6 Q Test for Bad Data ( Harris, Daniel, Exploring Chemical Analysis, 1 st , New York, 1997, 62) There was always somebody in my lab section with four thumbs. Freshmen at Phillips University perform an experiment in which they dissolve the zinc from a galvanized nail and measure the mass lost by the nail to tell how much of the nail was zinc. Several students performed the experiment in triplicate and pooled their results: It appears that Cheryl might be the person with four thumbs, because her value 7.8 looks out of line with the other data. Should the group reject the value 7.8 before averaging the rest of the data or should 7.8 be retained? Equation 6 The range is the total spread of the data. The gap is the difference between the questionable point and the nearest value. If Q calculated from Equation 6 is greater than Q in Table 4 , the questionable point should be discarded. For the numbers above, Q = 1.4/3.8 = 0.37. In Table 4, the critical value of Q at the 90% confidence limit is 0.38 for 12
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