Successful Trader's Cheat Sheet
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Deciding which money market mutual fund is right for you is important, but this should give you some food for thought, and help you get started as you begin your search. Remember that the benefits of a money market fund are the liquidity they provide while maintaining interest rates which are better than you’d find in a traditional savings account or money market account. Also keep in mind that yields change daily.

This is a bank run in the sense that there is a mismatch in maturities, and thus a money fund is a "virtual bank": the assets of money funds, while short term, nonetheless typically have maturities of several months, while investors can request redemption at any time, without waiting for obligations to come due. Thus if there is a sudden demand for redemptions, the assets may be liquidated in a fire sale, depressing their sale price.
Continuing investor anxiety as a result of the Lehman Brothers bankruptcy and other pending financial troubles caused significant redemptions from money funds in general, as investors redeemed their holdings and funds were forced to liquidate assets or impose limits on redemptions. Through Wednesday, September 17, 2008, prime institutional funds saw substantial redemptions.[14] Retail funds saw net inflows of $4 billion, for a net capital outflow from all funds of $169 billion to $3.4 trillion (5%).
When comparing money market mutual funds a few patterns arose. First, when it comes to expense ratios, or the annual fees you’ll pay to own the funds, 0.25% is a threshold that separates the top and bottom half of the money market mutual funds in my short list. When it comes to yields, 2.1% is a defining 7-Day Yield threshold. For YTD yields, 1.4% is a benchmark that separates the pack.
Over time, money market fund "depositors" felt more and more secure, and not really at risk. Likewise, on the other end, corporations saw the attractive interest rates and incredibly easy ability to constantly roll over short term commercial paper. Using rollovers they then funded longer and longer term obligations via the money markets. This expands credit. It’s also over time clearly long-term borrowing on one end, funded by an on-demand depositor on the other, with some substantial obfuscation as to what is ultimately going on in between.