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# Topic: Sample size

###### In the News (Thu 22 Feb 18)

 Some Sample Size Theory   (Site not responding. Last check: 2007-10-31) The behavior of the difference in sample means under the null hypothesis of equal population means is illustrated by the normal distributions on the left-hand side of displays (a) through (d) for sample sizes of 12, 24, 48, and 96 per group, respectively. The sample size that may be adequate to demonstrate that the population means are not all the same may be inadequate to demonstrate exactly where the differences occur. Instead, sample sizes are calculated by using a Bonferroni adjustment to the size of the test, that is, the nominal size of the test is divided by the number of comparisons that will be performed. www.tufts.edu /~gdallal/sizenotes.htm   (3158 words)

 Determining Sample Size 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). The fourth approach to determining sample size is the application of one of several formulas (Equation 5 was used to calculate the sample sizes in Table 1 and Table 2). Where n is the sample size, N is the population size, and e is the level of precision. edis.ifas.ufl.edu /PD006   (2171 words)

 Sample Size Calculator The mathematics of probability proves the size of the population is irrelevant, unless the size of the sample exceeds a few percent of the total population you are examining. This means that a sample of 500 people is equally useful in examining the opinions of a state of 15,000,000 as it would a city of 100,000. Population size is only likely to be a factor when you work with a relatively small and known group of people (e.g., the members of an association). www.geocities.com /profemery/samplesize.htm   (1126 words)

 Managers' Monitoring Manual | Setting Sample Size Sample sizes will be defined by what statistical test you use to analyze the data. Consequently, to calculate your sample size or to estimate the statistical power of your sampling regime you must use equations or simulations specific to the statistical analysis technique. While the other factors that affect sample sizes are set based on your desired need for precision and the smallest degree of change you want to detect, the mean and variance are factors that are set by the animals being sampled. www.pwrc.usgs.gov /monmanual/samplesize.htm   (2318 words)

 Questionnaire sample size calculator When you take only a small sample size, you have a high risk of not being able to recognise that more the 85% really are at least 'very satisfied' even when they are. If you move the alpha slider to 8.5%, the sample size has reduced to 106 because you have increased your tolerance to fog to the extent that are comfortable not being able to distinguish 89.75% from 85%. If you move the slider to 3%, the sample size has increased to 1355 and your tolerance of fog has reduced to the extent that you want to be able to distinguish 86.82% from 85%. www.netgm.com /tools/useful_stuff/questionnaire_sample_size.html   (2384 words)

 Survey Sample Size   (Site not responding. Last check: 2007-10-31) Many of you may have seen formulae for calculating the sample size or sampling error for a research project, but it can be difficult to see how to translate these equations into practical applications. That is to say, N = 384 is the sample size to use if we want observed percentages to be within plus or minus 5 percent of the unknown population parameter. If you wanted to always have a sampling error of plus or minus 5 percent and always have the most conservative estimate of your response distribution (that is, assume a 50/50 split in responses to your questions), your sample would always be 381. www.ryerson.ca /~mjoppe/ResearchProcess/SurveySampleSize.htm   (1149 words)

 Sample Size We do not want to use the sample deviation S at this point because S may be an underestimate of s. So we require a random sample of size 389 from the 600 members of the population. Suppose we want a maximum margin of error of 20 when finding a 98% confidence interval for the average GRE of Graduate School applicants, where a minimum score of 1300 out of a possible 1600 is required. www.wku.edu /~david.neal/statistics/misc/sampsize.html   (898 words)

 New View of Statistics: Sample Size Formulae   (Site not responding. Last check: 2007-10-31) Sample sizes may therefore have to be doubled or quadrupled when effects are represented by psychometric or other variables that have modest (~0.7) validity. Read the earlier section on sample size for an experiment for an overview of the role of typical error in sample-size estimation, and for an important detail about the conditions in a reliability study aimed at estimating sample size. Certainly, nearly doubling the group size from the usual 400 would be a good start in this example, because as we've already seen on this page, that would be equivalent to a p value of 0.01 and a Type II error of 10%, instead of the usual 0.05 and 20%. www.sportsci.org /resource/stats/ssdetermine.html   (3764 words)

 Sample Size   (Site not responding. Last check: 2007-10-31) The frequency of the event being studied, the size of the effect or the difference that you are trying to detect, the design of the study, and the sample size all affect the power of a study. This is because a larger sample size is needed to detect changes in rare occurrences or small values. This more complicated sampling strategy would have a smaller SE than there would be in study in which a simple random sample of people were assigned to the sides of the bed to get up from, regardless of their usual habits. www.familypractice.ubc.ca /research/samplesize2.htm   (1246 words)

 Sample size calculator   (Site not responding. Last check: 2007-10-31) The size of the population is not required, but if known gives a smaller sample size. Sample size required for estimating a proportion with absolute margin of error Sample size required for estimating a proportion with relative margin of error www.berrie.dds.nl /Sample_size.html   (242 words)

 Sample size calculations online. For two means, considering statistical power and alpha levels, usefull for analysis of ... The sample size for the pairwise t-test is calculated. Now, the problem is, that the formulae available for doing sample size calculation for population analysis are in the case of proportions meant for the chi-square and the Fisher, the one is not very appropriate for this type of data, the other impossible. It is customary to study the changes in a crosstable, with the numbers of 'A' and 'B' before and after the intervention in the two marginals and the changing respondents inside the table, with a diagonal of non-changes (cells aa and bb) and a diagonal of changers (cells ab and ba). home.clara.net /sisa/sampshlp.htm   (2687 words)

 Sample Size: Surveys Rather than survey a simple random sample of the population (which might have the survey team visiting every village), it is usually more practical to take a simple random sample of villages and then take a random sample of individuals from each village. A cluster sample is always less precise than a simple random sample of the same size, but it is usually a lot less expensive to obtain. Many of the sample size calculations for complex surveys involve estimates of quantities that are often unavailable. www.tufts.edu /~gdallal/size2.htm   (1087 words)

 Stats: Quick sample size calculations The sample size that provides 80% power for detecting a halving of risk is actually 553 per group, not 625. This ratio is called the effect size or the standardized effect size.Your sample size per group is 16 divided by the square of the effect size. The sample size that provides 80% for detecting a two week shift in breast feeding duration is actually 394, not 400. www.childrens-mercy.org /stats/size/quick.asp   (1692 words)

 Sample Size   (Site not responding. Last check: 2007-10-31) If samples are taken from each of "a" populations, then the small letter "n" is used to designate size of the sample from each population. When there are samples from more than one population, N is used to indicate the total number of subjects sampled and is equal to (a)(n). from the second, etc. The total number of subjects sampled would still be indicated by N. When correlations are computed, the sample size (N) refers to the number of subjects and thus the number of pairs of scores rather than to the total number of scores. davidmlane.com /hyperstat/A104571.html   (176 words)

 Sample size   (Site not responding. Last check: 2007-10-31) Note that the proportion that the sample is of the population is not a major factor in the estimate's accuracy. This is because accuracy of inferences from samples to populations Hence, the proportion that the sample is of the population has little practical effect on its accuracy. www.janda.org /c10/Lectures/topic09/L34-samplesize.htm   (911 words)

 SAMPLE MEDICINES Several cases have been reported where the samples have fallen into the hands of children, the latter partaking of them and becoming dangerously ill. In one case it was only the prompt services of a physician that saved the life of the little one. The danger of leaving samples of any sort in places where children and ignorant persons can get hold of them is apparent to all us, and unless samples can be delivered with descriptive literature by mail or otherwise, to responsible individuals, the practice should be refrained from. Samples were not the only means used by companies to garner public attention. www.bottlebooks.com /Sample%20Medicines/sample_medicines.htm   (2099 words)

 Determination of Sample Size At the planning stage of a statistical investigation the question of sample size (n) is critical. The aim of applying any one of the following sample size determinates is at improving your pilot estimates at feasible costs. Moreover, determination of sample size for mean could also be used for other unimodal populations including those with discrete random variables, such as proportion, provided there is sufficient pilot observations (say, over 30). home.ubalt.edu /ntsbarsh/Business-stat/otherapplets/SampleSize.htm   (1167 words)

 MaCorr Inc. Sample Size Optimization These are: sample size, percentage of sample that picked a particular answers and population size. The larger your sample, the more sure you can be that their answers truly reflect the opinion of the population. For this reason, the population size is ignored when it is "large" or unknown. www.macorr.com /ss_methodology.htm   (813 words)

 Sample Size For example, a sample size of 677 Inuit people is necessary in Nunavik in order to obtain a cv of 16.5% for a proportion of 0.10 of the Inuit population. The last two columns of the table show what the total sample size (Inuit persons plus non-Inuit persons) would be if the proposed design of collecting data for both Inuit and non-Inuit people is chosen, and the cv's of the corresponding estimates. The strategy was then to estimate the number of adults required in the sample in order to meet these requirements and apply the same sampling fraction to the children. www.iser.uaa.alaska.edu /projects/Living_Conditions/sample_size.htm   (726 words)

 Survey Sample Size It is usual to have some sub-objectives dropped due to sample size limitations. Sample size calculations for the estimation of a proportion, e.g., proportion of stream length that meets a designated use depend on: This strategy greatly increases the likelihood of attaining the desired sample sizes and fulfillment of the objectives. www.epa.gov /nhrlsup1/arm/designpages/monitdesign/sample_size.htm   (389 words)

 GraphPad StatMate sample size example StatMate presents a table showing the tradeoff between sample size, power, and the effect size that you will be able to detect as statistically significant. We already know that the mean number of alpha2-adrenergic receptors is about 250, so a sample size of 12 in each group has 80% power to detect a 35% (86.4/250) change in receptor number. Each curve is for a different power, and shows the relationship between the sample size you could choose for each group (X) and the difference you would then detect as "significant" (Y). www.graphpad.com /statmate/samplesize3.htm   (1187 words)

 Sample Size Calculator by Raosoft, Inc. Calculate a sample size or margin of error, with detailed interpretations of the statistics and the underlying algorithm. If you create a sample of this many people and get responses from everyone, you're more likely to get a correct answer than you would from a large sample where only a small percentage of the sample responds to your survey. where N is the population size, r is the fraction of responses that you are interested in, and Z(c/100) is the critical value for the confidence level c. www.raosoft.com /samplesize.html   (369 words)

 LSA: A Solution to Plato's Problem From each article we took a sample consisting of (usually) the whole text or its first 2,000 characters, whichever was less, for a mean text sample length of 151 words, roughly the size of a rather long paragraph. The variation appears to be largely determined by the size of the dictionaries sampled, and to some extent by the way in which words are defined as being separate from each other and by the testing methods employed. Conveniently, this corpus is nearly the same in both overall size, five million words, and in number of word types, 68,000, as our encyclopedia sample (counting, for the encyclopedia sample, singletons not included in the SVD analysis), so that no correction for sample size, which alters word frequency distributions, was necessary. lsi.telcordia.com /lsi/papers/PSYCHREV96.html   (20008 words)

 Stats: Selecting an appropriate sample size   (Site not responding. Last check: 2007-10-31) Someone sent me an email asking for the sample size needed to detect a 10% shift in the probability of recurrence of an event after one of two different surgical procedures is done. The sample size calculations for this type of study are a bit tricky. I was asked if this was an adequate sample size and whether there was a problem because the median survival time was unavailable for some of the subgroups. www.childrens-mercy.org /stats/size.asp   (2152 words)

 Marketing Calculator: Sample Size Calculator This tool lets you calculate the sample size needed for market research projects for any size of target population taking into account the desired confidence level and interval. Population: The sample size depends on the size of the target population and the homogeneity of the market. Hence, the overall sample size for a study with distinctive market segments is much larger than for homogeneous segments. www.steinermarketing.com /calc_sample_size.htm   (319 words)

 Managers' Monitoring Manual | References for Setting Sample Size   (Site not responding. Last check: 2007-10-31) The distribution, size, and trends of the saddlebilled stork (Ephippiorhynchus senegalensis) population in South Africa. Sampling larval fish populations: choice of sample number and size. On the importance of sampling variance to investigations of temporal variation in animal population size. www.pwrc.usgs.gov /monmanual/samplesizerefs.cfm   (828 words)

 Del Siegle - Sample Size The size of the population and the amount of error the researcher is willing to tolerate is what determines the size of the sample. The table that follows was developed for situations where the researcher wants to come within 5 percentage points (with 95 percent certainty) of what the results would have been if the entire population had been surveyed. sample size calculator that allows you to enter your preferences. www.gifted.uconn.edu /siegle/research/Samples/samsize.html   (117 words)

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