PS296 Chapter 8: One and Two Tailed Tests
One and Two Tailed Tests
Two Tailed Test:
-Nondirectional Test
-specify before experiment that we will reject a given percentage (say 5%) of the extreme
outcomes, both extremely high and extremely low
-if we reject lowest 2.5% and highest 2.5%, we are actually rejecting H0 a total of 5% of the
time when it is actually true
-that is a = .05
-when we reject extremes in both tails
Disadvantage:
-a score that would fall into 5% rejectio region of a one-tailed test may not fall into rejection
region of corresponding two tailed tests
-this is bc now we only reject 2.5% in each tail
-more common bc investigator may have no idea what data will look like and has to be
prepared for any eventuality
-also, investigators are reasonably sure the data will come out one way but want to cover
themselves in the event that they are wrong
One Tailed Test:
Document Summary
Specify before experiment that we will reject a given percentage (say 5%) of the extreme outcomes, both extremely high and extremely low. If we reject lowest 2. 5% and highest 2. 5%, we are actually rejecting h0 a total of 5% of the time when it is actually true. A score that would fall into 5% rejectio region of a one-tailed test may not fall into rejection region of corresponding two tailed tests. This is bc now we only reject 2. 5% in each tail. More common bc investigator may have no idea what data will look like and has to be prepared for any eventuality. Also, investigators are reasonably sure the data will come out one way but want to cover themselves in the event that they are wrong. We reject h0 for only the lowest (or only the highest) scores. Make prediction of the direction in which the individual will differ from the mean.
