Figure 4
The diagram centers around a document icon labeled “Seed reference”. The Seed reference appears at the center of the diagram along a vertical line. At the bottom of the diagram, a horizontal arrow labeled “Time” runs from left to right with the labels “Older” on the left and “Newer” on the right, with an arrowhead located at Newer. The vertical line intersects the horizontal line in the middle. A leftward arrow points from the Seed reference to a document icon labeled “Cited reference (direct backward citation tracking)” located on the left of the diagram. Above the Seed reference along the vertical line are three document icons labeled “Co-citing reference (indirect citation tracking)”. One icon is positioned near the center-right above the Seed reference, one directly above the Seed reference, and the third toward the center-left. Three downward curved arrows point from these icons to the Cited reference (direct backward citation tracking). To the right of the Seed reference is a document icon labeled “Citing reference (direct forward citation tracking)”. A leftward arrow points from this icon to the Seed reference. Below the Seed reference along the vertical line are three document icons labeled “Co-cited reference (indirect citation tracking)”. One icon is positioned near the center-right below the Seed reference, one directly below the Seed reference, and the third toward the center-left. Three downward curved arrows point from the Citing reference toward these three icons for Co-cited reference at the bottom.Citation tracking method. Source: Adapted from Hirt et al. (2020)