SamplingSamplingA sample is a small number of A sample is a small number of

individuals representing a larger individuals representing a larger group.group.

Samples and PopulationsSamples and Populations

A A samplesample in a research study is a relatively in a research study is a relatively small number of individuals about whom small number of individuals about whom information is obtained. The larger group information is obtained. The larger group to whom the information is then to whom the information is then generalized is the generalized is the populationpopulation..

Why use samples?Why use samples?

Although the best data comes from Although the best data comes from studying an entire population, samples are studying an entire population, samples are used because they are smaller and less used because they are smaller and less unwieldy. It can be too time consuming unwieldy. It can be too time consuming and expensive to study an entire and expensive to study an entire population.population.

Defining the populationDefining the population

Whether a researcher is drawing a sample Whether a researcher is drawing a sample or is studying an entire population, the or is studying an entire population, the population needs to be defined. This population needs to be defined. This helps focus the research.helps focus the research.

Target vs. accessible populationsTarget vs. accessible populations

The The targettarget population is the population a population is the population a researcher would like to generalize to. researcher would like to generalize to. Often this isn’t possible, so the Often this isn’t possible, so the accessibleaccessible population is used. For example, a population is used. For example, a researcher might want to target all male researcher might want to target all male elementary teachers in the United States, elementary teachers in the United States, but actually collects data from the male but actually collects data from the male elementary teachers in Hawaii.elementary teachers in Hawaii.

Random vs. nonrandom samplingRandom vs. nonrandom sampling

Random sampling is completely based on Random sampling is completely based on chance. For example, one might identify chance. For example, one might identify all members of a population, (n=250) write all members of a population, (n=250) write their names on separate pieces of paper, their names on separate pieces of paper, and then draw 25 names out of a hat to and then draw 25 names out of a hat to determine who is actually to be included in determine who is actually to be included in the study.the study.

Nonrandom samplingNonrandom sampling

In a nonrandom sample, members are In a nonrandom sample, members are selected on the basis of a particular set of selected on the basis of a particular set of characteristics, rather than a random characteristics, rather than a random chance of being included.chance of being included.

Simple random sampleSimple random sample

In a simple random sample, each and In a simple random sample, each and every member of a population has an every member of a population has an equal and independent chance of being equal and independent chance of being selected. selected.

Table of random numbersTable of random numbers

A table of random numbers is used to A table of random numbers is used to identify the people to be included in a identify the people to be included in a sample. These are usually found in sample. These are usually found in statistics books, or can be generated by statistics books, or can be generated by some calculators and computers.some calculators and computers.

Stratified random sampleStratified random sample

In stratified random sampling, subgroups In stratified random sampling, subgroups within a target population are identified to within a target population are identified to be included in proportion to the numbers in be included in proportion to the numbers in which they exist in the population. For which they exist in the population. For example, a researcher studying example, a researcher studying aggressive behavior in dog breeds found aggressive behavior in dog breeds found in Hawaii would want to include a sample in Hawaii would want to include a sample of registered breeds in the proportion they of registered breeds in the proportion they are found in the state.are found in the state.

Cluster samplingCluster sampling

In situations where simple random In situations where simple random sampling isn’t possible, as is often the sampling isn’t possible, as is often the case in schools, groups or case in schools, groups or clustersclusters are are identified for inclusion in research. For identified for inclusion in research. For example, a researcher might choose to example, a researcher might choose to study all of the students in some specific study all of the students in some specific classes.classes.

Two stage random samplingTwo stage random sampling

This technique combines random This technique combines random sampling with cluster sampling. It allows a sampling with cluster sampling. It allows a bigger group to be targeted for bigger group to be targeted for generalization.generalization.

Systematic sampling with a random Systematic sampling with a random startstart

In this procedure, a random number is In this procedure, a random number is generated to identify the first member generated to identify the first member selected for a sample, and then every selected for a sample, and then every nnth th member of the population is selected for member of the population is selected for inclusion. For example, the first member inclusion. For example, the first member selected in a population of 500 might be # selected in a population of 500 might be # 412, and then every 7412, and then every 7thth person is chosen: person is chosen: 419, 426, 433, 440, 447, and so on. When 419, 426, 433, 440, 447, and so on. When you pass 500, you loop back to the you pass 500, you loop back to the beginning.beginning.

Sampling ratioSampling ratio

This is the proportion of individuals This is the proportion of individuals selected for a study. For example, you selected for a study. For example, you might select to study ten percent of the might select to study ten percent of the population. The ratio is defined as the population. The ratio is defined as the sample size divided by the population size.sample size divided by the population size.

Convenience sampleConvenience sample

When it isn’t possible to draw a random or When it isn’t possible to draw a random or systematic nonrandom sample, a systematic nonrandom sample, a researcher might choose to study the researcher might choose to study the individuals who are available. This is individuals who are available. This is known as a convenience sample.known as a convenience sample.

Purposive samplingPurposive sampling

A purposive sample is one identified on A purposive sample is one identified on the basis of specific characteristics the basis of specific characteristics identified by the researcher. For example, identified by the researcher. For example, if a researcher wanted to study all of the if a researcher wanted to study all of the foreign-born teachers in a school district, foreign-born teachers in a school district, he or she would try to identify all of those he or she would try to identify all of those individuals and include only them. individuals and include only them.

External validityExternal validity

Since the entire point of sampling is to Since the entire point of sampling is to generalize the results to a larger generalize the results to a larger population, researchers need to be sure population, researchers need to be sure their work actually does represent the their work actually does represent the population. The extent to which population. The extent to which information can be generalized to a larger information can be generalized to a larger population is known as external validity.population is known as external validity.

Representative samplesRepresentative samples

A representative sample provides the most A representative sample provides the most accurate portrayal of the population being accurate portrayal of the population being studied.studied.

Replication studiesReplication studies

A replication study follows the format of a A replication study follows the format of a previous study, but uses a new group of previous study, but uses a new group of subjects or a new set of conditions or both.subjects or a new set of conditions or both.

Ecological generalizabilityEcological generalizability

This term refers to the degree to which a This term refers to the degree to which a study can be generalized to a different set study can be generalized to a different set of conditions. For example, researchers of conditions. For example, researchers studying rural schools might have difficulty studying rural schools might have difficulty generalizing their results to urban schools.generalizing their results to urban schools.

DataData

Data is a plural word that refers to the Data is a plural word that refers to the kinds of information researchers collect. kinds of information researchers collect. Data should be followed by a plural verb, Data should be followed by a plural verb, such as “Data are” or “Data were”.such as “Data are” or “Data were”.

InstrumentationInstrumentation

The process of preparing to collect data is The process of preparing to collect data is called instrumentation. It involves the called instrumentation. It involves the selection of the method by which data will selection of the method by which data will be collected, as well as the procedures be collected, as well as the procedures and conditions for collecting them.and conditions for collecting them.

ValidityValidity

This term refers to the defensibility of the This term refers to the defensibility of the inferences a researcher can make from a inferences a researcher can make from a study using an instrument. study using an instrument.

ReliabilityReliability

Reliability refers to consistency of results. Reliability refers to consistency of results. If a study is repeated, will it yield similar If a study is repeated, will it yield similar findings? A good example of reliability findings? A good example of reliability might be having three different people might be having three different people grading students’ essays. Will all three of grading students’ essays. Will all three of them agree on what constitutes an A, B, them agree on what constitutes an A, B, C, etc? Or will their scoring vary widely? C, etc? Or will their scoring vary widely? If there is a large variety, the grades would If there is a large variety, the grades would not be reliable.not be reliable.

ObjectivityObjectivity

This characteristic refers to the absence of This characteristic refers to the absence of subjective bias on the part of the subjective bias on the part of the researcher. For example, political analyst researcher. For example, political analyst with a particular ideological bent might with a particular ideological bent might conduct a poll differently from one who conduct a poll differently from one who has no affiliation. has no affiliation.

Different types of instrumentsDifferent types of instruments

Researcher instruments are used by the Researcher instruments are used by the researcher to collect data; a tally sheet or rubric researcher to collect data; a tally sheet or rubric are examples.are examples.

Subject instruments are completed by the Subject instruments are completed by the subject. A survey questionnaire is an example.subject. A survey questionnaire is an example.

Informant instruments are completed by Informant instruments are completed by knowledgeable participants providing knowledgeable participants providing information in addition to that collected by information in addition to that collected by researchers and given by subjects.researchers and given by subjects.

Selecting instrumentsSelecting instruments

Instruments may be selected in one of two Instruments may be selected in one of two ways. Either a researcher locates one that ways. Either a researcher locates one that has been developed by another person, or has been developed by another person, or he/she designs a new one. The he/she designs a new one. The advantage of selecting existing ones is advantage of selecting existing ones is that they have often been field tested for that they have often been field tested for reliability and validity. reliability and validity.

Collecting dataCollecting data

Data may be collected in a variety of ways. Data may be collected in a variety of ways. Respondents might give written Respondents might give written responses, or they might perform a task. responses, or they might perform a task. Doing a miscue analysis on a student is an Doing a miscue analysis on a student is an example of a performance analysis. example of a performance analysis.

Rating scalesRating scales

The difference between observation and rating The difference between observation and rating is that when a researcher rates a subject, he or is that when a researcher rates a subject, he or she is making a judgment of some type. On the she is making a judgment of some type. On the other hand, when a researcher makes an other hand, when a researcher makes an observation, he or she is merely recording observation, he or she is merely recording behavior and not judging it. For example, a behavior and not judging it. For example, a rating might be that a girl made 3 baskets in 20 rating might be that a girl made 3 baskets in 20 attempts, thus scored 2 on a scale of poor to attempts, thus scored 2 on a scale of poor to good on free throws, while an observation would good on free throws, while an observation would just note the number of baskets/attempts.just note the number of baskets/attempts.

Researcher instrumentsResearcher instruments

Interview schedulesInterview schedules Tally sheetsTally sheets Performance checklistsPerformance checklists Anecdotal recordsAnecdotal records Time-and-motion logsTime-and-motion logs

Subject instrumentsSubject instruments

QuestionnairesQuestionnaires Self-checklistsSelf-checklists Attitude scalesAttitude scales Personality inventoriesPersonality inventories Achievement testsAchievement tests Aptitude testsAptitude tests Performance testsPerformance tests Projective devicesProjective devices Sociometric devicesSociometric devices

ScoresScores

Raw scores are the initial scores obtained Raw scores are the initial scores obtained on a test. The number right out of a total on a test. The number right out of a total number of questions is an example.number of questions is an example.

Derived scores have been scaled to show Derived scores have been scaled to show their relative position with respect to other their relative position with respect to other raw scores. raw scores.

Derived scoresDerived scores

Percentile ranksPercentile ranks Age/grade equivalenceAge/grade equivalence Standard scoresStandard scores

Norm-referenced vs. Criterion Norm-referenced vs. Criterion referencedreferenced

A norm-referenced test is developed to A norm-referenced test is developed to provide scores that replicate a normal provide scores that replicate a normal curve among the population tested. Thus, curve among the population tested. Thus, among a population taking the test, half of among a population taking the test, half of the people should score above average the people should score above average and half below.and half below.

A criterion referenced test is based on a A criterion referenced test is based on a goal and an identified percentage is goal and an identified percentage is targeted to reach that goal.targeted to reach that goal.

Measurement scalesMeasurement scales

A nominal scale, the simplest scale, identifies A nominal scale, the simplest scale, identifies groups by a number, e.g. “1” for male and “2” for groups by a number, e.g. “1” for male and “2” for female.female.

An ordinal scale provides an rating from most to An ordinal scale provides an rating from most to least. A Likert scale is an ordinal scale.least. A Likert scale is an ordinal scale.

An interval scale is an ordinal scale that has the An interval scale is an ordinal scale that has the addition of equal distances between the points. addition of equal distances between the points. IQ is measured using an interval scale.IQ is measured using an interval scale.

A ratio scale is an interval with a true zero and is A ratio scale is an interval with a true zero and is rarely used in educational measurement.rarely used in educational measurement.