How can you determine if you are using a two tailed test vs a one tailed test? For example, for the below, which test would I use?

Question

Based on this data, at α =.01, can we conclude that the proportion of democrats who favor “junk food" tax is more than 5% higher than proportion of republicans who favor such a tax?

Answer 1

For one-tailed test, you are only looking for deviation in one direction, indicated by your alternative hypothesis.

Ho: a = b
Ha: a > b or a < b

For a two-tailed test, you are just looking for any difference.

Ho: a = b
Ha: a ≠ b

I hope this helps.