Degree Of Variability The third criterion, the degree of variability in the attributes being measured refers to the distribution of attributes in the population. Distribution of Means for Repeated Samples. Woeste, DeanĢ Determining Sample Size Using A Sample Size Of A Similar Study Figure 1. Florida Cooperative Extension Service / Institute of Food and Agricultural Sciences / University of Florida / John T. For information on obtaining other extension publications, contact your county Cooperative Extension Service office. Israel, associate professor, Department of Agricultural Education and Communication, and extension specialist, Program Evaluation and Organizational Development, Institute of Food and Agricultural Sciences (IFAS), University of Florida, Gainesville The Institute of Food and Agricultural Sciences is an equal opportunity/affirmative action employer authorized to provide research, educational information and other services only to individuals and institutions that function without regard to race, color, sex, age, handicap, or national origin.
#Stereology determining sampling precision series
This document is Fact Sheet PEOD-6, a series of the Program Evaluation and Organizational Development, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. This risk is reduced for 99% confidence levels and increased for 90% (or lower) confidence levels. Such samples with extreme values are represented by the shaded areas in Figure 1. There is always a chance that the sample you obtain does not represent the true population value. In other words, this means that, if a 95% confidence level is selected, 95 out of 100 samples will have the true population value within the range of precision specified earlier (Figure 1). In a normal distribution, approximately 95% of the sample values are within two standard deviations of the true population value (e.g., mean).
Furthermore, the values obtained by these samples are distributed normally about the true value, with some samples having a higher value and some obtaining a lower score than the true population value. The key idea encompassed in the Central Limit Theorem is that when a population is repeatedly sampled, the average value of the attribute obtained by those samples is equal to the true population value. The Confidence Level The confidence or risk level is based on ideas encompassed under the Central Limit Theorem. Thus, if a researcher finds that 60% of farmers in the sample have adopted a recommended practice with a precision rate of ±5%, then he or she can conclude that between 55% and 65% of farmers in the population have adopted the practice. This range is often expressed in percentage points, (e.g., ±5 percent), in the same way that results for political campaign polls are reported by the media. The Level Of Precision The level of precision, sometimes called sampling error, is the range in which the true value of the population is estimated to be. SAMPLE SIZE CRITERIA In addition to the purpose of the study and population size, three criteria usually will need to be specified to determine the appropriate sample size: the level of precision, the level of confidence or risk, and the degree of variability in the attributes being measured (Miaoulis and Michener, 1976).
This paper reviews criteria for specifying a sample size and presents several strategies for determining the sample size. Interested readers may obtain a more detailed discussion of the purpose of the study and population size in Sampling The Evidence Of Extension Program Impact, PEOD-5 (Israel, 1992). Israel 2 Perhaps the most frequently asked question concerning sampling is, "What size sample do I need?" The answer to this question is influenced by a number of factors, including the purpose of the study, population size, the risk of selecting a "bad" sample, and the allowable sampling error. 1 Fact Sheet PEOD-6 November 1992 Determining Sample Size 1 Glenn D.