To estimate how much time something takes on average you need to repeat and measure it many times. For instance if you had a clear unambiguous spec of what the

To estimate how much time something takes on average you need to repeat and measure it many times. For instance if you had a clear unambiguous spec of what the the software must do, you could give it to 10 different teams and measure how long each took. If you then modify the spec even slightly you could make it impossible to create such a program no matter how long you work on it. But every project has its own different spec. We don't know how long it will take until we try to do it at least once. Researches might study how long it takes on average to implement a given specific spec by different teams. But they can not estimate how much it would take to implement any arbitrary spec because - nobody has every tried to implement that spec before. And studying that tells you very little about how much time and effort is needed to implement any other arbitrary spec.