急問when do we use one or two tailed test in Hypothesis Test?

1-tail /2-tail is related to Ha.

Ho usually is " = "

When Ha is " not = " , we use 2-tail.

When Ha is " >" we use 1-tail.

Whaen Ha is " < ", we also use 1-tail.

It relates to the setting of Ha.

If Ha <> (NOT EQIUAL) 50 (say), then it is two tailed test.

If Ha > / < 50 (say), then it is one tailed test.

You may ask when do we set the above hypothesis, then you should refer to the question.