NCERT Solutions
Class 11 Maths
Sequences and Series

Ex.Misc.Q.7
The mean and standard deviation of a group of 100 observations were found to be 20 and 3, respectively.
Later on, it was found that three observations were incorrect, which were recorded as 21, 21 and 18.
Find the mean and standard deviation if the incorrect observations are omitted.
Number of observations (n) = 100
Incorrect mean (x ) = 20
Incorrect standard deviation (σ) = 3
=> 20 = (1 ÷ 20) × Σi=1100 xi
=> Σi=1100 xi = 20 × 100 = 2000
So, incorrect sum of observations = 2000
Correct sum of observations = 2000 – 21 – 21 – 18 = 2000 – 60 = 1940
Correct mean = correct sum ÷ (100 - 3) = 1940 ÷ 97 = 20
Standard deviation σ = √ [(1 ÷ n) × Σi=1n xi2 – (Σi=1n xi)2 ÷ n2]
=> 3 = √ [(1 ÷ n) × incorrect Σ xi2 – (x )2]
=> 3 = √ [(1 ÷ 100) × incorrect Σ xi2 – (20)2]
=> 9 = (1 ÷ 100) × incorrect Σ xi2 – 400
=> incorrect Σ xi2 = 100(9 + 400)
=> incorrect Σ xi2 = 40900
So, correct Σi=1n xi2 = incorrect Σi=1n xi2 – 212 - 212 - 182
= 40900 – 441 – 441 - 324
= 39694
So, Correct Standard deviation = √ [{correct Σ xi2 ÷ n} – (correct mean)2]
= √ [(39694 ÷ 97) – (20)2]
= √ [409.216 – 400]
= √9.216
= 3.036