For The Journal Business people complain loudly about junk science in the law. By junk science, they mean expert testimony that, at times, seems to be invented rather than scientific.
Fact witnesses testify to what they see and hear.
They don't offer opinions. There are, of course, a few exceptions. Eyeball witnesses can estimate the speed of a car in an accident or the value of the family home or other owned real estate. For the most part, however, the law confines people to testimony about what they actually saw or heard. Expert witnesses can express opinions.