Leo Tolstoy famously opens his novel Anna Karenina with the words, "All happy families are alike; each unhappy family is unhappy in its own way." Statisticians have derived from here something they've come to call the "Anna Karenina principle": roughly, the idea that success in any endeavor requires that everything go right, but remove a single important prerequisite, and your endeavor will fail. [1] Say the preconditions of success are A, B, C, and D.
Then in every successful case, all of: A, B, C, and D, will be in place, but each failure may have its own profile. Lack of success may be due to the absence of A, or B, or C and D, and so on.
Read more on psychologytoday.com