ISSUES RELATED TO SAMPLING Why Sample? Probability vs. Non-Probability Samples Population of Interest Sampling Frame.

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ISSUES RELATED TO SAMPLING Why Sample? Probability vs. Non-Probability Samples Population of Interest Sampling Frame

Types of Random Samples Simple Random Sampling Systematic Sampling with a Random Start Stratified Random Sampling Multi-Stage Cluster Sampling

Types of Non-Random Samples Purposive Sample Quota Samples Snowball Sample Available Subjects

SAMPLE SIZE The required sample size depends principally on two things: 1. the heterogeneity of the population in question; 2. the degree of accuracy required in conclusions. (Gray and Guppy p. 157)

CALCULATING SAMPLE SIZE Efficient sample sizes can be calculated if you know how accurate the results must be as well as how much variation exists in the population. The necessary level of accuracy depends on the kinds of consequences or decisions that are to be based on the research results.

Estimating the variability in a population is more difficult. The primary reason for doing a survey is to learn something about a population, and so knowledge of variability in the population is usually not readily available. (Gray and Guppy p. 160)

There are some methods that can be used to estimate variability: 1. Ask experts. You can ask people knowledgeable about a population to estimate rates of variability for key variables. 2. Use a pilot test. From a very small, random sample of the population, you can calculate measures of variability to use in determining sample sizes. 3. Use previous results. Sometimes the results of earlier research can be used to estimate variability. 4. Make an educated guess. As a last resort, estimate the lowest and highest values (i.e. the range) on a key variable and divide this range by four. (Gray and Guppy p. 160)

SOME PRACTICAL CONSIDERATIONS IN CALCULATING SAMPLE SIZE 1. Response rates. 2. Subgroup Analysis. 3. Cost.

Formula for Calculating Sample Size for Estimating a Proportion ( B ):

Formula for Calculating Sample Size for Estimating a Mean ( : ):