The ongoing war of words between Vanguard Group founder John Bogle and investment guru Jeremy Siegel over the merits of competing indices took to the airwaves this week. After a summer of competing Wall Street Journal op-eds, the duo debated on CNBC, Bogle arguing that there is nothing wrong with cap-weighted indexing, and Siegel, a Wharton School finance professor, claiming he has a better way. Siegel is affiliated with exchange-traded fund startup WisdomTree Investments, whose products feature “fundamental indexing,” weighting stocks by dividends. Appealing to historical data, Siegel said a value bias, provided by fundamental indexing “in the long run do better with less volatility.” Bogle countered, “People do too much data-mining to look back and see what has done well. But if it’s done well in the past there’s no guarantee it will do well in the future.”