I ask because most people use a PM for training and I haven't seen anything that suggests a whole lot of accuracy is needed. Its certainly nice to have but I'm not at all certain that it results in a fitter faster athlete.
Are you sure you are not confusing accuracy
ANY tool - no matter what it is, be it a powermeter or a drill or simple ruler - must have some accuracy in order to be useful. It does not necessarily have to be extremely precise, but some precision is necessary.
Easiest two examples: a ruler and a hammer.
A ruler is a measuring tool, correct? It tells you the dimensions of an object. You rely upon it's accuracy (ie, the lines on the ruler don't move), even if it doesn't have precision (the ruler you may have only goes to cm, not mm, or the lines on the ruler themselves have thickness, however the thickness is constant) If you had an inaccurate ruler, one where the lines don't remain at fixed locations on the ruler or if the lines are not placed at reliable, consistent intervals along the length of the ruler, would you still use that ruler?
A hammer. A hammer is a really simply tool - it is a weighted object with a blunt surface that enables object to be driven by transfer of force into or through a surface. You rely heavily upon its accuracy, not its precision, but you take the accuracy for granted. What would an inaccurate hammer look like? The head of the hammer would not have a level or hardened surface. Even a ball-pean hammer wouldn't have a ball end with any consistency of shape. The weighted end of the hammer might fluctuate throughout the swing of the hammer, meaning you may not necessarily have the force you intended to be placed at the end of the hammer's swing, but it may randomly end up closer to your hand. That is an inaccurate hammer. Would you use it?
A scale. We use scales a lot here at WW. We love them. We rely upon their accuracy, and if we have a nice scale it would have high precision as well. Let's see what an inaccurate scale would look like:
-You weigh yourself on a scale, say it comes out to 150lbs.
-Tomorrow you weigh yourself on the same scale, it comes out to 120lbs.
-Two days later you weight yourself on the same scale, it comes out to 140lbs.
That is an wildly inaccurate scale - there is no way, unless you lost a limb and grew part of it back like a lizard, that you would lose that much weight in 24hrs and gain half of it back in the next 24hrs.
However let's say you have a mildly inaccurate scale, one within a few percentage of reality. That scale tells you you are 151lbs the first day, 150lbs the second, and 151.5 the third. Is it in accurate? A little bit, but it is within an acceptable range of inaccuracy.
The issue being claimed against the Stages powermeter is too much inaccuracy. Can some inaccuracy be acceptable to use for training purposes? Yes. If it is too inaccurate however, to the point of being unreliable, then it is not good.
What came through from Rainmaker's test was that he compared Stages powermeter readings to other powermeters. Sort of like putting rulers next to each other. They may not each read "224 watts" - one might say "224" another might read "220" and another might read "225" but they are within an acceptable range of accuracy. Stages comes along and says "220" initially... "GREAT!" you think... but 5 minutes later, the other powermeters are reading 250, 252, 251 respectively, and Stages reads "140" That points to Stages having too much inaccuracy.
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