This measurement can also be made with a goniometer eyepiece, in which a row of parallel double-marks are used instead of the cross threads.
The fact they blinded the goniometer was significant, as they reduced bias.
Another negative aspect is that when the testers took the measurement they gave the goniometer to the model to record their readings.
All subjects used the same goniometer for all three measurements.
goniometer heads may not be enough for some severe cases.