One-Tailed or Two-Tailed

Question

An experiment studied the effect of diet on blood pressure. Researchers randomly divided 54 healthy adults into two groups. One group received a calcium supplement. The other received a placebo. Blood pressure was measured at the end of one month.

is this one tailed or two tailed and why

Answer 1

I take it here that what we are trying to measure is "does a calcium supplement effect blood pressure?" or a null-hypothesis of "a calcium supplement in one's diet will not effect blood pressure."

A one-tailed test is used when you are only looking to see deviation in one direction; ie, we expect if there is an effect, we only care if it is *higher* or *lower* than the placebo.

A two-tailed test is used when you want to see if there is an effect in either direction (ie you care if it either is lower or higher than the placebo group).

From your wording here, which only asks if there is an effect-not if it it raises blood pressure (or alternately lowers blood pressure), i would say this is looking for a two-tailed test, because that's how you'd find an effect on either end.

That said, I'm basing this purely off of how you've worded the prompt, so take with a grain of salt!

Here's a link to a more thorough explanation: https://stats.oarc.ucla.edu/other/mult-pkg/faq/general/faq-what-are-the-differences-between-one-tailed-and-two-tailed-tests/