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Statistics (chapter 8 (overview (A sample survey selects a sample from the…

- - - - an SRS doesn’t favor any part of the population. That’s why an SRS is a better method of choosing samples than convenience or voluntary response sampling.
      - The Simple Random Sample applet makes choosing an SRS very fast. If you don’t use the applet or other software, you can randomize by using a table of random digits. In fact, software for choosing samples starts by generating random digits, so using a table just does by hand what the software does more quickly.
      - We can trust results from an SRS, as well as from other types of random samples that we will meet later, because the use of impersonal chance avoids bias.
      - how to read a random saple table
    - - larger random samples give more accurate results than smaller random samples.
      - statistical inference: Provides methods for drawing conclusions about a population from sample data.
  - - - Choose the strata based on facts known before the sample is taken. For example, a population of election districts might be divided into urban, suburban, and rural strata. A stratified design can produce more precise information than an SRS of the same size by taking advantage of the fact that individuals in the same stratum are similar to one another.
    - - Analysis of data from sampling designs more complex than an SRS takes us beyond basic statistics. But the SRS is the building block of more elaborate designs, and analysis of other designs differs more in complexity of detail than in fundamental concepts.
  - - - undercoverage: When some groups in the population are left out of the process of choosing the sample.
      - A sample survey of households, for example, will miss not only homeless people but also prison inmates and students in dormitories. An opinion poll conducted by calling landline telephone numbers will miss households that have only cell phones as well as households without a phone. The results of national sample surveys therefore have some bias if the people not covered differ from the rest of the population.
    - - nonresponse: When an individual chosen for the sample can't be contacted or refuses to participate.
      - Nonresponse to sample surveys often exceeds 50%, even with careful planning and several callbacks. Because nonresponse is higher in urban areas, most sample surveys substitute other people in the same area to avoid favoring rural areas in the final sample.
    - - People know that they should take the trouble to vote, for example, so many who didn’t vote in the last election will tell an interviewer that they did. The race or sex of the interviewer can influence responses to questions about race relations or attitudes toward feminism.
      - Answers to questions that ask respondents to recall past events are often inaccurate because of faulty memory. For example, many people “telescope” events in the past, bringing them forward in memory to more recent time periods.
      - Wording effects wording effects are the most important influence on the answers given to a sample survey. Confusing or leading questions can introduce strong bias, and variations in wording can greatly change a survey’s outcome. Even the order in which questions are asked matters.
  - - - call screening is now common. A large majority of American households have answering machines, voicemail, or caller ID, and many use these methods to screen their calls. Calls from polling organizations are rarely returned.
      - the percent of cell-phone-only households is increasing rapidly.
    - - It is possible to collect large amounts of survey data at lower costs than traditional methods allow. Anyone can put survey questions on dedicated sites offering free services; thus large-scale data collection is available to almost every person with access to the Internet. Furthermore, web surveys allow delivery of multimedia survey content to respondents, opening up new realms of survey possibilities that would be extremely difficult to implement using traditional methods. Some argue that eventually web surveys will replace traditional survey methods.
      - Although web surveys are easy to do, they are not easy to do well. Three major problems are voluntary response, undercoverage, and nonresponse.
      - Undercoverage is a serious problem for even careful web surveys, because about 25% of Americans lack Internet access and only about 70% have broadband access. People without Internet access are more likely to be poor, elderly, minority, or rural than the overall population, so the potential for bias in a web survey is clear.
      - web surveys work well only for restricted populations, for example, surveying students at your university using the school’s list of student email addresses.