The HJ test is a modified version of the Baek and Brock (1992) test for conditional independence,
with critical values based on asymptotic theory. To motivate the test statistic it is convenient to restate
the null hypothesis in terms of ratios of joint distributions. Under the null the conditional distribution
of Z given (X, Y ) = (x, y) is the same as that of Z given Y = y only, so that the joint probability
density function fX,Y,Z (x, y, z) and its marginals must satisfy
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