ETFs and funds work just as well. I would just try to avoid using general index funds like an S&P500 fund, because the individual stock movements dampen the volatility.
"When using Puri’s spreadsheet and making one trade on the S&P 500 ETF (Ticker: SPY) on the first trading day of each year from 2001 to 2008, the result is that his system did beat Buy & Hold. The compounded annualized return of buying and holding the SPY index only, was 1.7%, while Puri’s system delivered 2.0%. Better results may be had with individual stocks, other ETFs, or closed-end funds."