I've included data in the article for rolling SnP500 returns since inception. Data shows that if you invest for the long term (10-20 Years), SnP returns 20% P.A (Averaged) at best, and about 5% P.A depending on when you entered. Hence, even if there's a couple of negative years, there is data to prove they you are indeed able to average over 8% in the long run.