Chapter 8 Sampling

1. Chapter 8Sampling Book Authors: Daniel R. Montello Paul C. Sutton Prepared for: GEOG 4020, Geographic Research Methodology University of Denver , Department of Geography

2. Chapter 8 Outline • Section 1: Sampling Frames and Sampling Designs • Sampling frames • Sampling designs • Nonprobability sampling • Probability sampling • Section 2: Implications of Sampling Frames and Designs • Nonparticipation and volunteer biases • Section 3:Spatial Sampling From Continuous Fields • Spatial interpolation • Section 4: Sample Size • Concept application GEOG 4020

3. Sampling • Geographers prefer general to idiosyncratic truths • With unlimited resources we cannot conduct a study that includes every case, place, time and variable; the answer is sampling • Sampling – a way of selecting a subset from the entire set of entities of interest, known as the population • Distinction between sample and population: sample is a subset of the larger group or phenomenon you are interested in studying GEOG 4020

4. Section 1Sampling Frames and Sampling Designs GEOG 4020

5. Sampling Frames • Samples are drawn from a population; however, the whole population is rarely available for sampling • Sampling frame – the subset of the population available for sampling • Useful data to geographers: • Tax records • Phone books • Census data • Maps Population Sampling Frame Sample GEOG 4020

6. Sampling Design • Sampling design – the procedure used to identify cases from the sampling frame to go into the sample • Two important types of sampling design: • Nonprobability sampling – probability of a particular case being selected is unknown beforehand • Probability sampling - probability of a particular case being selected is unknown beforehand GEOG 4020

7. Nonprobability Sampling • Convenience sampling – researcher accepts all available cases • Snowball sampling – researcher uses an existing case in the sample to locate additional cases GEOG 4020

8. Probability Sampling • Simple random sampling – Each member of the sampling frame has an equal chance of being selected in the sample • Systematic random sampling – First member is randomly selected, then every “nth” member after is selected • Stratified random sampling – Sampling frame is segmented into subsets called strata based on a variable. • Does not reduce sampling bias on average, but does guarantee that a single sample is unbiased on the stratification variable GEOG 4020

9. Probability Sampling • Cluster sampling – Researcher selected one or two variables out of convenience and then randomly samples within that area • Multistage area sampling – spatially hierarchical stratifed sampling Spatial extent of sampling frame GEOG 4020

10. Section 2Implications of Sampling Frames and Designs GEOG 4020

11. Implications of Sampling Frames and Designs • Representativeness – the degree to which the smaller set resembles the larger set • Generalizability – what larger set can we validly draw conclusions about from the evidence of the smaller set? • Various forms of nonprobability sampling are common • Can never attain true probability sampling of a population GEOG 4020

12. Nonparticipation and Volunteer Biases • Nonparticipation bias – a threat to the representativeness and generalizability of research when participants are given the opportunity to not participate • Volunteer bias – self-selection bias because cases get into studies by selecting themselves GEOG 4020

13. Section 3Spatial Sampling From Continuous Fields GEOG 4020

14. Continuous Field Sampling • Geographers sample locations within continuous fields and measure properties of those locations • Organize or break continuous space into discrete objects for study • Transects – Create linear features in space • Quadrats – Disaggregate space into quadrants and randomly sample points from within each quadrat • Nonindependent spatial sampling – samples are preferentially selected from one area over another GEOG 4020

15. Spatial Interpolation • Spatial interpolation – a form of educated guessing an unknown value between known values Accuracy Interpolation procedure Point density and distribution GEOG 4020

16. Section 4Sample Size GEOG 4020

17. Sample Size • Larger samples are more likely to be representative of the sampling frame • Cost of obtaining larger sample must be considered • Exploratory phases of research focus on a few cases • Very useful for refining research tools • For studies attempting to identify causal relationships 20-30 cases is the minimum GEOG 4020

18. Concept Application • What are populations, sample frames and samples? • What is the distinction between probability and nonprobability sampling designs, and what are the implications of this distinction for research? • What is sample representativeness, and how does it influence generalizability? • What are some implications of sampling from sets of discrete objects versus continuous fields? • What are the competing motivations for larger and smaller samples? GEOG 4020