The acronym INVEST helps to remember a widely used set of criteria, or checklist, to determine the quality of a user story. If the story is unable to meet any of these criteria, the team may want to rephrase it, or even consider a rewrite (which often means writing a new story card).
A user story should contain the following criteria:
“I” ndependent (of everything else)
“N” egotiable (not a specific contract for features)
“V” aluable (or vertical)
“E” stimable (to a certain degree)
“S” mall (so as to fit the emphasis)
“T” estable (in principle, even if there isn’t a test for it yet)