How do you determine to use a one tailed or two tailed level of signifigance? What in the question do you base this off of?

If the null hypothesis is *μ* = *k*, it’s a 2-tail test.

If the null hypothesis is *μ* ≤ *k*, or the null hypothesis is *μ* ≥ *k*,it’s a 2-tail test.

If the null hypothesis is *μ* ≤ *k*, or the null hypothesis is *μ* ≥ *k*,it’s a 1-tail test. Also you would use a t-stat when the # of observation is ≤ 30. Correct ?