Duration is rated as either advertised or actual time off of the seat that the valve "sees". Advertised is the most used to sell the cam. Effective is mostly rated for effective use for flow numbers. I have seen manufacturers like Comp Cams use .020 (which is very useful for Race Motors). Your cam card and the people you talk to will use the .050 spec for an effective duration. It is widely known that the valve (especially intake) does not start to react to proper flow until it lifts itself off of the seat and it reaches this spec. .020 is useful and .050 is useful for degreeing the cam because of ramp speed and a lot of heads really do offer a flow that's effective at that start point. You can have a cam rated at .294 duration but doesn't start to do it's job until the valve raises off at .050 therefore you should consider that duration spec not advertised. Others can do a more effective job coming right off the seat at .020 or 010. It's experience and understanding designs of the profile to get your combo just right for your engine. This also helps you understand where to place centerlines etc, when degreeing this piece.