1 Ch. 4, Sampling: How to Select a Few to Represent the Many (Pt. 1) Neumann, pp

2 HOW AND WHY DO SAMPLES WORK? A proper, representative sample lets you study features of the sample and produce highly accurate generalizations about the entire population

3 The most representative samples use random selection The random process allows us to build on mathematical theories about probability Due to their use of random selection, probability samples are also called random samples

4 Sample, population, random sample sample: a small collection of units taken from a larger collection population: a larger collection of units from which a sample is drawn random sample: a sample drawn in which a random process is used to select units from a population

5 Sampling in qualitative research Qual & quant researchers both use sampling, but qualitative researchers have different goals than to get a representative sample of a large population Qualitative researchers believe a small collection of cases, units, or activities can illuminate key features of an area of social life Use sampling less to represent a population than to highlight informative cases, events, or actions Goal is to clarify and deepen understanding based on highlighted cases

6 FOCUSING ON A SPECIFIC GROUP: 4 TYPES OF NONRANDOM SAMPLES Random samples are difficult to conduct Researchers who cannot draw random samples use nonprobability sampling techniques Convenience sampling Quota sampling Purposive or judgmental sampling Snowball sampling

7 Convenience sampling convenience sampling: a nonrandom sample in which you use a nonsystematic selection method that often produces samples very unlike the population it’s cheap and fast, but of limited use with caution, can be used for preliminary phase of an exploratory study also called accidental or haphazard sampling

8 Quota sampling quota sampling: nonrandom sample in which you use any means to fill preset categories that are characteristics of the population Not as accurate as a random sample, but much easier and faster

9 Quota sampling: in steps 1) Identify several categories of people or units that reflect aspects of diversity in population you believe to be important -e.g., gender or age 2) Decide how many units to get for each category, i.e., what the “quota” will be 3) Select units by any method

10 Purposive or judgmental sampling purposive sampling: a nonrandom sample in which you use many diverse means to select units that fit very specific characteristics It’s like convenience sampling for a highly targeted, narrowly defined population Used in 2 types of situations: 1) to select especially informative cases 2) to select cases from a specific but hard-to- reach population

11 Snowball sampling snowball sampling: a nonrandom sample in which selection is based on connections in a preexisiting network It is a multistage technique Each person or case has a connection with the others also called network, chain-referral or reputational sampling

12 Examples of networks studied using snowball sampling Scientists around world investigating same issue Elites of a medium-sized city who consult with one another Drug dealers & suppliers in a distribution network People on a college campus who have had sexual relations with one another

13 COMING TO CONCLUSIONS ABOUT LARGE POPULATIONS sampling element: a case or unit of analysis of the population that can be selected for a sample e.g., a person, a group, an organization, a written document or symbolic message, or a social action or event (e.g., an arrest, a protest event, divorce, a kiss)

14 Universe, population, and target population – increasing degrees of specificity universe: the broad group to whom you wish to generalize your theoretical results e.g., all people in FL population: a collection of elements from which you draw a sample e.g., all adults in the Miami metro area target population: the specific population that you used e.g., all adults who had a permanent address in Dade country, FL in Sept 2007, and who spoke English, Spanish, or Haitian Creole

15 Use target population to create a list of its sampling elements, a sampling frame sampling frame: a specific list of sampling elements in the target population population parameter: any characteristic of the entire population that you estimate from a sample sampling ratio: the ratio of the sample size to the size of the target population

16 Why use random samples? They’re most likely to produce a sample that truly represents the population True random processes: 1) are purely mechanical or mathematical without human involvement 2) allow us to calculate the probability of outcomes with great precision

17 All samples contain a margin of error A random process makes it possible to estimate mathematically the degree of match between sample and population, or sampling error sampling error: the degree to which a sample deviates from a population

18 Key features of random samples 1) They’re based on an accurate sampling frame 2) They use a random selection process without subjective human decisions 3) They rarely use substitutions for sampling elements

19 Types of random samples Simple random samples Systematic sampling Stratified sampling Cluster sampling

20 Simple random samples In simple random sampling: First develop an accurate sampling frame Select elements from the frame based on a mathematically random selection procedure Locate the exact selected elements to be in your sample

21 Over many separate samples, the true population parameter is the most frequent result sampling distribution: a plot of many random samples, with a sample characteristic across the bottom and the number of samples indicated along the side The sampling distribution shows the same bell- shaped pattern whether your sample size is 1000 or 100 but the more samples drawn, the clearer the pattern

Example of sampling distribution Number of blue & white marbles that were randomly drawn from a jar of 5,000 marbles with 100 drawn each time, repeated 130 times for 130 independent random samples Blue marblesWhite marbles # of samples

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24 Systematic sampling If you lack tools to select a pure random sample, systematic sampling is a quasi- random method systematic sampling: an approximation to random sampling in which you select one in a certain number of sample elements; the number is from the sampling interval sampling interval: the size of the sample frame over the sample size, used in systematic sampling to select units

25 Stratified sampling Sometimes researchers want to include specific kinds of diversity in their sample, e.g., racial diversity stratified sampling: a type of random sampling in which a random sample is drawn from multiple sampling frames, each for a part of the population Because you control the relative size of each stratum rather then letting random processes control it, you can be sure your sample will be representative of strata Stratified sampling generally results in a slightly more representative sample than simple random sampling

26 Selecting a stratified sample 1) Divide population into subpopulations (strata) -To use this method, you must have info about strata in population (i.e., the population parameter). 2) Create multiple sampling frames, one for each subpopulation 3) Draw random samples, one from each sampling frame

27 Cluster sampling In some situations where there is no good sampling frame, you can use multiple-stage sampling with clusters A cluster is grouping of the elements in the final sample that you are interested in cluster sampling: a multistage sampling method in which clusters are randomly sampled, and then a random sample of elements is taken from sampled clusters

28 THREE SPECIALIZED SAMPLING SITUATIONS Random-Digit Dialing (RDD) Within-Household Sampling Sampling Hidden Populations

29 Random-digit dialing random-digit dialing: computer based random sampling of telephone numbers

30 Within-household sampling A household can be thought of as a cluster in which there can be multiple sampling elements or individuals To ensure random selection, create selection rules, and follow them consistently

31 Sampling hidden populations hidden population: a group that is very difficult to locate and may not want to be found and is therefore difficult to sample