Question
Table below shows the frequency $f$ with which $'x'$ alpha particles radiated from a diskette:
$x$ $0$ $1$ $2$ $3$ $4$ $5$ $6$ $7$ $8$ $9$ $10$ $11$ $12$
$f$ $51$ $203$ $383$ $525$ $532$ $408$ $273$ $139$ $43$ $27$ $10$ $4$ $2$
Calculate the mean and variance.

Answer

Mean, $\overline{\text{x}}=\frac{\sum\text{f}_{\text{i}}\text{x}_{\text{i}}}{\sum\text{f}_{\text{i}}}=\frac{10078}{2600}=3.88$
$x_i$
$f_i$
$f_ix_i$
$\text{x}_{\text{i}}-\overline{\text{X}}$
$\big(\text{x}_{\text{i}}-\overline{\text{X}}\big)^2$
$\text{f}_{\text{i}}\big(\text{x}_{\text{i}}-\overline{\text{X}}\big)^2$
$0$
$51$
$0$
$-3.88$
$15.05$
$767.55$
$1$
$203$
$203$
$-2.88$
$8.29$
$1682.87$
$2$
$383$
$766$
$-1.88$
$3.53$
$1351.99$
$3$
$525$
$1575$
$-0.88$
$0.77$
$404.25$
$4$
$532$
$2128$
$0.12$ 
$0.014$
$7.448$
$5$
$408$
$2040$
$1.12$
$1.25$
$510$
$6$
$273$
$1638$
$2.12$
$4.49$
$1225.77$
$7$
$139$
$973$
$3.12$
$9.73$
$1352.47$
$8$
$43$
$344$
$4.12$
$16.97$
$729.71$
$9$
$27$
$243$
$5.12$
$26.21$
$707.67$
$10$
$10$
$100$
$6.12$
$37.45$
$374.5$
$11$
$4$
$44$
$7.12$
$50.69$
$202.76$
$12$
$2$
$24$
$8.12$
$65.93$
$131.86$
 
$\sum\text{f}_{\text{i}}=\text{N}=2600$
$\sum\text{f}_{\text{i}}\text{x}_{\text{i}}=10078$
 
 
$\sum\text{f}_{\text{i}}\big(\text{x}_{\text{i}}-\overline{\text{X}}\big)^2=9448.848$
Variance, $\sigma^2=\frac{\sum\text{f}_{\text{i}}\big(\text{x}_{\text{i}}-\overline{\text{X}}\big)^2​​}{{\text{N}}}=\frac{9448.848}{2600}=3.63$

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