Based on your financial expertise, provide your response or viewpoint on the given financial question or topic. The response format is open.

Empirical performance data of ETFs and Mutual Funds tracking identical Indexes?
Why don't you look at the actual funds and etfs in question rather than seeking a general conclusion about all pairs of funds and etfs?  For example, Vanguard's total stock market index fund (VTSAX) and ETF (VTI).  Comparing the two on yahoo finance I find no difference over the last 5 years visually.  For a different pair of funds you may find something very slightly different.   In many cases the index fund and ETF will not have the same benchmark and fees so comparisons get a little more cloudy.  I recall a while ago there was an article that was pointing out that at the time emerging market ETF's had higher fees than corresponding index funds.  For this reason I think you should examine your question on a case-by-case basis.  Index fund and ETF returns are all publicly available so you don't have to guess.