How to extract error bars from a graph
A meta-analysis needs the spread as well as the mean, and the spread is usually only in the figure. Here is how to get both.
What comes back
- Each point or bar with its value.
- The low and the high end of its error bar, as values, in columns of their own.
- Bars that are longer one way than the other keep their two ends.
- Error bars along x are read too.
Steps
- Drop in the chart.
- Each error-bar end is drawn back as a short cap: check the caps against the chart.
- Download Excel: the ends are in the y low and y high columns.
How exact it is
When a PDF holds the chart as a drawing (most figures in papers do), the values are read from the drawing itself. Nothing is estimated from pixels, so a point is as precise as the file is.
From pictures, charts with error bars are the harder ones: in our tests they were read fully right less often than charts without. An end that does not sit on a mark is flagged for you to look at.
Questions
Does it say whether a bar is a standard error or a confidence interval?
No. It reads where the bar ends. What the bar stands for is in the figure's caption.