The WIP-to-SWIP ratio is a simple comparison of a process, line or cell’s actual work-in-process count versus its standard work-in-process inventory (SWIP). Among other things, a process’ target condition reflects the consistent execution of standardized work, including SWIP maintenance (which is why it should be a leader standard work audit point). No SWIP maintenance, no standardized work adherence. Accordingly, the target WIP-to-SWIP ratio is 1.
A ratio less than 1 indicates that, at least during the time of the WIP count, the WIP level is insufficient. A ratio greater than 1 reflects a situation in which there is excess WIP. Daily or even more frequent tracking on a run chart or plan versus actual type chart can be incorporated within the natural work team’s visual metrics. Of course, this does not preclude an appropriate andon pull and response. Abnormal conditions should be quickly flagged and the root causes identified and addressed.
Depending upon the potential for normal variation in SWIP, the lean practitioner may want to apply tolerances to determine whether or not there is truly a problem. For example, if a cell’s SWIP is 4 units with strict one piece flow, a WIP count that does not exactly match that target is significantly abnormal. However, a line with 15 units of SWIP, may be “allowed” 15, plus 0, minus 3 units to accommodate normal fluctuation (due to work content variation) because the line has an internal FIFO lane with a maximum level of 5 and a minimum of 2.
The math follows: