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I'm just new in the field of t.tests so I thought I will make some excersises, but the point I can't get clear is when to use a lower or upper tail t.test. For example the following example:

3 people which have high blood pressure get a medicine for a period of 4 weeks to lower the blood pressure.

What I think when I read this is:

  • $H_0$: blood pressure keeps the same
  • $H_A$: blood pressure gets LOWER

So I would think I have t use a t.test whith lower tail(I think it's called this way), I work in R the most of the time so I would use alt = "less". But according to my teacher it sould be alt="greater", but why?

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This depends on how you compute the paired sample difference.

before - after => greater

after - before => less

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