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The sum of $100$ observations and the sum of their squares are $400$ and $2475$, respectively. Later on, three observations, $3, 4$ and $5$, were found to be incorrect . If the incorrect observations are omitted, then the variance of the remaining observations is
$8.25$
$8.50$
$8$
$9$
Solution
$\sum\limits_{i = 1}^{100} {{x_i}} = 400$ $\sum\limits_{i = 1}^{100} {x_i^2} = 2475$
Variance
${\sigma ^2} = \frac{{\sum {x_i^2} }}{N} – {\left( {\frac{{\sum {{x_i}} }}{N}} \right)^2}$
$ = \frac{{2475}}{{97}} – {\left( {\frac{{388}}{{97}}} \right)^2}$
$ = \frac{{2425 – 1552}}{{97}} = \frac{{873}}{{97}} = 9$
Similar Questions
Let the mean and variance of the frequency distribution
$\mathrm{x}$ | $\mathrm{x}_{1}=2$ | $\mathrm{x}_{2}=6$ | $\mathrm{x}_{3}=8$ | $\mathrm{x}_{4}=9$ |
$\mathrm{f}$ | $4$ | $4$ | $\alpha$ | $\beta$ |
be $6$ and $6.8$ respectively. If $x_{3}$ is changed from $8$ to $7 ,$ then the mean for the new data will be: