Perhaps you should try some spelling variations:
"improving basic performance series." » "improving basic performance studies.", "improving basic performance size.", "improving based performance series.", "improving music performance series.", "improving baseline performance series.", "improving air performance series.", "improve basic performance series."
"improving past performance series." » "improving past performance studies.", "improving past performance size.", "improving plant performance series.", "improving port performance series.", "improving transit performance series.", "improving fast performance series.", "improving post 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 basic performance series.") OR ("improving past performance series."))~.
• The words AND, OR and NOT may confuse the search; try adding quotes: "((\"improving basic performance series.\") OR (\"improving past performance series.\"))".
• Removing quotes may allow a broader search: (( improving basic performance series. ) OR ( improving past performance series. )).
• Adding a wildcard symbol may retrieve word variants: (("improving basic performance series.") OR ("improving past performance series."))*.