Average investment balances and interest rates are summarized below:

       
For the three months ended November 30, For the six months ended

November 30,
$ in millions 2012   2011 Change 2012   2011 Change
Average investment balances:
Funds held for clients $ 3,231.2 $ 3,107.1 4 % $ 3,266.6 $ 3,182.8 3 %
Corporate investments $ 773.9 $ 645.8 20 % $ 770.1 $ 646.3 19 %
 
Average interest rates earned (exclusive of net realized gains):
Funds held for clients 1.2 % 1.4 % 1.2 % 1.4 %
Corporate investments 1.0 % 0.9 % 1.0 % 0.9 %
 
Total net realized gains $ 0.1 $ 0.1 $ 0.3 $ 0.2
 

Our investment strategy focuses on protecting principal and optimizing liquidity. Yields on high quality instruments remain low, negatively impacting our income earned on funds held for clients and corporate investments. A substantial portion of our portfolio is invested in high credit quality securities with AAA and AA ratings, and with A-1/P-1 ratings on short-term securities.

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