Why is FACS so important when it comes to measurements with a computer?

Validity is an issue that any measurement method must assure. How can you tell whether the measurements you make with your computational method are related to useful, important, or relevant aspects of facial behavior? You have to relate the new measurements to other facial measurements or to additional observations. You must either show the correspondence of the new facial measurements to some other measurements, such as FACS, that already have some established validity, or conduct a series of behavioral science research studies that show a correspondence between your new measurements and the concepts that underlie your measurements. This second alternative can be quite expensive.