metrics." » "improving co performance measures.", "improving ceo performance metrics.", "improving do performance metrics.", "improving core performance metrics.", "improving eco performance metrics.", "improving crop performance metrics.", "improving ict performance metrics."
"improving tax performance series." » "improving tax performance studies.", "improving tax performance size.", "improving box performance series.", "improving cta performance series.", "improving dam performance series.", "improving trade performance series.", "improving data performance series.", "improving care performance series."

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