Model Answers to 08 ANOVA in Six Sigma Statistics using Minitab 17, Green Belt Edition.

Exercise 8.5.3  ANOVA GLM

Compo has got a new toy. It’s an injection molding machine for making phone cases. Compo loads the machine with plastic granules and then sets the temperature and pressure and the molding machine forms the phone cases.

As Compo is prone to dropping things he wants to ensure the strength of the cases is maximised. He sets up an experiment where he varies three factors, the grade of the plastic granules, temperature of the heater and the pressure used for the extrusion. In this experiment Strength is the response and the factors are called Temp, Press and Material. The levels for Temp and Press are set using a dial which has fixed settings. This makes all the factors fixed. In the experiments Temp has 3 levels; 100, 110 &115 degc. Press has 3 levels; 7, 10 and 13 psi and there are two types of plastic granules called A & B.

Analyse the data in File 08 ANOVA.xlsx worksheet Ex 8.5.3 and answer the questions shown below. The data was collected randomly and is recorded in time order.

  1. Form the Main Effects Plot and Interactions plot. Can you guess which factors and interactions are significant?
  2. Which Factors and Interactions are significant when you use the ANOVA GLM ?
  3. What is the Regression Equation for this model? And how much of the variation in Strength does it account for?
  4. What settings would maximise the strength of the phone cases?
  5. DO the VIFs indicate any problems with the model?
  6. Do the residuals and Lack-of-Fit Test indicate any issues with your model ?

Set-Up 1

Analysis 1

1.  Click Stat<<ANOVA<< Main Effects Plot

2. Complete the menu as shown below and then click OK.

From the y-axis range exhibited between levels we can safely say that Pressure has the most effect on Strength, then Temp and then Material.

At this time we cannot say which of the factors are statistically significant.

Within all three sub-plots we can see that the lines are not parallel. This indicates that there are interactions present but we cannot say if they are significant at this time.

Set-Up 2

1.  Click Stat<<ANOVA<< Interactions Plot.

2. Complete the menu as shown below and then click OK.

Analysis 2

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