The tensile strength of Portland cement is being studied. Four different mixing techniques can be used economically.

A completely randomized experiment was conducted and the following data were collected:

Mixing Technique Tensile Strength (lb/in2)

1 3129 3000 2865 2890 2 3200 3300 2975 3150 3 2800 2900 2985 3050 4 2600 2700 2600 2765

(a) Test the hypothesis that mixing techniques affect the strength of the cement. Use ???? = 0.05.

(b) Construct a graphical display as described in Section 3.5.3 to compare the mean tensile strengths for the four mixing techniques. What are your conclusions?

(c) Use the Fisher LSD method with ???? = 0.05 to make comparisons between pairs of means.

(d) Construct a normal probability plot of the residuals.

What conclusion would you draw about the validity of the normality assumption?

(e) Plot the residuals versus the predicted tensile strength.

Comment on the plot.

(f) Prepare a scatter plot of the results to aid the interpretation of the results of this experiment.