We all know that strength of a chain is as good as the strength of its weakest link. If a group of persons run together holding each other hand forming a chain, the speed of the group is determined by the speed of the slowest person. There is one such person/ link/ process/ resource that determines/ restricts the overall performance or the output of the system. If we strengthen any link in the chain other than the weakest one, the chain strength doesn't increase. Similarly, if we replace someone other than the slowest person in the group by a faster person, the overall speed doesn't improve. But, if the slowest person in the group is replaced by someone faster, the speed of the group improves.
This is what Theory of Constraints says. There will be a constraint. We need to identify that. To improve the performance, we need to work on that. We may have to augment resources on that and monitor. When it no longer remains a constraint, something else becomes a constraint. If we replace the slowest person with a faster one, next slowest determines the overall speed.
Here is an interesting exercise to identify constraint and see how constraint can change as we take actions to handle it.
There are 7 interlinked processes in a production system namely, P1, P2, P3, P4, P5, P6 and P7. Output of P1 and P2 are mixed in the ratio of 1:1.5 in P5. Output of P3 and P4 are mixed in the ratio of 1:2 in P6. Finally, output of P5 and P6 are mixed in the ratio of 2:3 in P7. Capacities of these processes are 200, 400, 200, 300, 300, 240 and 700 liters/day respectively. Volume of the content doesn't change during any of the processes. That means 10 liters from P1 mixed with 15 liters from P2 gives 25 liters in P5.
Find the capacity of this production system. Which process is restricting the final output? Where we need to work if we want to increase the overall output? Which process is the constraint right now? Change the capacity of P1 so that present constraint changes. What should be the minimum capacity of P6 in the original question to change the constraint?
This exercise takes us through all the stages described in the Theory of Constraints.