Chapter 12 Sample Surveys Population: The entire group of individuals that we wish to learn something about. Sample: A subset of the population

Sampling frame is a list of individual from whom a sample is drawn Sampling frame is a list of individual from whom a sample is drawn. The list may not be a physical list. If a computer program is used to generated phone numbers to be include in a survey, then the sampling frame is all potential numbers that the computer has be programmed to generate.

A Sample Survey is a study that asks individuals drawn from some population questions in the hope of learning something about the entire population. Surveys that are design to influence a particular response is called response bias. For example, saying "Given that at the age of 18 people are old enough to fight and die for your country, don't you think they should be able to drink alcohol as well?" yields a response bias. It is better to say "Do you think 18 year olds should be able to drink alcohol?”

Some individuals of the population of interest may not be in the sampling frame. This would be undercoverage bias. If the sampling frame is phone numbers, then people who do not have phones can not be selected. If the computer only generates landlines, then people who only own cell phones can not be selected. Homeless people are often excluded from sampling frames.

To avoid bias, a sample should be selected using some form of randomization. A simple random sample (SRS) of size n is a sample that every possible subset of size n of the population has an equally likely chance of being selected. Stratified random sample is a when the population is divided in to subgroups, or strata, and then a SRS is selected from each strata.

Cluster sample is when an entire group, usually from the same location, is selected by random. Multistage sample is when a few geographic regions are selected by random. Then within the geographic regions, a few counties are selected at random. Then within the counties a few blocks are selected at random.

Systematic sample is when individuals are selected systematically from a sampling frame. For example, every tenth person from a list is selected. Convenience sample is when a group is picked because it is easy or inexpensive to select. They are usually biased.

Voluntary response is a common biased sampling method Voluntary response is a common biased sampling method. People with stronger feelings are more like to participate in a voluntary response survey. Non-response bias is when individuals who are selected to be in a sample but are not able to be reached or choose not to participate.

A census is when the entire population is included in the study A census is when the entire population is included in the study. Numerical information from a population is called a parameter. µ σ Numerical information from a sample is called a statistic. _ x

A sample is said to be representative if the statistics computed from the sample accurately reflects the population parameter. Even with the best care to select a representative sample, sample statistics will differ from each other. This is called sampling variability.