How To Interpret Error Bar Plots
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in a publication or presentation, you may be tempted to draw conclusions about the statistical significance of differences between group means by looking at whether the error bars overlap. Let's look at two contrasting examples. What can you conclude when standard error bars do overlapping error bars not overlap? When standard error (SE) bars do not overlap, you cannot be sure that the
Standard Error Bars Excel
difference between two means is statistically significant. Even though the error bars do not overlap in experiment 1, the difference is not statistically
How To Calculate Error Bars
significant (P=0.09 by unpaired t test). This is also true when you compare proportions with a chi-square test. What can you conclude when standard error bars do overlap? No surprises here. When SE bars overlap, (as in experiment
How To Draw Error Bars
2) you can be sure the difference between the two means is not statistically significant (P>0.05). What if you are comparing more than two groups? Post tests following one-way ANOVA account for multiple comparisons, so they yield higher P values than t tests comparing just two groups. So the same rules apply. If two SE error bars overlap, you can be sure that a post test comparing those two groups will find no statistical significance. However error bars standard deviation or standard error if two SE error bars do not overlap, you can't tell whether a post test will, or will not, find a statistically significant difference. What if the error bars do not represent the SEM? Error bars that represent the 95% confidence interval (CI) of a mean are wider than SE error bars -- about twice as wide with large sample sizes and even wider with small sample sizes. If 95% CI error bars do not overlap, you can be sure the difference is statistically significant (P < 0.05). However, the converse is not true--you may or may not have statistical significance when the 95% confidence intervals overlap. Some graphs and tables show the mean with the standard deviation (SD) rather than the SEM. The SD quantifies variability, but does not account for sample size. To assess statistical significance, you must take into account sample size as well as variability. Therefore, observing whether SD error bars overlap or not tells you nothing about whether the difference is, or is not, statistically significant. What if the groups were matched and analyzed with a paired t test? All the comments above assume you are performing an unpaired t test. When you analyze matched data with a paired t test, it doesn't matter how much scatter each group has -- what matters is the consistency of the changes or differences. Whether o
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Graphpad.com FAQs Find ANY word Find ALL words Find EXACT phrase What you can conclude when two error bars overlap (or don't)? FAQ# 1362 Last Modified 22-April-2010 It is tempting to look at whether two error bars overlap or not, and try to reach http://www.graphpad.com/support/faqid/1362/ a conclusion about whether the difference between means is statistically significant. Resist that temptation (Lanzante, https://www.researchgate.net/post/Can_someone_advise_on_error_bar_interpretation_confidence_T_95_and_standard_deviation 2005)! SD error bars SD error bars quantify the scatter among the values. Looking at whether the error bars overlap lets you compare the difference between the mean with the amount of scatter within the groups. But the t test also takes into account sample size. If the samples were larger with the same means and same standard deviations, the error bar P value would be much smaller. If the samples were smaller with the same means and same standard deviations, the P value would be larger. When the difference between two means is statistically significant (P < 0.05), the two SD error bars may or may not overlap. Likewise, when the difference between two means is not statistically significant (P > 0.05), the two SD error bars may or may not overlap. Knowing whether SD error bars how to interpret overlap or not does not let you conclude whether difference between the means is statistically significant or not. SEM error bars SEM error bars quantify how precisely you know the mean, taking into account both the SD and sample size. Looking at whether the error bars overlap, therefore, lets you compare the difference between the mean with the precision of those means. This sounds promising. But in fact, you don’t learn much by looking at whether SEM error bars overlap. By taking into account sample size and considering how far apart two error bars are, Cumming (2007) came up with some rules for deciding when a difference is significant or not. But these rules are hard to remember and apply. Here is a simpler rule: If two SEM error bars do overlap, and the sample sizes are equal or nearly equal, then you know that the P value is (much) greater than 0.05, so the difference is not statistically significant. The opposite rule does not apply. If two SEM error bars do not overlap, the P value could be less than 0.05, or it could be greater than 0.05. If the sample sizes are very different, this rule of thumb does not always work. Confidence interval error bars Error bars that show the 95% confidence interval (CI) are wider than SE error bars. It
? Hi everyone, I have a question regarding interpret my result and I need some help? I need to know whether the difference between two samples is significant or not ? sample 1 Average 43.4 std 0.52 confidence.T 0.83 sample2 : Average 45.88 std.v 0.24 conf.t 0.39 using confidence 95 % and alpha 0.05 and as I understand I can pick any of confidence 95 or 99 or 90 without any intention. - I have made error bar using custom value of Std of each sample on a graph but I do not know whether they are overlap and no significant difference or what? please any suggestion. Topics Basic Statistical Analysis × 420 Questions 154 Followers Follow Basic Statistics × 276 Questions 79 Followers Follow Basic Statistical Methods × 401 Questions 93 Followers Follow Standard Deviation × 239 Questions 19 Followers Follow Jun 20, 2015 Share Facebook Twitter LinkedIn Google+ 0 / 0 All Answers (9) Ronald E. Goldsmith · Florida State University If you provide the sample sizes for both samples, you can calculate the t-test of the difference and the confidence intervals for each mean using an online calculator. Jun 21, 2015 Khalid Al · Thank you very much for your help, each sample has been repeated four times and then average has been taken . could you please provide me by link of this and i will try but I am afraid that i can not interpret my result. waiting your response thanks alot for your time Jun 21, 2015 Jochen Wilhelm · Justus-Liebig-Universität Gießen "I need to know whether the difference between two samples is significant or not ?" This is not a thing that is answered by statistics! This can only be judged, based on what actions are taken based on rejecting or accepting some hypothesis. Statistics can calculate a "p value", what is sometimes called "(statistical) significance" (the part "statistical" is actually important because this has nothing to do with com