Mean values on a bar chart or line graph are often shown with error bars. These show the variability of the data used to calculate each mean.The bars may show the range (highest to lowest value) or the mean plus and minus one standard deviation. Always check which is shown.
Long error bars show that the data are widely spread, so the mean is less reliable. Short error bars show that repeat readings were close together.If the standard deviation bars of two means overlap, the difference between the means is probably due to chance and is unlikely to be significant.If the bars do not overlap, the difference is more likely to be real. However, a statistical test is needed to decide whether the difference is significant.In an exam, use the error bars when evaluating a conclusion, for example: "the standard deviations overlap, so the difference in mean growth between the two fertilisers may be due to chance".
Long error bars show that the data are widely spread, so the mean is less reliable. Short error bars show that repeat readings were close together.If the standard deviation bars of two means overlap, the difference between the means is probably due to chance and is unlikely to be significant.If the bars do not overlap, the difference is more likely to be real. However, a statistical test is needed to decide whether the difference is significant.In an exam, use the error bars when evaluating a conclusion, for example: "the standard deviations overlap, so the difference in mean growth between the two fertilisers may be due to chance".