Approximations for the Percentage Points of the Chi-Squared Distribution
Jerrold H. Zar · Journal of the Royal Statistical Society Series C (Applied Statistics) · 1978
Sixteen formulae for approximating percentage points of X2 were examined at each of 15 significance levels. The formula of Wilson and Hilferty is fairly good at most significance levels, but the best approximations without resorting to computer capability are obtained using the Cornish‐Fisher expansion or an empirically derived improvement of the Severo‐Zelen modification of the Wilson‐Hilferty formula.