Perhaps you should try some spelling variations:
"improving tax performance contract." » "improving tax performance contracts.", "improving box performance contract.", "improving cta performance contract.", "improving dam performance contract.", "improving trade performance contract.", "improving data performance contract.", "improving care performance contract."
"improving urban performance series." » "improving urban performance studies.", "improving urban performance size.", "improving bank performance series.", "improving loan performance series.", "improving human performance series."

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

• Performing a fuzzy search may retrieve terms with similar spellings: (("improving urban performance series.") or ("improving tax performance contract."))~.
• If you are trying to use Boolean operators, they must be ALL CAPS: (("improving urban performance series.") OR ("improving tax performance contract."))*.
• Removing quotes may allow a broader search: (( improving urban performance series. ) or ( improving tax performance contract. ))*.