1. Do people who drink the caffeine equivalent of at least two cups of coffee (Group A) do worse on tests than those who drink less than one cup (Group B)?
2. Do people do better on tests when they drink the caffeine equivalent of at least two cups of coffee (Condition A) than when they drink less than one cup (Condition B)?
Notice that both studies described are getting at the same research question (Does drinking at least two cups of coffee worsen tests results?). But, the first example studies two different populations (those people who drink the caffeine equivalent of at least two cups of coffee and those people who drink less than one cup), while the second example studies the same people twice, under two different conditions or treatments (first when people drink the caffeine equivalent of at least two cups of coffee, and then when they drink less than one cup).

Statement
Advantage
Not an Advantage
The repeated-measures t test uses half the number of participants.


The repeated-measures t test can handle more than two conditions.


In the repeated-measures t test the participants in one treatment are not substantially different from the participants in another.


The repeated-measures t test can handle categorical data.