For example, FPX does not always add new IPOs to its lineup right away. In theory, Twitter could go public on November 15 and not find its way into FPX for a couple of months.

“Newly public companies are typically included in equity market indices on a delayed basis, until they become seasoned in trading markets. Once included, these companies are underrepresented in core equity portfolios. Using a portfolio of unseasoned newly public companies to complement core equity benchmarks, investors can obtain more comprehensive exposure to the total equity market,” according to Renaissance.

The firm noted it took an average of 123 days after their IPOs for LinkedIn (NYSE: LNKD) and Visa (NYSE: V) to be added to the Russell 3000. Linked is still not a member of the S&P 500.

FPX can also hold stocks for up to 1,000 past the IPO date. The Renaissance Capital ETF limits holdings to a two-year stay. Both ETFs cap holdings at a 10% weight.