For the following studies, describe the truncation condition and determine whether the study involves left or right truncation:
a. Gerontologists investigated the survival rates of elderly residents in a retirement community. Ages at death were recorded, as well as the ages at which individuals entered the retirement community. Describe the truncation condition, and determine whether the study involves left or right truncation.
b. A study on the time to complete a PhD program in statistics at a university used data on students who had received a PhD prior to June 2009. Only data on students who had received a PhD by this date were available to the researchers. Describe the truncation condition, and determine whether the study involves left or right truncation.