◼ Sampling is the process of selecting a number of
individuals from a population, preferably in a way that the individuals are representative of the larger group from which they were selected. ◼ A sample is any group on which information is obtained.
Defining the Population ◼ A population refers to all the members of a particular group. ◼ The first task in selecting a sample is to define the population of interest. ◼ In Educational Research, the population of interest is a group of persons who possess certain characteristics. ◼ A target population is the actual population that the researcher would like to generalize. ◼ Considered rarely available ◼ The accessible population would be the group that is available (realistic choice)
Two Main Types of Sampling ◼ Sampling may be either random or non-random ◼ Random sampling is a method of selecting subjects from a population by chance, so that biases do not alter the sample. ◼ The 3 most common ways of obtaining this type of sample are: ◼ Simple Random Sampling ◼ Stratified Random Sampling ◼ Cluster Sampling
Simple Random Sampling ◼ A Simple Random Sampling is a sample selected from a population in such a manner that all members have an equal chance of being selected ◼ If the sample is large, it is the best method to obtain a sample representative of the population from which it has been selected ◼ The larger the sample size, the more it is likely to represent the population ◼ Any differences that occur are the result of chance rather than bias on the part of the researcher ◼ Disadvantages of this method are: 1) the difficulty of performing the sampling and, 2) this method does not ensure that subgroups are present in the sampling in the same proportion as they are in a population
Stratified Random Sampling ◼ A Stratified Random Sampling is a sample selected so that certain characteristics are represented in the sample in the same proportion as they occur in the population ◼ The term strata refers to sub-groups ◼ The advantage of stratified random sampling is that it increases the likelihood of representation, especially if the sample size is small ◼ It virtually ensures that any key characteristics of individuals in the population are included in the same proportions in the sample size ◼ The disadvantage is that it requires still more effort on the part of the researcher
Cluster Random Sampling ◼ A Cluster Random Sampling is a sample obtained by using groups as the sampling unit (cluster), rather than individuals ◼ There are instances where it is not possible to select a sample of individuals from a population ◼ This is considered more effective with large numbers of clusters ◼ Advantages include more efficient and easier to implement in schools ◼ Its disadvantage is that there is a great chance of selecting a sample that is not representative of the population
Two-Stage Random Sampling ◼ This method selects groups randomly and then chooses individuals randomly from these groups. ◼ This becomes a combination of a cluster random sampling with individual random sampling. ◼ Considered less time consuming but allows for a good representation of the groups at random.
Nonrandom Sampling Methods ◼ There are 3 main types of nonrandom sampling methods used in Educational Research ◼ A Systematic Sample is a sample obtained by selecting every nth name in a population ◼ A Convenience Sample is any group of individuals that is conveniently available to be studied ◼ Are not considered representative of the population and should be avoided, if possible ◼ A Purposive Sample is a sample selected because the individuals have special qualifications of some sort, or because of prior evidence of representation ◼ Personal judgment is used for selection purposes ◼ A major disadvantage is that the researcher’s judgment could be in error
Sample Size ◼ The question remains as to what constitutes an adequate sample size. ◼ Samples should be as large as a researcher can obtain with a reasonable expenditure of time and energy. ◼ The recommended minimum number of subjects are as follows for the following types of studies: ◼ 100 for a Descriptive Study ◼ 50 for a Correlational Study ◼ 30 in each group for Experimental and Causal-Comparative Study
The use of 15 subjects per group should probably be replicated