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PROBABILITY AND NONPROBABILITY SAMPLING 269

ulation with extremely low values on the variable of interest, the sample mean
–
X will be much lower than the true population mean µ. If our sampling design
–
and sample size are right, however, the sample mean X will be within close
range of the true population mean µ. Thus, through appropriate sampling
designs, we can ensure that the sample subjects are not chosen from the
extremes, but are truly representative of the properties of the population. The
more representative of the population the sample is, the more generalizable are
the findings of the research. Recall that generalizability is one of the hallmarks
of scientific research, as we saw in Chapter 2.
Though in view of our concern about generalizability we may be particular
about choosing representative samples for most research, some cases may not
call for such concern for generalizability. For instance, at the exploratory stages
of fact finding, we may be interested only in “getting a handle” on the situation,
and therefore limit the interview to only the most conveniently available people.
The same is true when time is of the essence, and urgency in getting informa-
tion overrides in priority a high level of accuracy. For instance, a film agency
might want to find out quickly the impact on the viewers of a newly released
film exhibited the previous evening. The interviewer might question the first 20
people leaving the theater after seeing the film and obtain their reactions. On the
basis of their replies, she may form an opinion as to the likely success of the film.
As another example, a restaurant manager might want to find the reactions of
customers to a new item added to the menu to determine whether or not it has
been a popular and worthwhile addition. For this purpose, the first 15 people
who partook of the special item might be interviewed, and their reactions
obtained. In such cases, having instant information may be more gainful than
obtaining the most representative facts. It should, however, be noted that the
results of such convenience samples are not reliable and can never be general-
ized to the population. We will now discuss the different types of sampling
designs, bearing in mind the following points in the determination of the choice.

1. What is the relevant target population of focus to the study?
2. What exactly are the parameters we are interested in investigating?
3. What kind of a sampling frame is available?
4. What is the sample size needed?
5. What costs are attached to the sampling design?
6. How much time is available to collect the data from the sample?

PROBABILITY AND NONPROBABILITY SAMPLING

There are two major types of sampling designs: probability and nonprobability
sampling. In probability sampling, the elements in the population have some
known chance or probability of being selected as sample subjects. In nonprob-
ability sampling, the elements do not have a known or predetermined chance of