Grubbs Test for an Outlier

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  

Cheryl s value 7.8 looks out of line from the other data. A datum that is far from the other points is called an outlier. Should the group reject 7.8 before averaging the rest of the data or should 7.8 be retained  

We answer this question with the Grubbs test. First compute the average ( ) and the standard deviation (5) of the complete data set (all 12 points in this example)  

If Gcaicuiatsd Stable, reject the ques-tionable point. 

Gcaicuiaied = I questionable value -mean j Is. If Gcaicuiated tabie. the value in question can be rejected with 95% confidence. Values in this table are for a one-tail test, as recommended by the American Society for Testing and Materials (ASTM). 

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In order to use Grubbs test for an outlier, that is to test Ho all measurements come from the same population, the statistic G is calculated ... 

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