 An observational study observes individuals and measures variable of interest but does not attempt to influence the responses.  Often fails due to lurking variables.  An experiment, on the other hand, deliberately imposes some treatment on individuals in order to observe their responses.  Is the preferred method of gaining knowledge since it allows us to control lurking variables. AP Statistics, Section 5.1, Part 12

 The entire group of individuals that we want information about is called the population.  A sample is a part of the population that we actually examine in order to gather information.  The units are the individuals on which the experiment is performed. AP Statistics, Section 5.1, Part 13

 Sampling involves studying a part in order to gain information about the whole.  Sample Frame is the actual set of units from which a sample has been drawn.  A census attempts to contact every individual in the entire population. AP Statistics, Section 5.1, Part 14

 Getting a portion of the population is not difficult.  Getting a good unbiased sample is difficult.  Creating a plan to do this is called “sample design”. AP Statistics, Section 5.1, Part 15

 Voluntary response sample (example: Call in opinion polls).  The problem with call in opinion polls is that the people who answer the polls tend to have strong opinions, especially strong negative opinions.  This sample is biased; this sample is not representative of the population. AP Statistics, Section 5.1, Part 16

 Convenience sample (example: Mall intercept interviews)  Convenience sampling may not get you access to all the people in the population.  Interviewers often avoid people who may make them feel uncomfortable.  This sample is biased; this sample is not representative of the population. AP Statistics, Section 5.1, Part 17

 The design of a study is biased if it systematically favors certain outcomes.  You are conducting an observational study on how many students in you class like your math teacher. State if the following is an example of:  Convenience sampling  Voluntary Response sampling  A good sampling design  Not even sampling AP Statistics, Section 5.1, Part 18

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 The easiest way to sample is to use a “simple random sample”  A simple random sample (SRS) of size n consists of n individuals from the population chosen in such a way that every set of n individuals has an equal chance to be the sample actually selected.  Avoids favoritism by the selectors and self- selection by the respondents. AP Statistics, Section 5.1, Part 110

 Choose an SRS in two steps:  Step 1: Label. Assign a numerical label to every individual in the population. ▪ Use the smallest amount of digits possible. ▪ Each label must contain the same amount of digits.  Step 2: Random Assignment. ▪ Random number table (Table B) ▪ Random number generator (RandInt in the TI-83) AP Statistics, Section 5.1, Part 111

AP Statistics, Section 5.1, Part 112

AP Statistics, Section 5.1, Part 113

AP Statistics, Section 5.1, Part 114

AP Statistics, Section 5.1, Part 115 A method of selecting members from a larger population according to a random starting point and a fixed, periodic interval. Typically, every “nth” member is selected from the total population. Ex: A cop at a checkpoint decides to speed check every 10 th car that passes him.

 To select a stratified random sample, first divide the population into groups of similar individuals, called strata. Then choose a separate SRS in each stratum and combine these SRSs to form the full sample. AP Statistics, Section 5.1, Part 116

 Convenience – it is easier to sample smaller groups.  Coverage – representation from each group is assured.  Precision – gives estimates closer to the true value of the population than an SRS. AP Statistics, Section 5.1, Part 117

 Good sampling technique uses random selection to reduce the possibility of bias. AP Statistics, Section 5.1, Part 118

AP Statistics, Section 5.1, Part 119

 Undercoverage occurs when some groups in the population are left out of the process of choosing the sample.  Surveys of households leave out homeless people, prison inmates and college dorms…  Opinion polls by telephone leaves out people with out phones, or are not listed…  Nonresponse occurs when an individual chosen for the sample can’t be contacted or does not cooperate. AP Statistics, Section 5.1, Part 120

 Response bias. Respondents may lie if they feel uncomfortable telling the truth.  The interviewer’s attitude towards some answers may influence how the respondents answer.  Ex. The interviewer wears an anti-abortion shirt while asking people about their views on abortion. AP Statistics, Section 5.1, Part 121

 Wording of questions. “It is estimated that disposable diapers account for less than 2% of the trash in today’s landfills. In contrast, beverage containers, third-class mail and yard wastes are estimated to account for about 21% of the trash in landfills. Given this, in your opinion, would it be fair to ban disposable diapers?” AP Statistics, Section 5.1, Part 122

 “Do you own or have you ever own a fur coat?” AP Statistics, Section 5.1, Part 123

 We want to make inferences about the population as a whole.  We can’t afford to talk to everyone.  Even though two samples, following the same design most probably will give us different results, those results are reasonable estimates of the population as a whole AP Statistics, Section 5.1, Part 124

 Properly designed samples avoid systematic bias, but their results are rarely accurate and vary from sample to sample. (Sampling Error)  Large random sample give more precise results than smaller sample. AP Statistics, Section 5.1, Part 125