  Scientific knowledge develops through cumulative discoveries that build on,
contradict, contextualize, or correct prior findings. Scientists and
journalists often communicate these incremental findings to lay people through
visualizations and text (e.g., the positive and negative effects of caffeine
intake). Consequently, readers need to integrate diverse and contrasting
evidence from multiple sources to form opinions or make decisions. However, the
underlying mechanism for synthesizing information from multiple visualizations
remains underexplored. To address this knowledge gap, we conducted a series of
four experiments (N = 1166) in which participants synthesized empirical
evidence from a pair of line charts presented sequentially. In Experiment 1, we
administered a baseline condition with charts depicting no specific context
where participants held no strong belief. To test for the generalizability, we
introduced real-world scenarios to our visualizations in Experiment 2, and
added accompanying text descriptions similar to on-line news articles or blog
posts in Experiment 3. In all three experiments, we varied the relative
direction and magnitude of line slopes within the chart pairs. We found that
participants tended to weigh the positive slope more when the two charts
depicted relationships in the opposite direction (e.g., one positive slope and
one negative slope). Participants tended to weigh the less steep slope when the
two charts depicted relationships in the same direction (e.g., both positive).
Through these experiments, we characterize participants' synthesis behaviors
depending on the relationship between the information they viewed, contribute
to theories describing underlying cognitive mechanisms in information
synthesis, and describe design implications for data storytelling.
