> you need to use a food scale or something that measure the volume of food

Isn't that obvious? Basic high school science projects would have students using measuring devices. Are you saying that it's common for nutritional studies to tell people to eyeball their portions and that is then used as actual data?

I see from the article "Nutritional epidemiology studies typically ask people to keep a food diary or complete questionnaires about their intake over the past 24 hours, a week, or even several months." I find that hard to believe. How could any study like that be taken seriously? That's like having someone stand at a street corner for an hour and observe the population to then come up with an average BMI for the neighbourhood.