Even if you can quantify something, it may be hard to get the data. It may be easy to get data on loan approval and race, but hard to get data on credit-worthiness. You'll see a positive correlation is you don't use a randomized selection procedure, since very sick people will also get more powerful medicine. You observe a positive correlation between money and output. You need a randomized selection procedure to get good results.
Married males make more money than unmarried males.