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SD, SEM, and 95% CI are three different error bars answering three different questions, and they're not interchangeable. The standard error vs standard deviation confusion is where most figures go wrong: SEM is always the smallest, so it makes data look tighter than it is, which is why reviewers flag it most. This piece explains what each bar actually claims, shows one dataset drawn three ways, and tells you which to use, and when to just report SD with your n.