Though statistics can be misapplied and limited in scope, I frequently look at average annual returns for the investment reports of plan allocations. Generally 5-10% is typical over the twenty years I have been in this business.
A handful of the first annual plan reports I delivered for 2017 had a 17% average annual return. Another plan had 19%. So based on this small sample of different plans with different funds in their portfolio, some pooled, some individual accounts, 2017 was for some an exceptional year for gains. With 2018 much less robust the two years together are still better than average. Over time, though not without risk, equity investments seem to generate more gains.