Chapter 4: How to Get a Good Sample Flashcards
sample survey
a subgroup of a large population is questioned on a set of topics. The results from the subgroup are used as if they were representative of the larger population, which they will be if the sample was chosen correctly.
experiment
measures the effect of manipulating the environment in some way.
randomized experiment
the manipulation is assigned to participants on a random basis.
explanatory variable
the factor of an experiment being manipulated
outcome variable/ response variable
the result of the explanatory variable
observational study
resembles an experiment except that the manipulation occurs naturally rather than being imposed by the experimenter. For example, we can observe what happens to people’s weight when they quit smoking, but we can’t experimentally manipulate them to quit smoking.
case-control study
A special type of observational study is frequently used in medical research that serves as a called a case-control study, it is an attempt to include an appropriate control group.
meta-analysis
is a quantitative review of a collection of studies all done on a similar topic. Combining information from various researchers may result in the emergence of patterns or effects that weren’t conclusively available from the individual studies.
case study
is an in-depth examination of one or a small number of individuals.
unit
individual or object to be measured
population/ universe
is the entire collection of units about which we would like information or the entire collection of measurements we
would have if we could measure the whole population.
sample
sample is the collection of units we actually measure or the collection of measurements we actually obtain.
sampling frame
is a list of units from which the sample is chosen.
Ideally, it includes the whole population.
sample survey
measurements are taken on a subset, or sample, of
units from the population.
census
a survey in which the entire population is measured
margin of error
measure of accuracy in the form of a number. As a general rule, the amount by which the proportion obtained from the sample will differ from the true population proportion rarely exceeds 1 divided by the square root of the number in the sample. This is expressed by the simple formula 1/√n, where the letter n represents the number of people in the sample. To express results in terms of percentages instead of proportions, simply multiply everything by 100.
probability sampling plans
sampling conditions where everyone in a population has a specified chance of making it into the sample
simple random sample
sampling conditions in which every conceivable group of people of the required size has the same chance of being the selected sample.
random numbers
Random numbers can be found in tables designed for that purpose, called “tables of random digits,” or they can be generated by computers and calculators.
stratified random sample
sample is collected by first dividing the population of units into groups (strata) and then taking a simple random sample from each
strata
when a population of units falls into natural groups
systematic sampling plan
With this plan, you divide the list into as many consecutive segments as you need, randomly choose a starting point in the first segment, then sample at that same point in each segment.
random digit dialing
This method results in a sample that approximates a simple random sample of all households in the United States that have telephones. The method proceeds as follows. First, they make a list of all possible telephone exchanges, where the exchange consists of the area code and the next three
digits. Using numbers listed in the white pages, they can approximate the proportion of all households in the country that have each exchange. They then use a computer to generate a sample that has approximately those same proportions. Next, they use the same method to randomly sample banks within each exchange, where a bank
consists of the next two numbers. Phone companies assign numbers using banks so that certain banks are mainly assigned to businesses, certain ones are held for future
neighborhoods, and so on. Finally, to complete the number, the computer randomly generates two digits from 00 to 99.
Once a phone number has been determined, a well-conducted poll will make multiple attempts to reach someone at that household. Sometimes they will ask to
speak to a male because females are more likely to answer the phone and would thus be overrepresented.
multistage sampling plan
Many large surveys, especially those that are conducted in person rather than over the telephone, use a combination of the methods we have discussed. They might stratify by region of the country; then stratify by urban, suburban, and rural; and then choose a random sample of communities within those strata. They would then divide those communities into city blocks or fixed areas, as clusters, and sample some of those. Everyone on the block or within the fixed area may then be sampled. This is called a multistage sampling plan.
