The flow of knowledge is often from practice to science. For instance, it was known that aspirin could alleviate pain for many decades before scientists discove

The flow of knowledge is often from practice to science. For instance, it was known that aspirin could alleviate pain for many decades before scientists discovered why. Engineers can make their work science-based because the sorts of phenomena they are dealing with are well-understood by the scientists. With psychological phenomena, the scientists have a much poorer understanding, and so you have to go by what experiment shows works.