Why is external jugular vein not preferred for the evaluation of jugular venous pressure?

External jugular vein has a valve at the point of entry into subclavian vein to prevent backflow of blood from subclavian vein which usually has a higher pressure. Moreover, external jugular vein is not directly in line with right atrium and superior vena cava. That is why it is taught that jugular venous pressure and pulsations should be assessed from the pulsations of the internal jugular veins. Beginners in clinical medicine often look at the external jugular vein for this purpose as it is easy to see. Of course, distension of external jugular vein does occur when the right atrial pressure is elevated. That is because drainage into subclavian vein becomes difficult, resulting in passive elevation of pressure in the external jugular vein.