![]() ![]() The example states, "Suppose we observe that, in our sample of 50 commuters, the average length of travel to work is 30 minutes with a population standard deviation of 2.5. The Excel Help file for CONFIDENCE has been rewritten for Excel 2003 and for Excel 2007 because all earlier versions of the Help file gave misleading advice on interpreting results. Note: See the article on NORMSINV for a discussion of computational differences in NORMSINV.įor more information, click the following article number to view the article in the Microsoft Knowledge Base:Ĩ26772 Excel statistical functions: NORMSINV Interpretation of the results of CONFIDENCE Values of alpha have to be much smaller than that, for example 0.0000001, before round-off errors in NORMSINV are likely to be noticed. In practice, alpha is generally set to 0.05, 0.01, or maybe 0.001. Inaccuracies in NORMSINV generally occurred for values of its argument very close to 0 or very close to 1. This does not mean that you should lose confidence in CONFIDENCE for earlier versions of Excel. Therefore, CONFIDENCE may return different (and improved) results in these later versions of Excel, because CONFIDENCE relies on NORMSINV. No changes were made directly to CONFIDENCE, but NORMSINV was improved in Microsoft Excel 2002, and then more improvements were made between Excel 2002 and Excel 2007. Cell A7 shows the same value because a call to CONFIDENCE(alpha, sigma, n) returns the result of computing: Note: In Excel 2007, with the pasted range of cells selected, click Format in the Cells group on the Home tab, and then click AutoFit Column Width.Ĭell A6 shows the value of CONFIDENCE. To illustrate the CONFIDENCE function, create a blank Excel worksheet, copy the following table, and then select cell A1 in your blank Excel worksheet. A 95% or 0.95 confidence interval corresponds to alpha = 1 – 0.95 = 0.05. ![]() ![]() You want to compute a 95% confidence interval for the population mean. You test IQs for a sample of 50 students in your local school and obtain a sample mean of 105. Example of usageĪssume that intelligence quotient (IQ) scores follow a normal distribution with standard deviation 15. Typically, alpha is a small probability, such as 0.05. Sigma is a positive number, and n is a positive integer that corresponds to the sample size. Parameters: Alpha is a probability and 0 < alpha < 1. Observations in the sample are assumed to come from a normal distribution with known standard deviation, sigma, and the number of observations in the sample is n. The confidence interval is a range of values that are centered at a known sample mean. The CONFIDENCE(alpha, sigma, n) function returns a value that you can use to construct a confidence interval for a population mean. The meaning of a confidence interval is frequently misinterpreted, and we try to provide an explanation of valid and invalid statements that can be made after you determine a CONFIDENCE value from your data. This article describes the CONFIDENCE function in Microsoft Office Excel 2003 and in Microsoft Office Excel 2007, illustrates how the function is used, and compares the results of the function for Excel 2003 and for Excel 2007 with the results of CONFIDENCE in earlier versions of Excel. ![]()
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