double expected_value = xt->row_tot[r] * xt->col_tot[c] / xt->total;
double residual = *mp - expected_value;
double sresidual = residual / sqrt (expected_value);
double expected_value = xt->row_tot[r] * xt->col_tot[c] / xt->total;
double residual = *mp - expected_value;
double sresidual = residual / sqrt (expected_value);