Let's assume that we have computed T-ratio for some data: $t = -0.9054 $ And our $\alpha = 0.05 $ Now, we would like find rejection region for this left-tail. The author wrote, that rejection region is: $(-\infty, -z_{\alpha}]$. Where $z_\alpha $ is $\alpha$ -quantile in normal distribution.
I don't understand why did author use Normal Distribution, after all t comes from T-Distribution.
Thanks in advance.