A statistical model is often a parametrized family of probability density functions or probability mass functions {displaystyle f(x| heta )} f(x| heta ). A simple-vs.-simple hypothesis test has completely specified models under both the null and alternative hypotheses, which for convenience are written in terms of fixed values of a notional parameter {displaystyle heta } heta :
Under the Wald statistical test, the maximum likelihood estimate {displaystyle {hat { heta }}} {hat { heta }} of the parameter(s) of interest {displaystyle heta } heta is compared with the proposed value {displaystyle heta _{0}} heta _{0}, with the assumption that the difference between the two will be approximately normally distributed. Typically the square of the difference is compared to a chi-squared distribution.