Crush Ratio = Thin Pipe / WhatGoesIn
The way value streams are presented often implies that the flow is smooth and all work flowing through the value stream gets done. This may be close to the truth in manufacturing, but it certainly should not be the case in solution development. The Rock Crusher assumes that flow is initially turbulent and must be stabilized and throttled. Since what goes in, must come out, we provide two ways out of the Rock Crusher: the thin pipe or the waste gate. These two ways out give rise to a metric we call the crush ratio, a measure of how many of the rocks entering the Rock Crusher are ultimately pulled through the thin pipe to create a solution increment.
Like all metrics, the crush ratio is just an indicator and not an explanation. It is one more piece of data that may help us create a hypothesis of what is happening in the value stream.
To learn more about Rock Crusher metrics, see chapter 13 Is it Working? Rock Crusher Metrics