# Risk-free rate for ex-post evaluation of investment strategy

When evaluating the strategy ex-post using e.g. Sharpe ratio, what should one use as the risk-free rate? Let's suppose I am using a 1Y sample of weekly returns, sampled between 2012-01-01 and 2012-12-31. Should I use the 1Y risk-free rate as of 2012-01-01, an average of the 1W risk-free rates as of the beginning of each week in 2012, or subtract each 1W risk-free rate from the corresponding strategy return each week before averaging/calculating variance?

My intuition was to do the last, but on the other hand with the interest rates floating, this hardly makes it the "risk-free" option?

-