Chapter 3, Problem 9CS

Electronic devices contain electric circuits etched into wafers made of silicon. These silicon wafers are sealed with an ultrathin layer of silicon dioxide, in a process known as oxidation. This can be done with either new or recycled wafers.

In a study described in the book Statistical Case Studies for Industrial Process Improvement by V. Czitrom and P. Spagon, both new and recycled wafers were oxidized, and the thicknesses of the layers were measured to determine whether they tended to differ between the two types of wafers. Recycled wafers are cheaper than new wafer, so the hope was that they would perform at least as well as the new wafers. Following are 36 thickness measurements (in tenth s of a nanometer) for both new and recycled wafers.

Suppose that it is desired to use the type of wafer whose distribution has less spread. Write a brief paragraph that explains which type of wafer to use and why. Which measure is more useful for spread in this case, the standard deviation or the interquartile range? Explain.