You are working on a project to determine whether community crime prevention efforts are effective at reducing crime level. You collected data on the number of crimes for a sample of 9 gated communities that implemented the crime prevention efforts (experimental group) and 9 gated communities that did not receive the crime prevention efforts (control group). Your hypothesis is that the experimental group has a lower mean number of crimes than the control group (i.e., community crime prevention measures are effective). Using the 0.05 significant level, you are testing the null hypothesis.

Q: What is the critical value in this study?