$${\chi ^2}$$ is calculated by the help of formula

A. $${\chi ^2} = \frac{{\sum {{\left( {{\text{O}} - {\text{E}}} \right)}^2}}}{{\text{E}}}$$

B. $${\chi ^2} = \frac{{\sum \left( {{\text{O}} - {\text{E}}} \right)}}{{{{\left( {\text{E}} \right)}^2}}}$$

C. $${\chi ^2} = \frac{{\sum {{\left( {{\text{O}} - {\text{E}}} \right)}^2}}}{{\sum {\text{O}}}}$$

D. $${\chi ^2} = \frac{{\sum {{\left( {{\text{E}} - {\text{O}}} \right)}^2}}}{{\sum {{\left( {\text{O}} \right)}^2}}}$$

Answer: Option A