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Question

For a group of 200 candidates, the mean and standard deviations of scores were found to be 40 and 15 respectively. Later on it was discovered that the scores of 43 and 35 were misread as 34 and 53 respectively. Find the correct mean and standard deviation.

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Solution

We have:
n=200, X¯=40, σ =15

1nxi =X¯1200xi =40 xi =40×200 =8000 Since the score was misread, this sum is incorrect. Corrected xi =8000 -34-53+43+35 = 8000-7 =7993 Corrected mean =Correctedxi200 =7993200 =39.955


SD =σ = 15 Variance= 152 =225According to the formula,Variance =1nxi2-1nxi21200xi2-402 =2251200xi2-1600 =225xi2=200×1825 =365000 This is an incorrect reading.Corrected xi2 =365000-342-532+432+352 =365000-1156-2809+1849+1225 =364109


Corrected variance=1n× Correctedxi -Corrected mean2 =1200×364109 -39.9552 = 1820.545 -1596.402 = 224.14 Corrected SD =Corrected variance = 224.14 =14.97

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