Chapter 7 Special Topics Introduction to Sampling

What We Will Learn We will learn how to produce data in a manner which will be trustworthy. This will enable us to judge whether conclusions based on this data are reliable or not. Statistical techniques for producing data will allow is to apply Statistical Inference. Statistical Inference – this is the art of using results obtained from sampling to make inferences about populations of interest.

Important Definitions Population: a population in a statistical study is the entire group of individuals about which we want information. Sample: a sample is a part of the population from which we actually collect information used to draw conclusions about the population. Sampling: refers to the process of choosing a sample from the population. A census attempts to contact every individual in the entire population.

Preliminary Steps for Sampling One must state carefully exactly what the population of interest is. One must decide exactly what is to be measured. Example: How can a survey measure unemployment?

Bad Sampling Methods Convenience Sample: A convenience sample is a sample of individuals who are selected because they are members of a population who are most convenient to reach, such as people passing by in the street. Usually, such a sample cannot be trusted to be representative of the population. Example: The Inconvenient Truth about Convenience Samples.

Bias Bias: The design of a statistical study is biased if it systematically favors certain outcomes. When a sampling method is biased, not all the members of the population in question have the same chance to be included. If certain members of the population are not included in a study, the results of the study may not generalize back to the population in question. When you encounter the term “bias”, think of “missing the mark”.

Voluntary Response Sample A Voluntary Response Sample consists of people who choose themselves by responding to a general appeal. Examples: Call-in shows, internet polls, etc. Voluntary response samples are biased because people with strong opinions are most likely to respond. Example: Are Online Polls in Line?

Homework Worksheet for Sections 7.1 and 7.2.