WonderDrug Company wants to test the effectiveness of a new headache drug. What sampling technique (simple random, stratified random, cluster, convenience) is WonderDrug Company using in each of the following studies?
(a) It sends the new drug and the old drug to doctors nationwide who agree to prescribe the medicines. It provides the doctors with sheets to classify patients as male or female, and under 40 or over 40, and instructs the doctors to give either the new drug or the old drug to every other patient down each list of male/female and within that, under 40/over 40.
(b) It gives the new drug to physicians in one headache clinic to prescribe for its patients, and it gives the former drug to physicians in another headache clinic to prescribe for its patients.
(c) It advertises the purpose of the study in local newspapers, offering to compensate volunteers who take either the new drug or the former drug.
(d) It sends the new drug and the old drug to doctors nationwide who agree to prescribe the medicines, and instructs the doctors to alternate giving the new drug or the old drug to every other patient.