Lees score on his test was 90 The class average was 621 and

Lee\'s score on his test was 90. The class average was 62.1 and the standard deviation was 6.76. Is his score unusual? Why or why not?

Solution

His z score is

z = (x-u)/sigma = (90-62.1)/6.76 = 4.127218935

As his z score has absolute value greater than 2, then YES, HIS SCORE IS UNUSUAL. [ANSWER]

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If you use another standard, say, z > 3 to be considered unusual, please use that. Anyway, it will still be rare in that standard.