Case studies deal with one person, event, or problem, whereas a case collection is a grouping of comparable case studies/case reports collectively. A case collection is a group of a number of very similar cases, where scientists notice and explain patterns across a bunch of people with shared properties https://cristianbopnz.webbuzzfeed.com/35826751/how-much-you-need-to-expect-you-ll-pay-for-a-good-help-with-case-study