Population
The entire set of individuals of interest to a researcher. Although the entire population usually does not participate in a research study, the results from the study will be generalized to the entire population. Also known as target population.
sample
A set of individuals selected from a population, usually intended to represent the population in a research study.
Target population
A group defined by a researcher's specific interest. Individuals in a target population typically share one characteristic
accessible population
The easily available segment of a target population. Researchers typically select their samples from this type of population.
representativeness
The extent to which the characteristics of the sample accurately reflect the characteristics of the population.
biased sample
A sample with characteristics different from those of the population. If, for example, the individuals in a sample are smarter (or older or faster) than the individuals in the population, then the sample is biased.
selection bias
When participants or subjects are selected in a manner that increases the probability of obtaining a biased sample. A threat to external validity that occurs when the selection process produces a sample with characteristics that are different from those i
law of large numbers
In the field of statistics, the principle that states that the larger the sample size, the more likely it is that values obtained from the sample are similar to the actual values for the population.
sampling
The process of selecting individuals to participate in a research study.
sampling methods
The variety of ways of selecting individuals to participate in a research study. Also known as sampling techniques or sampling procedures.
probability sampling
A sampling method in which the entire population is known, each individual in the population has a specifiable probability of selection, and sampling is done using a random process based on the probabilities. Probability sampling has three important condi
random process
A procedure that produces one outcome from a set of possible outcomes. The outcome must be unpredictable each time, and the process must guarantee that each of the possible outcomes is equally likely to occur.
simple random sampling
A probability sampling technique in which each individual in the population has an equal and independent chance of selection. The obvious goal of a simple random sample is to ensure that the selection procedure cannot discriminate among individuals and th
principal methods of random sampling
.Sampling with replacement
.Sampling without replacement
Sampling with replacement
This method requires that an individual selected for the sample be recorded as a sample member and then returned to the population (replaced) before the next selection is made. This procedure ensures that the probability of selection remains constant thro
Sampling without replacement
As the term indicates, this method removes each selected individual from the population before the next selection is made. Although the probability of being selected changes with each selection, this method guarantees that no individual appears more than
Systematic sampling
A probability sampling technique in which a sample is obtained by selecting every nth participant from a list containing the total population after a random starting point.The size of n is calculated by dividing the population size by the desired sample s
stratified random sampling
A probability sampling technique that involves identifying specific subgroups to be included in the sample and then selecting equal-sized random samples from each pre-identified subgroup. The main advantage of a stratified random sample is that it guarant
proportionate stratified random sampling
A probability sampling technique that involves identifying specific subgroups to be included, determining what proportion of the population corresponds to each subgroup, and randomly selecting individuals so that the proportion for each subgroup in the sa
cluster sampling
A probability sampling technique involving random selection of groups instead of individuals from a population. Ex: Instead of selecting 300 students one at a time, the researcher can randomly select 10 classrooms (each with about 30 students) and still e
Combined-Strategy Sampling
Occasionally, researchers combine two or more sampling strategies to select participants. For example, a superintendent of schools may first divide his district into regions (e.g., north, south, east, and west), which involves stratified sampling. From th
nonprobability sampling
A method of sampling in which the population is not completely known, individual probabilities cannot be known, and the selection is based on factors such as common sense or ease with an effort to maintain representativeness and avoid bias.
convenience sampling
A nonprobability sampling method involving selection of individuals on the basis of their availability and willingness to respond; that is, because they are easy to get. Occasionally called accidental sampling or haphazard sampling.Convenience sampling is
quota sampling
A nonprobability sampling method; a type of convenience sampling involving identifying specific subgroups to be included in the sample and then establishing quotas for individuals to be sampled from each group.
Ex:in a sample of 30 preschool children by e