Singular vs. plural

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I have heard completely different things regarding this. Some sites say they are different and some sites say they are the same. I don't know which information is accurate. Does the plural form of a keyword also target the singular? While doing keyword research I have noticed that "good clinical practice" is searched for much more often than "good clinical practices". 865 searches vs. 94 searches. Now I already wrote all my copy using "good clinical practices". Should I change it to "good clinical practice", use both (although 1 is much better since "practice" is contained within the word than the other), or not worry about it "practices"? I still have trouble understanding this. Thanks.