Population vs. SamplePopulation vs. Sample The entire group of individuals The entire group of individuals that we want information about that we want information about is called the is called the populationpopulation..

A A samplesample is a part of the is a part of the population that we actually population that we actually examine in order to gather examine in order to gather information.information.

Example: Let’s say we want to find Example: Let’s say we want to find the average height of female the average height of female students at HHS. It is too much students at HHS. It is too much trouble to try to measure the height trouble to try to measure the height of each girl, so we go to 20 of each girl, so we go to 20 different homerooms and measure the different homerooms and measure the height of 5 girls in each of those height of 5 girls in each of those homeroom.homeroom.

The population of our study is The population of our study is “Females at HHS”.“Females at HHS”.

The sample of the study is the group The sample of the study is the group of 100 girls whose height we of 100 girls whose height we actually measured.actually measured.

Sample DesignSample Design Sample DesignSample Design is the term used is the term used to describe the method used to to describe the method used to choose the sample from the choose the sample from the population to survey.population to survey.

There are many ways to choose a There are many ways to choose a sample. Some methods are more sample. Some methods are more preferred than others.preferred than others.

Self-selected SampleSelf-selected Sample This methods allows the sample to This methods allows the sample to choose themselves by responding to a choose themselves by responding to a general appeal (volunteering to be general appeal (volunteering to be surveyed). surveyed).

Examples of Self-selected Sample: a Examples of Self-selected Sample: a call-in radio poll, an internet poll on call-in radio poll, an internet poll on a websitea website

Problems with Self-selected samples: Problems with Self-selected samples: bias – because people with strong bias – because people with strong opinions on the topic (especially opinions on the topic (especially negative opinions) are most likely to negative opinions) are most likely to respond.respond.

Convenience SamplingConvenience Sampling In a convenience sample individuals are In a convenience sample individuals are chosen because they are easy to reach.chosen because they are easy to reach.

Example: People conducting a survey go Example: People conducting a survey go to the mall and stop people who are to the mall and stop people who are shopping. This is convenient for the shopping. This is convenient for the person doing the survey but does not person doing the survey but does not guarantee that the sample is guarantee that the sample is representative of the population of the representative of the population of the study.study.

Convenience sampling also involves bias Convenience sampling also involves bias on the part of the interviewer.on the part of the interviewer.

Simple Random SamplesSimple Random Samples A random sample of size “n” A random sample of size “n” individuals from the population individuals from the population chosen in such a way that every chosen in such a way that every set of “n” individuals has an set of “n” individuals has an equal chance to be the sample equal chance to be the sample selected.selected.

Example: Putting everyone’s name Example: Putting everyone’s name in a hat and drawing 3 names to in a hat and drawing 3 names to participate in the study.participate in the study.

Systematic SampleSystematic Sample When a rule is used to select When a rule is used to select members of the population.members of the population.

Ex. Every third person on an Ex. Every third person on an alphabetized listalphabetized list

Stratified Random SampleStratified Random SampleTo select a stratified random To select a stratified random sample, first divide the population sample, first divide the population into groups of similar individuals, into groups of similar individuals, called STRATA. Then choose a called STRATA. Then choose a separate sample in each strata and separate sample in each strata and combine these to form the full combine these to form the full sample. Common example would be sample. Common example would be separating by gender or race first, separating by gender or race first, then selecting samples from each then selecting samples from each group.group.

Cluster SamplingCluster Sampling Cluster Sampling is a sampling technique used when "natural" groupings are evident in a statistical population. It is often used in marketing research. In this technique, the total population is divided into these groups (or clusters) and a sample of the groups is selected. Then the required information is collected from the elements within each selected group.

ExampleExample A researcher studies the academic progress of middle school students in Georgia and wanted to choose a cluster sample based on geographic location. First, the researcher divides the entire population of Georgia into clusters, or counties. Next, the researcher selects either a simple random sample or a systematic random sample of those clusters (or counties). He or she chose a random sample of 32 counties and he or she wanted a final sample of 10,000 students. The researcher would then select those 10,000 middle school students from those 32 counties either through simple (or systematic random sampling). This is an example of a cluster sample.