To use extremes, if 1% of a population has a PHD then holding a PHD may be a proxy for other desired things. If 99% of a population has a PHD then it's less useful as a proxy.
PS: On it's own a few years of post secondary education don't actually mean much, so a large chunk of the value is as a proxy.
To use extremes, if 1% of a population has a PHD then holding a PHD may be a proxy for other desired things. If 99% of a population has a PHD then it's less useful as a proxy.
PS: On it's own a few years of post secondary education don't actually mean much, so a large chunk of the value is as a proxy.