It is well known that acetaminophen lessens people's physical pain. A recent study suggests that acetaminophen can lower people's psychological pain as well (DeWall et al., 2010). Surprised by these findings, you decided to see if acetaminophen works for different types of psychological pain than those assessed in the original research. You obtained a sample of volunteers and gave half of them acetaminophen and the other half a placebo pill. Both groups of participants read socially painful stories and rated how painful the experience would be for them. Specifically, you want to know if the mean social pain rating of the acetaminophen group is statistically lower than the mean social pain rating of the control group. Stated differently, you want to know if the mean difference in social pain rating between these two samples is likely or unlikely to be due to sampling error. You arepredicting that the acetaminophen willreducesocial pain ratings; therefore, you used a one-tailed hypothesis test with = .05.

ndrug= 31,Mdrug= 213,SDdrug= 18.

ncontrol= 31,Mcontrol= 222,SDcontrol= 20.

QUESTION:Assuming that the null hypothesis is true (i.e., that acetaminophen does not affect social pain at all), what precise value would you expect for themean differencebetween the mean ratings of social pain for the acetaminophen sample and the placebo sample?

Precise expected value for themean differencebetween the two sample means if the null is true = ______.