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Representative Sampling

Representative Sampling . Presented at the AWDS Task Force’s Marketing Workshop Big Sky, Montana Friday, September 20, 2002 Len Singel, AWDS Coordinator. Overview. Definitions Reducing Error Sample Creation Examples. Definitions.

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Representative Sampling

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  1. Representative Sampling Presented at the AWDS Task Force’s Marketing WorkshopBig Sky, Montana Friday, September 20, 2002 Len Singel, AWDS Coordinator

  2. Overview • Definitions • Reducing Error • Sample Creation • Examples

  3. Definitions • Population – consists of all the units (individuals, households, organizations) to which you desire to generalize survey results • Sample Frame – list from which your sample is drawn

  4. Definitions • Sample – the set of respondents selected from a larger population for the purpose of a survey • Completed Sample – consists of all units that return completed questionnaires (instruments)

  5. Definitions • Coverage Error – results from every unit in the Survey Population not having a known, non-zero chance of being included in the sample • Sampling Error – the result of collecting data from a subset, rather than all of the members of the sampling frame

  6. Avoiding Coverage Error

  7. How to Reduce Coverage Error? • Make certain your list contains everyone in the Survey Population • Unique, non-repeating Sampling Units • Update & Properly maintain your list • Have your list contain other information that can be used to improve your survey

  8. How Large Should Your Sample be? • Sample does not have to be arbitrarily selected • ALL THAT IS REQUIRED: • Knowing about your study population AND • Knowing your survey objectives

  9. How Large Should Your Sample Be? • Sample Size Depends On: • How much sampling error can be tolerated • Population size • Variation of Population (50/50 or 80/20) • Amount of Confidence

  10. Does Size Matter

  11. Small Populations

  12. Larger Populations

  13. 3 Easy Steps to Sampling • Identify your Target Population • Put together a Population List • Select the Sample

  14. Hey, Wait a Minute….. I DON’T KNOW HOW TO SELECT THE SAMPLE!!!

  15. Sampling Methods • Probability • Simple Random Sampling (SRS) • Systematic Sampling • Nonprobability (Purposeful)

  16. Probability Sampling Method • Simple Random Sampling (SRS) • Each member of the target population has an equal chance of being selected • All elements are selected randomly – CAN BE CUMBERSOME!!! • Systematic Sampling • Members of the target population have an equal chance of being selected • Only the first element is selected randomly; rest selected systematically (every 5th, 10th, …)

  17. Purposeful Sampling Method • Depends on subjective judgment • Selected because it is convenient or typical • All members do not have the same chance of being selected

  18. Purposeful Sampling Method • Appropriate for: • Exploratory Research • Focus Groups • Organizing Committees • Building Networks • Do notuseif your goal is to learn about the larger population –Results are not generalizable!!

  19. Example 1: Successful Hunting Experience • Target Population: 1 million Deer Hunters • Survey Objectives: Learn about Hunting Success • Acceptable Sampling Error: + 3% • Confidence Level: 95% • Population Variation: 80/20

  20. Example 1:Successful Hunting Experience • Population List: Automated Licensing Database • Sample Size: 683 • Sampling Method: Systematic Sampling

  21. Example 2:Female Hunter Perceptions • You know you want to study female waterfowl hunters and their satisfaction levels. • But… • You do not know much about them AND • You do not know your survey objectives

  22. Example 2:Female Hunter Perceptions • Target Population: Female Waterfowl Hunters (n = 100?) • Survey Objectives: Explore their satisfaction levels • Acceptable Sampling Error: + 3% • Confidence Level: 95% • Population Variation: 50/50 or 80/20 ???

  23. Example 2:Female Hunter Perceptions • Source: Automated Licensing Database • Sample Method: ?? • Sample Size:??

  24. Example 2:Female Hunter Perceptions The Solution! • Conduct Exploratory Research • Increase Sampling Error to + 10% • Hold a series of focus groups (n = 50)

  25. Example 2:Female Hunter Perceptions • Enables you to… • Learn about Population Variation • Learn about General Characteristics • Define Survey Objectives • Select appropriate sample size to be surveyed.

  26. Review • What does sample size depend on? • What are the 3 steps to sampling? • Does size matter?

  27. Sources • Dillman, D.A. 2000. Mail and Internet Surveys: The Tailored Design Method. John Wiley & Sons, Inc. 464pp. • Salant, P. and D.A. Dillman. 1994. How to Conduct your Own Survey. John Wiley & Sons, Inc. 232pp.

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