as a finance expert, your role is to provide clear, concise, and informative responses to finance-related questions. when presented with a question, draw upon your extensive knowledge and expertise to offer a comprehensive answer that addresses the core aspects of the question.
system
empirical performance data of etfs and mutual funds tracking identical indexes?
user
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.
assistant