In this installment I would like to describe a first set of small experiments we can make using our simulation program, and show what we can learn out of it. This is by no means an exhaustive list – it serves to encourage you to try it out for yourselves.
First a quick description of the program: in order to make the effects more visible we made some changes to the original dice game by making the difference between the high and low variation stronger. Now, in the simulation, each workstation will process up to 50 pieces per round, the actual processed number being uniformly distributed between 1 and 50 for the high variance case and between 24 and 27 in the low variance case.
The average number of processed parts and the variance is, using the formulae for the uniform distribution, mean=(a+b)/2 and Variance=(b-a)^2/12 where a and b are the endpoints of the uniform distribution. This will give us for the high variance case a mean of (1+50)/2 = 25.5 and for the low variance case a mean of (24+27)/2=25.5 . The variances however will differ – in the first case we will have V=(50-1)^2/12 = 200 for the high and (27-24)^2/12=0.7 for the low variance case.
The simulation has a graphical UI that is shown in the graph. We can select the number of the workstations and the number of simulation runs in increments of 500. We can also chose the line feed strategy – that is the way the new orders come into the system. Random Feed means that they come in the same way, uniformly distributed, with high variance between 1 and 50 while Regular Mean Feed means that we feed the line with the average number of orders the line can handle 25.5 per run , with a low variance.
There is also a button called re-run which will just create a new run with the same settings as the previous run. This is for our first experiments the most important button because by re-running the same setup we can really see the influence of randomness.
We have 3 output graphs :
- The number of WIP for each run of the simulation. This is the number of work items that are somewhere inside the production line after the end of a run – the number of semi-finished goods.
- A graph describing the throughput of the line. This is a bit more tricky – in order to clearly see the differences we do not display the number of finished pieces per run but the deviation from the expected number of finished goods per run. So a value of 0 would roughly mean 25 finished pieces, a value of 20 means 25+20=45 finished pieces while a value of -20 means 5 finished pieces. While this takes a bit of getting used to, this graph shows the deviations from the expectation much more clearly then the raw numbers.
- The number of WIP at each workstation at the end of the simulation. This can give us an idea where we have an accumulation of semi-finished goods and this is often proposed as a way of finding the bottleneck on a line. We will soon know better.
So, let us run a first simulation! I set the number of workstations at 5 , for 1000 runs. Now, I would be interested in the effects of randomness, so I pick the worst scenario – highly variable demand and higly variable machines. The output is on the graph.
Remember, from traditional point of view this is a nicely balanced line. Still, after 1000 runs, we have almost 2000 pieces scattered around production (graph 1) and our throughput was more or less unpredictable between 20% (5 pieces) of the expected volume and 180% (45 pieces) .Obviously we had some good and some bad days without any chance of predicting what the next day will bring. By looking at the last graph we can readily identify the bottleneck – workstation 2 with 3 trailing quite close behind it. So, to improve the line performance we need to focus on these two, right?
Before investing resources on improvement work, press the Re-run button. You will obviously have different results, but very likely something similar will happen to what I see now – the bottleneck at 2 disappeared and this workstation now has the best performance, while workstation 1 accumulated the most WIP. Obviously the threat of starting an improvement project at their workstation worked wonders – however at workstation 1 people became complacent and need more supervision . Also, the WIP in general has evolved in a much better way, we had a long stretch of runs where the WIP actually decreased and we are now at about 1500 pieces instead of 2000 !
So, let us press Re-run again: the WIP trend is a complete mess now, steadily increasing to about 2500 at the end of the runs and the workstation 2 started to accumulate more WIP , however the workstation 1 is still the black sheep. Press again : workstation 1 has improved , however 2 became the bottleneck again, while our focus on the overall WIP trend payed off and we are again down at about 1500 pieces.
The one thing that did not change across all these simulations was the daily performance – it stayed just as variable in all 3 re-runs.
Of course this is all a joke – there is no way the simulation would react to the threat of an improvement project. What we see is the effect of random variations in the line – and the most visible is the random movement of the “bottleneck”. This is the core problem of the Theory of Constraints methodology by the way – we hear very often that the a line does not have a single bottleneck, but “moving bottlenecks”. This is the effect of random variations in the line.
The second visible effect is the accumulation of WIP. We would naively expect no WIP or very little, as the line is balanced on the average. In reality the random variations cause ever larger queues at random locations in our balanced line and the prediction of the factory physics formulae is that the WIP will continuously increase as a result of these random variations. Looking at the graphs, it does seem that the WIP is kind of levelling off and stabilizing at around 2000 pieces but this is an illusion. If you change the number of runs to 2000 and higher you will see that the increase is continuous.
The third, even more worrying effect is the high variability of the throughput. This line, though balanced, can give no guarantee of the number of pieces produced per day (run), despite the best efforts of the production people. So, what will happen in real life? Important orders will be moved forward (expedited) while less important ones will stay in the WIP – all this will create even more randomness and increase the negative effects we already see. I think this is not unlike many situations we see in production lines today.
As we are not trained to discern random effects we will necessarily come up with explanations for purely random phenomena – like my example of the threat of running an improvement project having positive effects or people becoming complacent. This is also something we see very often in real life.
So, how would you investigate our line further? Please feel free to try.