Q Test for Bad Data

Sometimes one datum is inconsistent with the remaining data. You can use the Q test to help decide whether to retain or discard a questionable datum. Consider the five results 12.53, 12.56, 12.47, 12.67, and 12.48. Is 12.67 a bad point To apply the Q test, arrange the data in order of increasing value and calculate Q. defined as 

The range is the total spread of the data. The gap is the difference between the questionable point and the nearest value. 

If Calculated Gtabie If16 questionable point should be discarded. In the preceding example. calculated = 0.11/0.20 = 0.55. In Table 4-5, we find CW = 0.64. Because Qcalculaled Qtabie the questionable point should be retained. There is more than a 10% chance that the value 12.67 is a member of the same population as the other four numbers. 

Some people would argue that you should never discard a datum unless you know that there was an error in the procedure that led to that particular measurement. Others would repeat the questionable measurement several more times to gain higher confidence that one measurement is really out of line (or not). The decision is yours and it is subjective. 

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