Confidence interval The range of values consistent with the data that is believed to encompass the actual or true population value Sauke TA Sex M 2000 How to Repost Statistics in Medicine Chd ed Philadelphia American College of Pesicians To draw conclusions about a study population researchers use samples that they assume truly represent the population The confidence interval CI is among the most reliable indicators of the soundness of their assumption A CI is the range of values within which the population value being studied is believed to fall Cls are reported in the results section of published research and are often calculated either for mean or proportion data calculation details are beyond the scope of this article A 95 CI which is the most common level used others are 90 and 99 means that if researchers were to sample numerous times from the same population and calculate a range of estimates for these samples 95 of the intervals within the lower and upper limits of this range will include the population value To illustrate the 95 CI of a mean value say that a sample of patients with hypertension has a mean blood pressure of 120 mmHg and that the 95 CI for this mean was calculated to range from 110 to 130 mmHg This might be reported as mean 120 mmHg 95 CI 110 130 mmHg It indicates that if other samples from the same population of patients were generated and intervals for the mean blood pressure of these samples were estimated 95 of the intervals between the lower limit of 110 mmHg and the upper limit of 130 mmHg would include the true mean blood pressure of the population Notice that the width of the Cl range is a very important indicator of how reliably the sample value represents the population in question If the Cl is narrow as it is in our example of 110 130 mmHg then the upper and lower limits the sample This sample mean value is probably a more reliable estimate of the true mean value of the population than a sample mean value with a wider CI of for example 110 210 mmHg With such a wide CI the population mean could be as high as 210 mmHg which is far from the sample mean of 120 mmHg In fact a very wide CI in a study should be a red flag it indicates that more data should have been collected before any serious conclusions were drawn about the population Remember the narrower the Cl the more likely it is that the sample value represents the population value NurseONE resources ON THIS TOPIC EBSCO MEDLINE FULL TEXT ARTICLES Hildebrandt M Vervolgyl E Bender R 2009 Calculation of NNTs in RCTs with time to event outcomes A literature review BMC Medical Research Methodology 9 21 Hildebrandt M Bender R Gehrmann U Blettner M 2006 Calculating confidence intervals for impact numbers BMC Medical Research Methodology 6 32 Altman D G 1998 Confidence intervals for the number needed to treat BM Clinical Research Ed 317 7168 1309 1312 MYILIBRARY Campbell M J Machin D Walters S L 2010 Medical statistics A textbook for the health sciences 4th ed Mateo M A Kirchhoff K T Eds 2009 Research for advanced practice nurses From evidence to practice