Sampling Design

Question 1

Describe the different between Probability and Non-probability Sampling Designs

Answer

Each of the probability and non-probability sampling designs has been discussed with examples in the book. Overheads provided can be used while discussing the sampling designs. An aspect that needs to be stressed is that the term "random" used in sampling, such as simple random sampling, does not mean haphazardness, but is used to denote a systematic pattern. When we talk of simple random sampling, we mean that every single element in the population has a known and equal chance of being selected. Students seem to confuse the term used in a statistical sense with the layperson's usage of the term to mean ill-arranged, or haphazard. Each of the sampling designs can be discussed with reference to the student population in a university setting. The simple random sampling, and the systematic sampling designs can be used with the university register of students as the sampling frame. Stratified random sampling can be used stratifying the student population along the lines of professional/nonprofessional schools, traditional/nontraditional students, or academic status (undergraduate/graduate), etc. Cluster sampling could use several classes in session, or several narrow specializations, etc. Area-sampling could be explained in terms of location of the campus -- north wing, south wing, etc. Double sampling would be resorted to when a subset of the students sampled are again approached for more information.

In the non-probability sampling designs, Convenience Sampling can be illustrated as the instructor talking to students who are readily available to ascertain how they are experiencing the course. Purposive  Judgment Sampling would constitute sampling the students who can supply the information needed. For instance, if research is conducted on the types of sexual harassment that students experience, it is only those who have undergone sexual harassment in some form or the other, who would be able to provide the needed information. Thus this group would constitute the purposive judgment sample. Such members can perhaps be reached at a workshop conducted on sexual harassment. Snowball sampling can be used to determine a sample of international students who may be experiencing difficulties understanding the language used in lectures.  An initial group may be identified from their assignments, and these students can be asked to indicate students who may be experiencing language difficulties.  Quota sampling can be used to obtain information regarding experiences of say, NZ Europeans, NZ Maoris, and international students, specifying a quota to be sampled from each group, so that minority groups are adequately represented. Another example would be to stipulate quotas from the various disciplines and then sampling the students from each discipline on a convenience basis.

Question2

What kinds of sampling designs would be used for the following:

a. A study to get a quick idea of the medical acceptability of a new aspirin substitute which cannot be dispensed over the counter without prescription.

b. A study involving a sample of 325 students in a university where 2,000 students are enrolled.

c. An investigation of the career salience of professionals in the fields of medicine, engineering, business, and law.

d. The generalisability of the attitudes of blue collar workers from a sample of 184, to the total population of 350 blue collar workers in the entire factory of a particular company.

Answers

a. Purposive Judgment Sampling.

b. A Systematic sampling design (using university listing of students).

c. A Stratified Random sampling with stratification along profession, gender, age, etc.

d. Simple random sampling because of the importance attached to high generalisability.