Chapter 5

How can you correct problems associated with convenience sampling?

Try to ensure that the samples are reasonably representative and not strongly biased Provide a clear description of how the sample was obtained and who the participants are Let everyone make their own judgments about representativeness

What is the Little Genius Problem?

When studying children, you may need to recruit parents whoare willing to participate in research Typically, they know a lot about research because they themselves are highly educated The issue: While our clinical samples may be representative, our comparative samples are typically of higher SES, have parents with higher education, and thus may look less like the population you're trying to generalize to

The process of selecting individuals for a study

Sampling

What are the two principle METHODS of random sampling?

Sampling with replacement Sampling without replacement

However, it is more likely that a biased sample is the result of ____, which means that the sampling procedure favors the selection of some individuals over others

Selection bias E.g., if we recruit from a university, we are likely to obtain a sample that is smarter, on average, than individuals in the entire population

The selection process is fair and unbiased, but there is no guarantee that the sample is representative.

Simple random 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 selection.

What are the 5 probability sampling methods?

Simple random sampling Systematic sampling Stratified random sampling Proportion stratified random sampling Cluster sampling Combined-strategy

The accuracy of a sample's representation increases in relation to the ____ of the sample size

Square root

Advantage of proportionate stratified random sampling?

*The sample can be constructed so that several variables (e.g., age, economic status, political affiliation, etc.) are represented in the sample in the same proportions in which they exist in the population

What are the negatives of this method?

A lot of extra work -Requires a lot of preliminary measurement before the study actually begins Discards many of the sampled individuals Can make it impossible for a researcher to describe or compare some subgroups or strata that exist within the population E.g., if a specific subgroup makes up only 1% of the population, they must only make up 1% of the sample. If there's 100 individuals in the sample, this means that there is only 1 person from the subgroup.

Dr. Near conducts an experiment on memory for individuals who are above the age of 65. Although there are millions of people above the age of 65, she selects a group of 25 to participate in the experiment. What name is given to the group of 25?

A sample

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

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

What name is given to the group of individuals from which researchers actually select participants for research studies?

Accessible population

A portion of the target population who are accessible to be recruited

Accessible population E.g., adolescents diagnosed with bulimia nervosa at local clinics and agencies

Researchers often begin a research study with a specific target for the level of ____

Accuracy E.g., a political poll may want a margin of error of 5% in determining voters' preferences between two candidates In this situation, it can be computed that the sample must have at least 384 individuals to be confident that the preferences observed in the sample are within 5% of the corresponding population preferences

Why is quota sampling NOT the same as stratified and proportionate stratified sampling?

Because it does not randomly select individuals from the population

The major threat to select a representative sample is ____

Bias

A ____ is one that has characteristics noticeably different from those of the population

Biased sample E.g., The individuals in a sample are smarter, older, faster... than the individuals in the population

How is the n calculated in systematic sampling?

By dividing the population size by the desired sample size E.g., population of 100 children, desired sample size of 25... size of n is 100/25 = 4

A researcher recruits a sample of 25 preschool children for a research study by posting an announcement in a local daycare center describing the study and offering a $10 payment for participation. What kind of sampling is the researcher using?

Convenience sampling

Most commonly used sampling method in behavioral science research It's easy Inexpensive

Convenience sampling

Researchers use participants those individuals who are easy to get People are selected on the basis of their availability and willingness to respond

Convenience sampling

A biased sample can occur simply by ____

Chance

What are the steps of random sampling?

Clearly define the population from which you want to select a sample List all the members of the population Use a random process to select individuals from the list (e.g., random number table)

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

Cluster 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.

A probability sampling technique involving random selection of GROUPS instead of individuals

Cluster sampling E.g., A researcher may want to obtain a large sample of 3rd grade students from the city school system. Instead of selecting 300 students at 1 time, the researcher can randomly select 10 classrooms (each with about 30 students) and still end up with 300 individuals in the sample

A teacher obtains a sample of children from a fifth-grade classroom by randomly selecting the third, fifth, and eighth rows and taking all of the students in those rows. What kind of sampling is being used?

Clustered sampling

What is the college sophomore problem?

College students in an introduction to psychology course have to participate in studies to get credit; therefore, we have the SAME students in dozens of our tasks Mainly an issue when you have typical adults do tasks to learn more about them or specific constructs, not when you're doing research on clinical populations The issue: college students may not be very representative

What are some potential solutions of the college sophomore problem?

Consider the makeup of your convenience sample Try to recruit from a diverse array of students Acknowledge the limitation and move on

An easy method for obtaining a sample, but the sample is probably biased.

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

Which of the following sample techniques is most likely to result in a biased sample? Simple random sampling Convenience sampling Proportionate stratified random sampling Systematic sampling

Convenience sampling

What are the 2 non-probability sampling methods?

Convenience sampling Quota sampling

The average difference between the sample mean and the population mean ____ as the sample size ____ However, there is a limited benefit from increasing sample size beyond a certain threshold of participants (usually beyond 25-30)

Decreases Increases

Steps of proportionate stratified random sampling?

Determine the desired size of the sample Randomly select from a subgroup in the population until you have a number corresponding to the percentage of that subgroup (e.g., freshman are 30% of college population, you want 10 participants in your sample, randomly select 3 freshman) Continue this process with each subgroup to obtain the full sample

Social media can be a useful tool to get people to participate in online surveys But BE CAUTIOUS Certain tools are better than others

Do not provide public links Screen participants and then email those who passed the screener with a unique survey link

Why is convenience sampling considered a WEAK form of sampling?

Does NOT require knowledge of the population Does NOT use a random process for selection Strong possibility that the sample is BIASED

What is the basic requirement for random sampling?

Each individual in the population has an equal chance of being selected *Equality

What is the second requirement for random sampling?

Each selection is independent of the others I.e., the choice of one individual does not influence or bias the probability of choosing another individual *Independence

Usually, target populations are not ____

Easily available (i.e., do not have access to every adolescent in the world with bulimia nervosa)

What is the goal of random sampling?

Ensure that the selection procedure cannot discriminate among individuals and thereby result in a non-representative sample

Another factor influence sample size in research is ____

Ethics If a sample is too large, then it is unnecessarily using extra subjects or participants, which can be viewed as unethical

Be cautious about ____ results of a study using the accessible population to the target population

Generalizing

What is the main advantage of stratified random sampling?

Guarantees that each of the different subgroups will be well represented with a relatively large group of individuals in the sample Thus, this type of sampling is appropriate when the purpose of the research study is to examine specific subgroups and make comparisons between them

To share/distribute flyers via social media, it must be approved in your ____ application

IRB

Steps of stratified random sampling

Identify the specific subgroups (or strata) to be included in the sample Select equal-sized random samples from each of the pre-identified subgroups, using the same steps as in simple random sampling Combine the subgroup samples into one overall sample

Because chance determines each selection, it is possible (although usually unlikely) to obtain a very distorted sample E.g., we could get a random sample of 10 males from a population of equal number of men and women What do researchers do to avoid this?

Impose additional restrictions on the random sampling procedure

Disadvantages of cluster sampling?

It can raise concern about the independence of individual scores Individuals within a cluster often have common characteristics or share common experiences that may influence the variables being measured

A ____ sample is probably more representative than a ____ sample

Large Small

____: 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

Law of large numbers

How large should the sample be to be representative?

No simple answer BUT there are some general guidelines to help choose a sample size

____ sampling: the odds of selecting a particular individual are UNKNOWN because the researcher does not know the population size and cannot list the members of the population

Non-probability

For situations in which the researcher cannot know the complete list of potential participants, what kind of sampling is necessary?

Non-probability sampling

Individuals in a target population typically share ____

One characteristic E.g., children of divorced parents, adolescents with bulimia nervosa

Bottom line: not everyone can participate, and the ____ of the study may depend on the way in which participants are selected

Outcome

Example of stratified random sampling?

Plan to select 50 individuals from a large introductory psychology class and want to ensure that psychology majors and nonmajors are equally represented Select a random sample of 25 students from the psychology majors in the class Select a random sample of 25 students who are not psychology majors Combine the two subgroup samples to produce the desired stratified random sample

What is proportionate stratified random sampling commonly used for?

Political polls, major public opinion surveys, etc.

The large group of interest

Population Typically, populations are huge (e.g., adolescents, men, women, or humans)

Samples tend to be similar to the ____ from which they are taken

Populations E.g., If you select a sample population for which the average age is 21 years, you will probably get a sample with an average age around 21 years

Small set of individuals who participate

Sample

____ sampling: the odds of selecting a particular individual are known and can be calculated

Probability E.g., probability selection of 1/100 for each person

What are the two basic categories of sampling methods?

Probability Non-probability

Guarantees that the composition of the sample (in terms of the identified strata) will be perfectly representative of the composition of the population, but some strata may have limited representation in the sample.

Proportionate stratified A sample is obtained by subdividing the population into strata and then randomly selecting from each stratum a number of participants so that the proportions in the sample correspond to the proportions in the population.

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 sample exactly matches the corresponding portion in the population

Proportionate stratified random sampling E.g.., College administration wants a sample of students that accurately represent the distribution of freshmen, sophomores, juniors, and seniors in the college population Sample is then selected so that it has exactly the same percentages for the 4 groups

A variation of quota sampling mimics ____

Proportionate stratified sampling

Allows a researcher to control the composition of a convenience sample, but the sample probably is biased.

Quota A sample is obtained by identifying subgroups to be included, then establishing quotas for individuals to be selected through convenience from each subgroup.

A non-probability 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 E.g., adjust the quotas to ensure that the sample proportions match a predetermined set of population proportions

Quota sampling

A researcher would like to select a sample of 50 people so that five different age groups are equally represented in the sample. Assuming the researcher DOES NOT KNOW the entire list of people in the population, which sampling technique should be used?

Quota sampling

Selection must be a ____ process, which ensures that every possible outcome is equally likely

Random

If each person in a large group has an equal chance of being included in an experiment, then what kind of sampling is being used?

Random sampling

The goal of a research study is to examine the ____, then generalize the results to the entire ____

Sample Population

Advantages of cluster sampling?

Relatively quick and easy way to obtain a large sample The measurement of individuals can be done in groups, which can greatly facilitate the entire research project

*Most populations are so large than the probabilities remain unchanged from one selection to the next, even when we don't ____ individuals

Replace

Although samples tend to be similar to their populations, most researchers would like more assurance that the samples in their research study are good representatives of the populations they want to study to increase the likelihood of obtaining a ____

Representative outcome

To generalize the results of a study to a population, the researcher must select a ____

Representative sample

How accurately we can generalize the results from a given sample to the population depends on the ____ of the sample

Representativeness This refers to how closely the sample mirrors or resembles the population

If possible, researchers should attempt to perform some preliminary statistical calculations to determine whether the expected results are likely to be ____

Statistically significant

Step ___: Identify the participants or subjects for the study, decide how they will be selected, and plan for their ethical treatment

Step four

A probability sampling technique that involves identifying specific subgroups to be included in the sample and then selecting equal-sized random samples for each pre-identified subgroup

Stratified random sampling

A researcher would like to describe and compare the attitudes of four different ethnic groups of students at a local state college. What kind of sampling would be best to obtain the participants for this study?

Stratified random sampling

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

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

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

Systematic sampling

The group defined by the researcher's specific interests

Target population

What are the negatives of stratified random sampling?

Tends to produce a distorted picture of the overall population -Subgroups may not be equally represented in the population Every individual in the population does NOT have an equal chance of being selected for the sample -Not a true random sample because all individuals in the population are not equally likely to be selected

What are the 3 important conditions of probability sampling?

The exact size of the population must be known and it must be possible to list all of the individuals Each individual in the population must have a specified probability of selection When a group of individuals are all assigned the same probability, the selection process must be unbiased so that all group members have an equal chance of being selected.

Why is systematic sampling less random than simple random sampling?

The principle of INDEPENDENCE is violated BUT ensures a high degree of representativeness E.g., if we select participant 11, we are biased in favor of choosing participant 15

What are some potential solutions of the little genius problem?

Try to recruit elsewhere instead of professors/students with kids Use Vanderbilt resources (with caution) Do community outreach Acknowledge limitation and move on

In non-probability sampling, the researcher does not use an ____ method of selection

Unbiased Thus, non probability sampling has a greater risk of producing a biased sample than does probability sampling

What is the Professional Participant Problem?

We often get the same people over and over again because we have the same resources You MIGHT get a more representative sample (lots of families find out about research, so it may not be as biased) but they are better at certain tasks than others due to repetition and may be familiar with your methods

When is stratified random sampling useful?

When a researcher wants to describe each individual segment of the population or wants to compare segments

Sampling _____ replacement: Individual selected is recorded as a sample member and then returned to the population Ensures that the probability of selection remains constant/independent selections (e.g., 1/100)

With

Sampling ____ replacement Individual selected is removed from the population before the next selection is made Ensures no individual appears more than once in a single sample However, the probability of selection changes (e.g., 1/100 → 1/99)

Without