Chapter 5 Sampling Learning Objectives 5 1 Identify

Chapter 5: Sampling

Learning Objectives 5 -1: Identify the circumstances that make sampling unnecessary and the reason they are rare. 5 -2: Identify the relation between the desired sample, the obtained sample, the sampling frame, and sample quality. 5 -3: Define and distinguish probability and nonprobability sampling and both techniques’ relationship to sample generalizability. Bachman, The Practice of Research in Criminology and Criminal Justice 7 e. SAGE Publishing, 2020. 3

Learning Objectives 5 -4: Define the major types of probability sampling and indicate when each is preferred. 5 -5: Explain when nonprobability sampling methods may be preferred. 5 -6: Describe the concept of sampling error and explain how its size is affected by the heterogeneity of the population and the fraction of population included in the sample. Bachman, The Practice of Research in Criminology and Criminal Justice 7 e. SAGE Publishing, 2020. 4

Sample Planning • The Purpose of Sampling – To generate a set of individuals or other entities that give us a valid picture of all such individuals or other entities – Population versus sample – Sample versus elements Bachman, The Practice of Research in Criminology and Criminal Justice 7 e. SAGE Publishing, 2020. 5

Sample Planning • Define Same Components and the Population – Sampling frame – Enumeration units – Sampling units Bachman, The Practice of Research in Criminology and Criminal Justice 7 e. SAGE Publishing, 2020. 6

Sample Planning • Evaluate Generalizability – Sample generalizability – Cross-population generalizability – External validity – Sampling error – Target population Bachman, The Practice of Research in Criminology and Criminal Justice 7 e. SAGE Publishing, 2020. 7

Sample Planning • Assess Population Diversity: Research in a Diverse Society – The social world and the people in it are just too diverse to be considered identical units in most respects. – Representative sample Bachman, The Practice of Research in Criminology and Criminal Justice 7 e. SAGE Publishing, 2020. 8

Sample Planning • Consider a Census – Researchers can bypass the issue of generalizability by conducting a census studying the entire population of interest rather than drawing a sample. Bachman, The Practice of Research in Criminology and Criminal Justice 7 e. SAGE Publishing, 2020. 9

Sampling Methods • Probability Sampling Methods – These methods randomly select elements and therefore have no systematic bias. – Both the size of the sample and the homogeneity (sameness) of the population affect the degree of error due to chance. Bachman, The Practice of Research in Criminology and Criminal Justice 7 e. SAGE Publishing, 2020. 10

Sampling Methods • Probability Sampling Methods: Simple Random Sampling – Requires a procedure that generates numbers or identifies cases strictly on the basis of chance a) Random number table b) Random digit dialing c) Replacement sampling Bachman, The Practice of Research in Criminology and Criminal Justice 7 e. SAGE Publishing, 2020. 11

Sampling Methods • Probability Sampling Methods: Systematic Random Sampling – Variant of simple random sampling – The first element is selected randomly from a list, and then every nth element is systematically selected thereafter. a) Sampling interval b) Periodicity Bachman, The Practice of Research in Criminology and Criminal Justice 7 e. SAGE Publishing, 2020. 12

Sampling Methods • Probability Sampling Methods: Stratified Random Sampling – Uses information known about the total population prior to sampling to make the sampling process more efficient a) Proportionate stratified sampling Bachman, The Practice of Research in Criminology and Criminal Justice 7 e. SAGE Publishing, 2020. 13

Sampling Methods • Probability Sampling Methods: Multistage Cluster Sampling – Can be useful when the sampling frame is not available – Cluster Bachman, The Practice of Research in Criminology and Criminal Justice 7 e. SAGE Publishing, 2020. 14

Sampling Methods • Nonprobability Sampling Methods: Availability Sampling – Elements are selected because they are available or easy to find. a) Also known as convenience sampling – Even though they are not generalizable, availability samples are often appropriate in research. Bachman, The Practice of Research in Criminology and Criminal Justice 7 e. SAGE Publishing, 2020. 15

Sampling Methods • Nonprobability Sampling Methods: Quota Sampling – Quotas are set to ensure that the sample represents certain characteristics in proportion to their prevalence in the population. – Researchers have no way of knowing if the sample is representative in terms of any other characteristics that quotas aren’t set for. Bachman, The Practice of Research in Criminology and Criminal Justice 7 e. SAGE Publishing, 2020. 16

Sampling Methods • Nonprobability Sampling Methods: Purposive or Judgement Sampling – Each sample element is selected for a purpose. – The researcher uses his or her own judgment about whom to select into the sample. Bachman, The Practice of Research in Criminology and Criminal Justice 7 e. SAGE Publishing, 2020. 17

Sampling Methods • Nonprobability Sampling Methods: Snowball Sampling – Useful for hard-to-reach or hard-to-identify populations for which there is no sampling frame – Identify one member of the population and speak to him or her, and then ask that person to identify others in the population and speak to them. Bachman, The Practice of Research in Criminology and Criminal Justice 7 e. SAGE Publishing, 2020. 18

Sampling Methods • Lessons About Sample Quality – Need to know what population the sample is supposed to represent – Need to know exactly how cases in the sample were selected – Need to be aware how researchers talk about the implications of their study Bachman, The Practice of Research in Criminology and Criminal Justice 7 e. SAGE Publishing, 2020. 19

Sampling Methods • Generalizability in Qualitative Research – Qualitative research often focuses on populations that are hard to locate or are very limited in size. – Availability and snowball sampling are often used. – Generalizability should not be ignored in qualitative research. Bachman, The Practice of Research in Criminology and Criminal Justice 7 e. SAGE Publishing, 2020. 20

Sampling Distributions • Estimating Sampling Error – Inferential statistics a) The tool for calculating sampling error Bachman, The Practice of Research in Criminology and Criminal Justice 7 e. SAGE Publishing, 2020. 21

Sampling Distributions • Estimating Sampling Error – Normal distribution a) Bell shape b) Symmetric – The shape of a normal distribution is produced by random-sampling error. – Sample statistic – Population parameter – Confidence intervals and limits Bachman, The Practice of Research in Criminology and Criminal Justice 7 e. SAGE Publishing, 2020. 22

Sampling Distributions • Estimating Sampling Error – Sample statistic is identical to the population parameter. a) We can have a lot of confidence that the value at the peak of the bell curve represents the norm for the entire population. – Confidence intervals and limits Bachman, The Practice of Research in Criminology and Criminal Justice 7 e. SAGE Publishing, 2020. 23

Open-Access Student Resources • • • Quizzes e. Flashcards SAGE journal articles Multimedia resources URL Bachman, The Practice of Research in Criminology and Criminal Justice 7 e. SAGE Publishing, 2020. 24