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Assessing the mean and a measure of error using a plot is a good initial look at the data, but is not sufficient to determine whether the difference is statistically significant. The choice of the error bar is important in this visual estimation, and the appropriate error term to plot is the standard error of the mean, not the standard deviation. The SD provides the variance in the sample, whereas the standard error of the mean provides an estimate of the true mean of the population from which the sample was taken. The statistical analysis makes an inference about the population based on the data from the sample; therefore, population estimates are the better method to assess differences. Frank Underwood From: Evidence based health (EBH) [mailto:[log in to unmask]] On Behalf Of Markos Kashiouris Sent: Tuesday, March 31, 2009 10:31 AM To: [log in to unmask] Subject: Question Frequently in basic research papers, data are graphically presented in a bar-format with one standard-deviation up and down represents the error bar. I have been working in a lab in the past. The data were presented in such way that if the SD error bars didn't overlap in the comparison groups eg cell counting etc the researchers presented their data as significant. These basic-science research papers are the basis of the pyramid which leads to animal studies and higher phase trials including human research. I was wandering if this reporting method is statistically justified and if not what the error bars shall include : 2 standard deviations each side (up or down from the bar), one SD each side of the bar or using the Standard Error? Thank you. Markos Kashiouris, MD PGY2 Resident in Internal Medicine