Impurities in the form of iron oxides lower the economic value and usefulness of industrial minerals, such as kaolins, to ceramic and paper-processing industries. A 24 experiment was conducted to assess the effects of four factors on the percentage of iron removed from kaolin samples ("Factorial Experiments in the Development of a Kaolin Bleaching Process Using Thiourea in Sulphuric Acid Solutions," Hydrometallurgy, 1997: 181-197). The factors and their levels are listed in the following table:

The data from an un-replicated 24 experiment is listed in the next table.

a. Calculate estimates of all main effects and two-factor interaction effects for this experiment.
b. Create a probability plot of the effects. Which effects appear to be important?

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Low High Factor Description Units Leve Level A H,SO Thiourea .10 L 0.0 5.0 30 C Temperature ℃ 70 D Time EN 150 Iron Extraction Test Extraction Run Test Run 28 51 12 21 41 27 48 ad AH abd exd acd bcd abcd ab 57 70 95 ac bx: abc