Selecting Research Participants Chapter 5

Population

Is 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 are generalized to the entire population

Sample

Is a set of individuals selected from a population and usually is intended to represent the population in a research study

Target Population

Is the group that is defined by the researcher's specific interests. Individuals in a target population typically share one charactereistic

Representativeness

of a sample refers to the extent to which the charactereistics of the sample accuartly reflect the characteristics of the population

Representative Sample

Is a sample with different characteristics from those of the population

Biased Sample

Is a sample with different characteristics from those of the population

Selection bias or Sampling bias

occurs when participants or subjects are selected in a manner that increases the probability of obtaining a baised sample

Sampling

Is the process of selecting individuals to participate in a research study

Probability Sampling

The entire population is known each individual in the population has a specifiable probability of selction, and sampling occurs by a random process based on the probabilities

Random Process

Is a procedure that produces one outcome from a set of possible outcomes. The outcome must be unpredictable each time and the process based on the probabilities

Nonprobability sampling

the population is not completely known, individual probabilities cannot be known and the sampling method is based on the factors such as common sense or ease, with an effort to maintian representativeness and avoid bias

Simple Random Sampling

A sample is obtained using a random process to select participants from a list containing the total population. The random process ensures that each individual has an equal and independent chance of selction

Strengths and Weaknesses For Simple Random Sampling

The selection process is fair and unbiased but there is no guarantee that the sample will be representative

Systematic Sampling

A sample is obtained by selecting every nth participant from a list containing the total population after a random start

Strengths and Weaknesses For Systematic Sampling

An easy method for obtaining an essentially random sample, but the selections are not really random or independent

Stratified Random Sampling

A sample obtained by dividing the population into subgroups (strata) and then randomly selecting equal numbers from each of the subgroups

Strengths and Weaknesses Stratified Random Sampling

Guarantees that each subgroup will have adequate representation but the overall sample is usually not representative of the population

Proportionate Stratified Sampling

A sample is obtained by subdividing the population inro strata and then randomly selecting from each strata a number of participants that is in proportion to the proportions in the population

Strengths and Weaknesses for Proportionate Stratified Sample

Gaurantees that the composition of the sample (in terms of the indentified strata) will be perfectly representative of the compostion of the poulation, but some strata may have limited representation in the sample

Cluster Sampling

Instead of selecting individuals, a sample is obtained by randomly selecting clusters (preexisting groups) from a list of all the clusters that exist within the population

Strengths and Weaknesses for Cluster Sampling

An easy method for obtaining a large relatively random sample, but the selctions are not really random or independent

Convenience Nonprobability Sampling

A sample obtained by selecting individual participants who are easy to get

Strengths and Weaknesses for Convenience Nonprobability Sampling

An easy method for obtaining a sample, but that sample will probably be biased

Quoto Nonprobability Sampling

A sample is obtained by indentifying sub-groups to be included, then establishing quotas for individuals to be selected through convenience from each subgroup

Strengths and Weaknesses for Quoto Nonprobability Sampling

Allows a researcher to control the compostion of a convenience sample, but the sample will prob be biased.