Principle (Efficiency): Pull is more efficient than push.
For a given WIP level, a pull system will attain a higher throughput level than will an equivalent (i.e., identical equipment) push line. The reason is that a pull system draws in WIP only when there is capacity to work on it. In contrast, a push system will sometimes force in WIP when the system is overloaded and therefore cannot work on it. This just elevates the average WIP level for little gain in throughput. Likewise, push will sometimes fail to put inventory into the system when it is needed, causing a loss of throughput. The net result is that because releases are better synchronized to the actual rate of the line, a pull system makes better use of its WIP.
A consequence of this law is that, all other things being equal, a pull
line will be more profitable than a push line. For example, suppose
profit is given by
Profit = p×throughput + h×WIP
where p is a unit profit (not including inventory holding
costs) and h is an annual inventory holding cost. Then, since
for any WIP level pull will achieve a higher throughput than push, it follows
that the profit of the push line must be higher than that of the pull line.
Click here for an illustration of this.
The following graph shows the qualitative relationship between push
and pull system in terms of the throughput versus WIP curve.
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