Their meaning depends on the statistic selected. Standard deviation describes spread in repeated observations, whereas standard error and confidence intervals address uncertainty in a summarized estimate. Because these quantities answer different questions, readers should not compare bars without checking which measure the figure uses and how it is labeled.
Overlap or separation provides a visual comparison of group summaries, but it does not by itself prove whether a statistically significant difference exists. The result depends on the defined statistical analysis, not solely on the distance between plotted extensions. Treating visual overlap as a significance test can therefore lead to incorrect conclusions.
A figure or caption should identify the statistic represented, such as standard deviation, standard error, or a confidence interval. This label explains whether the extensions describe observed spread or uncertainty around a summary. Clear identification also makes comparisons more transparent and prevents readers from assigning an unintended meaning to the same visual feature.
Start with repeated observations for each category, calculate the selected summary and its associated measure, and then extend the bar graph using that measure. The workflow is incomplete unless the statistic is identified in the figure or caption. This pairing connects the plotted extent with the intended variability or uncertainty.
They support comparisons among group means when results come from experiments, surveys, or observational studies. By displaying a measure around each category summary, they give readers additional context for judging differences and reliability. Their usefulness increases when the chosen statistic is stated clearly and visual comparisons are not treated as formal evidence of significance.
Interpretation should begin with the stated error measure, followed by consideration of the group summaries and the study context. Standard deviation, standard error, and confidence intervals convey different information, so the same apparent distance between bars can have different implications. Formal conclusions about statistical significance should not rely on overlap or separation alone.