$$
{p = \frac{fatalities\_current\_year + fatalities\_previous\_year}{population\_current\_year + population\_previous\_year}} {\qquad\qquad} {\left( 1 \right)}
$$   
$$
z = \frac{{\left( {\frac{fatalities\_current\_year}{population\_current\_year}}-{\frac{fatalities\_previous\_year}{population\_previous\_year}} \right)}}{\sqrt{{p}*{(1-p)}*{{\left( {\frac{1}{population\_current\_year}}+{\frac{1}{population\_previous_\_year}}\right)}}}} {\qquad\qquad} {\left( 2 \right)}
$$