A A SurveySurvey is a study of one or is a study of one or more characteristics of a group.more characteristics of a group.

Population vs. SamplePopulation vs. Sample

The entire group of individuals that we want The entire group of individuals that we want information about is called the information about is called the populationpopulation..

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

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

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

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

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

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

Self-selected SampleSelf-selected Sample

This methods allows the sample to choose This methods allows the sample to choose themselves by responding to a general appeal themselves by responding to a general appeal (volunteering to be surveyed). (volunteering to be surveyed).

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

Problems with Self-selected samples: bias – Problems with Self-selected samples: bias – because people with strong opinions on the because people with strong opinions on the topic (especially negative opinions) are most topic (especially negative opinions) are most likely to respond.likely to 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 to Example: People conducting a survey go to the mall and stop people who are shopping. the mall and stop people who are shopping. This is convenient for the person doing the This is convenient for the person doing the survey but does not guarantee that the sample survey but does not guarantee that the sample is representative of the population of the study.is representative of the population of the study.

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

Random SamplesRandom Samples

A random sample of size n individuals from A random sample of size n individuals from the population chosen in such a say that every the population chosen in such a say that every set of n individuals has an equal chance to be set of n individuals has an equal chance to be the sample selected.the sample selected.

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

Systematic SampleSystematic Sample

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

Stratified Random SampleStratified Random Sample

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

Representative SampleRepresentative Sample

A sample that accurately reflects the A sample that accurately reflects the characteristics of a population.characteristics of a population.

This is what we want!This is what we want!

Sample BiasSample Bias

A biased sample is a sample that is NOT A biased sample is a sample that is NOT representative of the population.representative of the population.

Example: if we are trying to find out Example: if we are trying to find out something about students at HHS and we something about students at HHS and we include students for KMHS in our survey then include students for KMHS in our survey then we’ve included people who are not in our we’ve included people who are not in our population.population.

Question BiasQuestion Bias

A question that is worded in such a way to A question that is worded in such a way to encourage a particular response is a biased encourage a particular response is a biased question.question.