Error Bar Overlap
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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 error bars reliability whether two error bars overlap or not, and try to reach a conclusion
Standard Error Bars Don't Overlap
about whether the difference between means is statistically significant. Resist that temptation (Lanzante, 2005)! SD error bars SD error bars error bars statistical significance 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 what does overlap in standard deviation mean also takes into account sample size. If the samples were larger with the same means and same standard deviations, the 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
Error Bar Overlay
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 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 d
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 not overlap? When overlapping error bars excel standard error (SE) bars do not overlap, you cannot be sure that the difference between large error bars two means is statistically significant. Even though the error bars do not overlap in experiment 1, the difference is not statistically significant (P=0.09 by
Sem Error Bars
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 2) you can be http://www.graphpad.com/support/faqid/1362/ 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 if two SE error bars do not https://egret.psychol.cam.ac.uk/statistics/local_copies_of_sources_Cardinal_and_Aitken_ANOVA/errorbars.htm 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 or not the error bars for each group overlap tells you nothing about theP
Forum Microsoft Office Application Help - Excel Help forum Excel Charting & Pivots Error bars overlap and are indistinguishable To get replies by our experts at nominal charges, follow this link to buy points and post your thread in our http://www.excelforum.com/excel-charting-and-pivots/508573-error-bars-overlap-and-are-indistinguishable.html Commercial Services forum! Here is the FAQ for this forum. + Reply to Thread Results 1 to 3 of 3 Error bars overlap and are indistinguishable Thread Tools Show Printable Version Subscribe to this Thread… Rate This http://mathbench.umd.edu/modules/prob-stat_bargraph/page08.htm Thread Current Rating Excellent Good Average Bad Terrible Display Linear Mode Switch to Hybrid Mode Switch to Threaded Mode 02-04-2006,05:57 PM #1 hadley View Profile View Forum Posts Registered User Join error bar Date 02-04-2006 Posts 2 Error bars overlap and are indistinguishable Hello, Does anyone have advice as to how to "nudge" a data series so that its error bars can be seen instead of overlapping with the error bars of another data series? Thanks. Register To Reply 02-04-2006,11:10 PM #2 Jon Peltier Guest Re: Error bars overlap and are indistinguishable In a line chart, you're kind of out of luck, since the data points need error bar overlap to line up with the categories. In an XY chart, you can add or subtract a small amount from the X value to slide the point over. - Jon ------- Jon Peltier, Microsoft Excel MVP Peltier Technical Services Tutorials and Custom Solutions http://PeltierTech.com/ _______ "hadley"
and found 6: Error bars 7: Practice with error bars 8: And another way: the standard error 9: The same graph both ways 10: Review map| <| >| home Another way to add info: the standard error Graphs using standard deviation (SD) tell you what a big population of fish would look like -- whether their sizes would be all uniform, or somewhat raggedy, or totally raggedy. Sometimes, though, you don't really care what a population looks like, you just want to know, did a treatment (like Fish2Whale instead of other competing brands) make a difference on average? In that case you measure a bunch of fish because you're trying to get a really good estimate of the average effect, despite whatever raggediness might be present in the populations. Let's say your company decides to go all out to prove that Fish2Whale really is better than the competition. They convert a supply closet into an acquarium, hatch 400 fish, and tell you to do a HUGE experiment. The whole idea of the HUGE experiment is to get a really accurate measurement of the effect of Fish2Whale, despite the natural differences such as temperature, light, initial size of fish, solar flares, and ESP phenomena. The return on their investment? Really small error bars. But how do you get small error bars? Just using 400 fish WON'T give you a smaller SD. A huge population will be just as "ragged" as a small population. Instead, you need to use a quantity called the "standard error", or SE, which is the same as the standard deviation DIVIDED BY the square root of the sample size. Since you fed 100 fish with Fish2Whale, you get to divide the standard deviation of each result by 10 (i.e., the square root of 100). Likewise with each of the other 3 brands. So your reward for all that work is that your error bars are much smaller: Why should you care about small error bars? Well, as a rule of thumb, if the SE error bars for the 2 treatme