A medical researcher wants to know how well a particular drug works at prolonging the lives of patients after diagnosis with Type I diabetes. He decides he needs 50 patients for his study. He realizes that he already has 50 patients who he has been treating for this disease over the past decade.

He decides to base his study on these patients because he has ready access to their records and a long-standing relationship with them that will make them more likely to participate fully in the trial.

a. What is the technical name for this type of sample?

b. How would this sampling strategy affect the researcher’s estimate of the life-extension effect associated with this drug?