Purposeful sampling selects information rich cases for indepth study. Size and specific cases depend on the study purpose.
There are about 16 different types of purposeful sampling. They are briefly described below for you to be aware of them. The details can be found in Patton(1990)Pg 169-186.
Extreme and deviant case sampling This involves learning from highly unusual manifestations of the phenomenon of interest, suchas outstanding successes, notable failures, top of the class, dropouts, exotic events, crises.Intensity sampling This is information rich cases that manifest the phenomenon intensely, but not extremely, such as good students,poor students, above average/below average.
Maximum variation sampling This involves purposefully picking a wide range of variation on dimentions of interest. This documents unique or diverse variations that have emerged in adapting to different conditions. It also identifies important common patterns that cut across variations. Like in the example of interviewing SJU students, you may want to get students of different nationalities, professional backgrounds, cultures, work experience and the like.
Homogenious sampling This one reduces variation, simplifies analysis, facilitates group interviewing. Like instead of having the maximum number of nationalities as in the above case of maximum variation, it may focus on one nationality say Americans only.
Typical case sampling It involves taking a sample of what one would call typical, normal or average for a particular phenomenon,
Stratified purposeful sampling This illustrates charecteristics of particular subgroups of interest and facilitates comparisons between the different groups.
Critical case sampling This permits logical generalization and maximum application of information to other cases like "If it is true for this one case, it is likely to be true of all other cases. You must have heard statements like if it happenned to so and so then it can happen to anybody. Or if so and so passed that exam, then anybody can pass.
Snowball or chain sampling This particular one identifies, cases of interest from people who know people who know what cases are information rich, that is good examples for study, good interview subjects. This is commonly used in studies that may be looking at issues like the homeless households. What you do is to get hold of one and he/she will tell you where the others are or can be found. When you find those others they will tell you where you can get more others and the chain continues.
Criterion sampling Here, you set a criteria and pick all cases that meet that criteria for example, all ladies six feet tall, all white cars, all farmers that have planted onions. This method of sampling is very strong in quality assurance.
Theory based or operational construct sampling. Finding manifestations of a theoretical construct of interest so as to elaborate and examine the construct.
Confirming and disconfirming cases Elaborating and deepening initial analysis like if you had already started some study, you are seeking further information or confirming some emerging issues which are not clear, seeking exceptions and testing variation.
Opportunistic Sampling This involves following new leads during field work, taking advantage of the unexpected flexibility.
Random purposeful sampling This adds credibility when the purposeful sample is larger than one can handle. Reduces judgement within a purposeful category. But it is not for generalizations or representativeness.
Sampling politically important cases This type of sampling attracts or avoids attracting attention undesired attention by purposisefully eliminating from the sample political cases. These may be individuals, or localities.
Convenience sampling It is useful in getting general ideas about the phenomenon of interest. For example you decide you will interview the first ten people you meet tomorrow morning. It saves time, money and effort. It is the poorest way of getting samples, has the lowest credibility and yields information-poor cases.
Combination or mixed purposeful sampling This combines various sampling strategies to achieve the desired sample. This helps in triangulation, allows for flexibility, and meets multiple interests and needs. When selecting a sampling strategy it is necessary that it fits the purpose of the study, the resources available, the question being asked and the constraints being faced. This holds true for sampling strategy as well as sample size.