Presentation 7-Sampling Flashcards
Sampling
All sampling involves attempting to make a judgment about a whole something—a bowl of soup, a brand of pizza, or an inmate population—based on an analysis of a part of the whole.
Scientific sampling, however, is considerably more careful and systematic than casual, everyday sampling.
Sample
sample is a subset of a population which should represent the main interest of the study. A population is any precisely defined set of people or collection of items which is under consideration.
A sample is drawn from a population which consists of all possible cases of whatever one is interested in studying. It consists of one or more elements selected from a population. (Population is the area of study/interest- age, women).
Purpose of sampling
When the subject of sampling is first encountered, a not uncommon question is, Why bother? Why not just study the whole group? A major reason for studying samples rather than the whole group is that the whole group is sometimes so large it is not feasible to study it.
For example, human service workers might be interested in learning about welfare recipients, the mentally ill, prison inmates, or some other rather large group of people. It would be difficult—and often impossible—to study all members of these groups. Sampling allows us to study a workable number of cases from the large group to derive findings that are relevant for all members of the group.
Another purpose of sampling
A second reason for sampling is that, as surprising as it may seem, information based on carefully drawn samples can be better than information from an entire group. This is especially true when the group being studied is extremely large. Between the decennial censuses, the Census Bureau conducts sample surveys to update population statistics and collect data on other matters. The quality of the data gathered by these sample surveys is actually superior to that of the census itself. The reason for this is that, with only a few thousand people to contact, the task is more manageable.
Better-trained interviewers can be used, greater control can be exercised over the interviewers, and fewer hard-to-find respondents are involved.
In fact, the Bureau of the Census even conducts a sample survey after each census as a check on the accuracy of that census.
Research being based on samples
Samples make possible a glimpse at the behavior and attitudes of whole groups of people, and the validity and accuracy of research results depend heavily on how samples are drawn.
An improperly drawn sample renders the data collected virtually useless.
An important consideration regarding samples is how representative they are of the population from which they are drawn.
Representative sample
A representative sample is one that accurately reflects the distribution of relevant variables in the target population. In a sense, the sample should be considered a small reproduction of the population .Imagine, for example, that you were interested in the success of unmarried teenage mothers in raising their children in order to improve the provision of services to these adolescents. Your sample should reflect the relevant characteristics of unmarried teenage mothers in your community. Such characteristics might include age, years of education, and socioeconomic status.
How is a sample representative
To be representative, the sample would have to contain the same proportion of unmarried teenage mothers at each age level, educational level, and socioeconomic status that exists in the community. In short, a representative sample should have all the same characteristics as the population. The representative character of samples allows the conclusions based on them to be legitimately generalized to the populations from which they are drawn. Before comparing the various techniques for drawing samples, we will define some of the major terms used in the field of sampling.
Sampling Terminology
A sample is drawn from a population, which infers to all possible cases of what we are interested in studying. In the human services, the target population is often people who have some particular characteristic in common, such as all Trinbagonians, all eligible voters, all school-age children, and so on. A population need not, however, be composed of people. Then, the target population will be all possible cases of whatever our unit of analysis is.
Sample/Population
A sample consists of one or more elements selected from a population. The manner in which the elements are selected for the sample has enormous implications for the scientific utility of the research based on that sample. To select a good sample, you need to define clearly the Population from which the sample is to be draw.Failure to define the population clearly can make generalizing from the sample observations highly ambiguous and result in drawing inaccurate conclusions.
The definition of a population should specify four things: content, units, extent, and time
content, units, extent, and time
Content of the population
First, the content of the population refers to the particular characteristic that the members of the population have in common. For Greenley and Schoenherr, the characteristic held in common by the members of their population was that they were health or social service agencies.
Second, the unit indicates the unit of analysis, which in our illustration is organizations rather than individuals or groups.
Although Greenley and Schoenherr collected data from practitioners and clients in the organizations, their focus was on comparing the performance of agencies.
Unit of analysis
Second, the unit indicates the unit of analysis, which in our illustration is organizations rather than individuals or groups.
Although Greenley and Schoenherr collected data from practitioners and clients in the organizations, their focus was on comparing the performance of agencies.
Extent of the population
Third, the extent of the population refers to its spatial or geographic coverage.
For practical reasons, Greenley and Schoenherr limited the extent of their population to health and social agencies serving one county in Wisconsin. It would not have been financially feasible for them to define the extent of their Population as all agencies in Wisconsin or the United States.
Time of the population
Finally, the time factor refer to the temporal period during which a unit would have to possess the appropriate characteristic in order to qualify for the sample. So with these four factors clearly defined, a population will normally be adequately delimited, and what is called a sampling frame can be constructed
Sampling frames
A sampling frame is a list of the population elements used to draw some types of probability sample.
A sampling frame is a listing of all the elements in a population In many studies, the actual sample is drawn from this listing. The adequacy of the sampling frame is crucial in determining the quality of the sample drawn from it. Of major importance is the degree to which the sampling frame includes all members of the population although there is an endless number of possible sampling frames depending on the research problem a few will lustrations will describe some of the intricacies of developing good sampling frames.
