Question

When would you use a one-sided alternative hypothesis?

Answer 1

In short: If the other side is not important or not possible.

Example:
You want to examine whether "brain gym" (a mixture of small mental and physical exercises) will improve your pupils' scores. They cannot harm their performances, as they're not tiring or confusing at all.

In that case you may set up a control group or do a before/after test, whatever.

• Your zero-hypothesis is that there will be no difference.
• Your alternative hypothesis is that there will be improvement.

Then you do all your testing and measuring and, following all the statistical rules, you decide whether the difference is significant or not.