The Cp / Cpk rice weighing challenge is a nice challenge in which the participants learn in a practical manner the meaning of cp and Cpk. The challenge is simple. You have to fill cups with rice using measuring spoons and determine the Cp / Cpk values of their weight. By using the PDCA cycle you will have to look for opportunities to improve the Cp/Cpk values.
Do you reach the 1.33 value?
What do we need?
In order to run the Cp / Cpk rice weighing challenge you will need:
- 1 kg dry rice per team
- Different measuring spoons. Any set will do. You could use for example a set with 1, 1/2, 1/4, 1/8 and 1/16 tbsp (15ml to 0.9375 ml). You will need preferable one set per team.
- plastic cups, 10 per team
- A cooking weighing scale (the scale has to be accurate within +/- 1 gram)
Divide the participants in groups of maximum 4 persons. You should have at least 2 groups.
Each team gets one measuring spoon. If you have two teams. One team gets the biggest available measuring spoon (1 tbsp) and the other team gets the smallest available measuring spoon. (1/16 tbsp). If you have more teams you can use also the in between spoons.
Give each team 10 plastic cups that they have to fill with rice.
rules and execution
Each team has to fill each cup with 5 tbsp of rice (= 75 ml). Don't go into detail whether they should level the spoon or not. This means that the team with the 1 tbsp spoon has to use 5 times the spoon and the team with the 1/16 spoon has to use 80 times the spoon. Keep the instruction vague. It gives room for improvement later on. Just tell them use X times the spoon. Where X depends on the measuring spoon you have given to the team.
the cups should be filled alternating between the different team members
The teams are only allowed to measure the cups once all the cups are filled. You can allow them to zero the scale with an empty cup fi they ask for it, but don't propose it to them. If they forget to do it you can use it as a nice example to explain the difference between Cp and Cpk.
The target is 60 grams +/- 10% (54 to 66). You can announce this before they start filling the cups. Please note that the target is based on a "standard" bag of rice where the volume is 1.25 liter per Kg. Depending on which rice you buy you can adjust the target. The volume and weight are often displayed on the packaging. Don't worry if they are not displayed. You can just use the target of 60 grams. It will only contribute to a bigger difference between the Cp and Cpk.
Let the teams calculate the Cp and Cpk and discuss the results together. Explain them that Cp and Cpk are 2 statistical parameters that indicate how stable the process is.
Which team scored the best result and why?
Explain the teams that we can use the Cp and Cpk values for root cause analysis.
The Cp and Cpk values gives us an idea on the variation of our process. The lower the variation the higher the Cp and Cpk values.
The difference between the Cp and Cpk values is an indication on how centered the process is. In other words how well are we hitting the target. The smaller the difference the closer we are on the target.
In round 2 the teams do exactly the same exercise. But they have to improve the Cp and Cpk values.
In order to do that they can use any measuring cup available and adapt their way of working if they want to. It will be allowed that only one team member fills all the cups.
Let the teams work together if you have only one set of measuring cups.
The teams have again to fill 10 plastic cups with rice.
They can only measure the cups after they are all filled.
Let the teams calculate the Cp and Cpk again and discuss the results together.
Was there an improvement? Why (or Why not?)
There are several lessons learned that you could observe during the Cp / Cpk rice weighing Challenge:
The process variation is influenced by different factors:
- The spoon size. The smaller the measuring spoon the more variation you will get in the process. And thus the Cp, Cpk value will be lower.
- different ways of working between team members. for example, one team member levels the spoon after filling it and another not.
- The accuracy of the scale.
Even if the process is performed with low variation the mean could be far away from the target. This will cause a difference between the Cp and Cpk values. There could be different reasons for this:
- not putting the scale on zero with an empty cup on
- not using "standard rice"
- not levelling the spoons after filling them in a consistent way
Shifting the mean is often easily done in this challenge. You can use a combination of measuring spoons to shift the mean.
In order to reduce the variation you have to standardize the way of working. You have to fill the cup with as less spoons as possible and level the spoons in order to have a consist amount of rice in them.
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