Perhaps you should try some spelling variations:
"the comparison series" » "the comparison studies", "the comparison string", "the comparison section"
"The comparison sites" » "The comparison systems", "The comparison system", "The comparison studies"
"The comparison test" » "The comparison tests", "The comparison theory"

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: (("the comparison series") OR ((("The comparison sites") OR ("The comparison test")))).
• The words AND, OR and NOT may confuse the search; try adding quotes: "((\"the comparison series\") or (((\"The comparison sites\") OR (\"The comparison test\"))))".
• Removing quotes may allow a broader search: (( the comparison series ) or ((( The comparison sites ) OR ( The comparison test )))).
• Adding a wildcard symbol may retrieve word variants: (("the comparison series") or ((("The comparison sites") OR ("The comparison test"))))*.