# Shared Flashcard Set

## Details

Soc 497 Sampling
Soc 497 Sampling Lecture 7
89
Sociology
Undergraduate 3
03/11/2013

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Term
 sociologists WHAT from a population to discover and understand social phenomena.
Definition
 sample
Term
 Two famous examples of sampling illustrate the importance of proper sampling techniques:
Definition
 1. 1936 Literary Digest presidential poll2. 1948 Gallup presidential poll
Term
 1936 Literary Digest presidential poll
Definition
 Editors mailed postcards to people in six states, names were selected from telephone directories & automobile registration lists.Doing so created a sampling frame problem: they only sampled the rich, and therefore did not have an accurate sample to predict the election.
Term
 1948 Gallup presidential poll
Definition
 The Gallup samplers used a method called quota sampling, which selects people to match a set of characteristics (e.g. the poor, those living in rural vs. urban environments).Predictions were based on old census data: the quotas they used were no longer valid indicators of the U.S. population, and Gallup incorrectly predicted Thomas Dewey defeating Harry Truman
Term
 There are two types of sampling methods:
Definition
 1. Nonprobability sampling2. Probability sampling
Term
 Nonprobability sampling
Definition
 represents techniques where samples are not selected by using probability theory.
Term
 Probability sampling
Definition
 are selected according to some sort of random assignment.
Term
 There are four types of nonprobability sampling:
Definition
 1. Reliance on available subjects2. Purposive (or judgmental) sampling3. Snowball sampling4. Quota sampling
Term
 Reliance on available subjects (nonprobability sampling)
Definition
 (also called convenience or haphazard sampling) involves using people in close proximity, or people who are gathered in one place at one time.It is generally considered one of the weaker forms of sampling.Strengths: easy and efficientWeaknesses: can be unrepresentative of general populations
Term
 Purposive (or judgmental) sampling (nonprobability sampling)
Definition
 units to be observed are selected on the basis of a researcher's judgment about which ones will be most useful or representative.One may want to interview the entire population of some limited group (e.g. directors of shelters for homeless adults).
Term
 Three guidelines should be followed when selecting a purposive sample:
Definition
 1. Informants should be knowledgeable about the cultural arena, situation, or experience being studied.2. Informants should be willing to talk.3. Informants should be representative of the range of points of view.
Term
 In purposive sampling, interviews should be selected until two criteria are met:
Definition
 1. Completeness2. Saturation
Term
 Completeness
Definition
 Interviews should continue until a researcher is confident subjects have provided an overall sense of the meaning of a concept, theme, or process.
Term
 Saturation
Definition
 Interviews should continue until one is confident that they are learning little that is new from subsequent interviews.
Term
 Snowball sampling
Definition
 a nonprobability sampling method whereby each person interviewed is asked to suggest additional people for interviewing.
Term
 Snowball samples are appropriate for
Definition
 studying difficult to identify or difficult to locate populations
Term
 Snowball sampling can be problematic for the following reasons:
Definition
 1. It can result in samples with questionable representativeness.2. Initial contacts may shape the entire sample and foreclose access to some members of the population of interest.
Term
 Quota sampling
Definition
 is a type of nonprobability sampling in which units are selected into a sample on the basis of pre-specified characteristics, so that the total sample will have the same distribution of characteristics assumed to exist in the population being studied.
Term
 Quota sampling begins with
Definition
 a matrix that describes characteristics of a target population.
Term
 Quota sampling is similar to probability sampling, but has two inherent issues:
Definition
 1. Quota frame problems2. Selection of sample elements may be biased
Term
 1. Quota frame problems
Definition
 ▪ Proportions that different cells in a matrix represent must be accurate.▪ It is often difficult to get up-to-date information for this purpose.
Term
 Selection of sample elements may be biased.
Definition
 E.g. an interviewer may be instructed to interview 5 people who meet a given set of characteristics, yet still avoid certain people that are representative of the issue at hand.
Term
 In all research designs, sociologists distinguish between WHAT andinformants.
Definition
 respondents
Term
 Respondents
Definition
 allow a researcher to construct a composite picture of the group respondents represent.
Term
 Informants
Definition
 s are people well versed in a social phenomenon one wishes to study and who are willing to tell what they know about it.
Term
 Probability sampling
Definition
 is a general term for samples selected in accord with probability theory.
Term
 Probability samples are often used for
Definition
 large-scale surveys
Term
 Probability sampling can be a very effective tool in research if...
Definition
 it is done properly.
Term
 Nonprobability sampling cannot
Definition
 guarantee that a sample is representative of a population.
Term
 Probability sampling is useful
Definition
 because it helps ensure that the same variations that exist in a populationare represented in a sample.
Term
 Sampling bias
Definition
 occurs when subjects selected for a study are not typical nor representative of a larger population.
Term
 Bias is often WHAT. When one selects a sample based on some background characteristic, they ALWAYS introduce some sort of bias into their sample.
Definition
 unintentional
Term
 Some degrees of bias are more acceptable than others
Definition
 depending on one’s research design
Term
 Samples must be WHAT of the population from which they are selected
Definition
 representative
Term
 Representativeness
Definition
 refers to the quality of a sample of having the same distribution of characteristics as the population from which it was selected.
Term
 A sample is representative of the population from which it is selected
Definition
 if all members of the population have an equal chance of being selected in the sample.
Term
 There are two advantages to probability sampling:
Definition
 1. Probability samples are typically more representative than other types of samples because biases are avoided.2. Probability theory permits researchers to estimate the accuracy or representativeness of a sample.
Term
 In order to understand probability sampling one needs to differentiate WHAT and populations:
Definition
 elements
Term
 elements
Definition
 A unit of which a population is composed and which is selected in a sample.
Term
 population
Definition
 A specified aggregation of elements in a study.
Term
 In probability sampling, one uses random selection
Definition
 to select a set of elements from a population that accurately portray the total population from which the elements are selected.
Term
 Random selection ensures that each element has an
Definition
 equal chance of selection.
Term
 Probability theory
Definition
 provides the basis for estimating parameters of a population.
Term
 parameter
Definition
 is the summary description of a given variable in a population.E.g. the mean unemployment rate or age distribution in L.A. are both parameters of the L.A. population.
Term
 When researchers generalize from a sample, they use
Definition
 sample statistics to estimate population parameters.
Term
 An action that involves elements of probability is called an
Definition
 event or trial.ie ▪ Dealing a card.▪ Tossing a coin.▪ Spinning a roulette wheel.
Term
 The result of an event or trial is an
Definition
 outcome.
Term
 An outcome represents the characteristic
Definition
 of the event/trial. For example…▪ Drawing an ace or a seven or a king, etc.▪ A coin landing on heads or tails.▪ A ball landing on double zero (00) in roulette.
Term
 robability is used to predict what kind
Definition
 of samples are likely to be obtained from a population.
Term
 Thus, probability establishes a connection between samples and populations. Inferential statistics rely on this connection when they use
Definition
 sample data as the basis for making conclusions about populations.
Term
 probability of A=
Definition
 number of outcomes classifed as A--------------------------------total number of possible outcomes
Term
 Two conditions must be met for a sample to be random:
Definition
 1. Every individual in a population has an equal chance of being selected.2. When more than one individual is being selected, the probabilities must stay constant (i.e. there must be sampling with replacement).
Term
 As one selects a bigger sample,
Definition
 the sample mean approaches the population mean.
Term
 The larger the sample
Definition
 the more accurate its estimation of the population from which it was drawn.
Term
 Although samples can have different means,
Definition
 the sample means should be close to the population mean.
Term
 The sample means (M1 , M2 , M3 …) should cluster around
Definition
 the population mean (μ).
Term
 Thus, the distribution of sample means tends to form a normal shape with
Definition
 an expected value of μ.
Term
 Any individual sample mean probably will not be identical to its population mean. Therefore,
Definition
 sampling error exists when sampling.
Term
 Sampling error
Definition
 represents the degree of error to be expected of a given sample design. In other words, there is “error” between M and μ.Some sample means will be relatively close to μ and others will be relatively far away.
Term
 The standard error provides a measure
Definition
 of the standard distance between M and μ.
Term
 The law of large numbers:
Definition
 The larger the sample size (n), the closer the sample means should be to the population mean (μ).
Term
 The formula for standard error reflects the intuitive relationship between
Definition
 standard deviation, sample size, and “error:”
Term
 As sample size increases
Definition
 standard error decreases.
Term
 As sample size decreases
Definition
 standard error increases.
Term
 A confidence level
Definition
 is the estimated probability that a population parameter lies within a given confidence interval.
Term
 A confidence interval
Definition
 is the range of values within which a population parameter is estimated to lie.
Term
 A sampling frame
Definition
 is a list of units that compose a population from which a sample is selected.
Term
 If a sample is to be representative of a population, it is essential
Definition
 that the sampling frame include all members of the population.
Term
 There are different ways to sample using probability theory, including…
Definition
 1. Simple random sampling2. Systematic sampling3. Stratified sampling
Term
 Simple random sampling is a type
Definition
 of probability sampling in which the units composing a population are assigned numbers.
Term
 Simple random sampling is beneficial in many ways, but it is
Definition
 not necessarily the most accurate sampling method.
Term
 Simple random sampling has
Definition
 A set of random numbers is generated and the units having those numbers are included in the sample.
Term
 Systematic sampling
Definition
 is a type of probability sampling in which every kth unit in a list is selected for inclusion in a sample.
Term
 Generally, systematic sampling is
Definition
 more accurate than simple random sampling.
Term
 Two terms are frequently used in connection with systematic sampling:
Definition
 1. Sampling interval2. Sampling ratio
Term
 Sampling interval
Definition
 is the standard distance between elements selected from a population in the sample.
Term
 Sampling ratio
Definition
 is the proportion of elements in a population that are selected to be in a sample.
Term
 Stratified sampling is a modification of
Definition
 either simple random or systematicsampling.
Term
 Stratification
Definition
 refers to the grouping of units composing a population into homogenous groups (strata) before sampling.
Term
 Stratified sampling is slightly more accurate
Definition
 than simple random sampling.
Term
 Cluster sampling
Definition
 is a multistage sampling technique in which natural groups are sampled initially with the members of each selected group being sub-sampled afterward.
Term
 Cluster sampling is used when it is
Definition
 not practical or possible to create a list of all elements that compose a target population.
Term
 Cluster sampling is efficient, but
Definition
 less accurate.
Term
 In multistage cluster sampling,
Definition
 stratification techniques can refine and improve a sample being selected.
Term
 By stratifying a sample in multistage cluster sampling,
Definition
 sampling error can be reduced.
Term
 Probability proportionate to size (PPS) sampling
Definition
 is a type of multistage cluster sample in which clusters are selected not with equal probabilities, but with probabilities proportionate to their sizes—as measured by the number of units to be sub-sampled.
Term
 PPS is a more sophisticated form of
Definition
 cluster sampling.
Term
 Depending on the nature of one’s population, one may wish to sample a
Definition
 disproportionate amount of one or more elements, or use weighting to sample.
Term
 Weighting regards
Definition
 assigning different weights to cases that were selected into a sample with different probabilities of selection.
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