August 2004 www.pqsystems.com Vol. 6, No. 8
 Quality Quiz Professor Cleary’s Quality Quiz is the most-frequently-visited feature of the PQ Systems Quality eLine each month, with David Schwinn’s Six Sigma and More column not far behind. Last month, Marge Orrine (nee Marge N. Avera), was lost in the middle. That is, she applied the term “average” without understanding various measures of central location (mean, median, mode). She failed to make a breakthrough on her goal of saving Natural Dairy Products lots of money by addressing a cost-wasting process and bringing about improvement, but she is so far undaunted in her quest for a Black Belt. Since Marge spent a good deal of effort in collecting data related to the weight of sticks of butter emerging from NDP’s packaging line, she wants to use the same data in a way that will impress her boss and help her reach her own goal. “Standard deviation ought to do it,” she muses, reading a description of this statistical concept. Taking the first three numbers of her data set (7, 10, 13), she enters them on her calculator programmed to figure standard deviation. The response is 3, but she has no idea what this means. Marge’s old statistics textbook shows a formula for deriving standard deviation: Miles Aweigh, the head of Marge’s department, notices her calculation and asks her what it means. Marge carefully explains: “Standard deviation takes each data point and subtracts it from the mean, squaring it, then dividing it by the sample size, minus one.” Miles nods his head thoughtfully, then asks why she doesn’t take the log of each deviation. “That’s obvious,” she responds. “The log is only half of the semi-log.“ Is she right, in her explanation of this calculation, or is she offering specious math drivel? Yes, Marge's explanation is correct. No, she is offering specious math drivel.