How do we resolve this dilemma? Time to Fill does tell us something important, yet we cannot be sure whether a change to this number is due to our own efforts or the effects of the overall job market.
The most frequently used analogy when it comes to measuring HR is that of driving a car without a speedometer: how would you drive if you did not know your speed? The deeper we look into HR measurement the less satisfying this analogy becomes. The basic premise that you need information to perform well is correct. However, when you have a speedometer and you are going too fast, your actions are obvious - you take your foot of the gas pedal. This direct link between information and action is not the case for HR.