When to perform a hypothesis test on difference of means? I'm reading a research paper found at this link.


Although overall spending was higher
in traditional plans than HDHPs for both populations, OOP spending was substantially higher for those in HDHPs compared with traditional plans, especially for those with SU/MH disorders (\$573 vs \$280).

Is this a two-sample t-test that was performed? Also, why is it that they performed the test (I'm assuming) on the difference of the means and not something like the sum?

Test differences in means when the interest is in how much higher or lower one mean is from the other. For instance, we might want to know if a new medical treatment results in a longer life expectancy than an older treatment. A simple analysis might test the mean life expectancy of patients who received each treatment.
