AP Statistics Section 5.1 Designing Samples. In an observational study , we simply observe individuals and measure variables , but we do not attempt to influence the responses. In an experiment , we impose some treatment on individuals to observe their responses.
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A political scientist wants to know what percent of the voting-age population consider themselves conservatives. What would prevent the political scientist from asking every individual of voting age their political affiliation?
time & money
The entire group of individuals that we want information about is called the __________.An attempt to collect data from every individual in a population is a called a _______.
The smaller part of the population that we actually examine in order to gather information is called a _______. The sample is the part from which we draw conclusions about the whole.
Sampling method refers to the process used to choose the sample from a population. Poor sampling methods can produce misleading conclusions, as the following example illustrates.
TV news and sports programs like to conduct call-in polls of public opinion. Telephone companies charge for these calls. The ABC program Nightline asked whether the United Nations should continue to have its headquarters in the U.S. More than 186,000 callers responded, and 67% said “No.” Why might you question the validity of such a survey?
Only those people with real strong opinions will call.
voluntary response sample.
A properly designed sample showed that 72% of adults wanted the UN to stay! People who go to the trouble to respond to an open invitation are not representative of the entire adult population.
Voluntary response sampling and convenience sampling choose samples that are almost guaranteed not to represent the entire population. These sampling methods display ____, or systematic error. A sampling method is biased if it systematically ______ certain outcomes.
The remedy for bias in choosing a sample is to allow _________________ to do the choosing. Choosing a sample by chance attacks bias by giving all individuals an ______ chance to be chosen. The use of chance to select the sample is the essential principle of statistical sampling.
The simplest way to use chance to select a sample is to place names in a hat (the _________) and draw out a handful (the ______). This is the idea of 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 sample of size n is equally likely to be chosen.
Example: We need to pick 6 students from a class of 36. The students are sitting in 6 rows with 6 desks in each row. Determine if each sample is a SRS.
a) Seat the students alphabetically and pick the first row. ______b) Randomly assign students to seats and pick the 3rd row. ______c) Alphabetize the students’ names and pick every 6th name. ______d) Alphabetize the students’ names and have a computer or calculator pick 6 names. ______e) Randomly sort the students’ names and pick every 6th name on the list. ______f) Arrange the students in any order and use a random digit table. ______
A table of random digits is a long string of the digits 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 with these two properties:1. Each entry in the table is equally likely to be any of the 10 digits 0 through 9.2. The entries are independent of each other. ( i.e. knowledge of one part of the table gives no information about any other part)
Identify any numbers you would ignore.
Take the digits in the table in singlets, pairs, triplets, etc.
Example: We want to interview a sample of 5 business clients out of a population of 30 business clients in order to find ways to improve client satisfaction. We will use a random digit table to randomly select the five clients.
Let’s start with line 130, which is:69051 64817 87174 09517 84534 06489 87201 97245Group the random digits into groups of 2: 69 05 16 48 17 87 17 40 95 17 84 53 40 64 89 87 20 19 72 45
69 05 16 48 17 87 17 40 95 17 84 53 40 64 89 87 20 19 72 45Cross out any numbers that are 31 or more.Cross off any repeats of 01 to 30 if they occur.Continue the process until you have 5 two digit numbers.