By market yield, we mean the average interest rate of all the bonds that constitute a fund's portfolio. It is as simple as that, and there is no need for you to waste too much time analyzing historical performance data. Of course, most fund companies will tell you that this is an oversimplification, and that your portfolio manager will add value by playing the yield curve. In theory, this is true. But in practice, it seems to be rarely the case.
Fund managers know that playing the yield curve is a dangerous game.
They also know that there is a scarce supply of investment grade of corporate and government bonds in the market. So they tend to keep stable portfolios. This is why there is a very limited potential for value-added from active portfolio management. The best source, as usual, is the fund company itself. The market yield reflects the actual interest that you will be collecting, as determined by the market price you pay for the bond.