We explore the ability of humans to compare strings, an error-prone activity which occurs in critical contexts (e.g., bank wire transfers), where it is not always possible to trust a machine to help. We hypothesize that understanding human cognitive and perceptual limitations can improve how people verify whether two character strings are identical. We vary the strings' relative positions and presentation formats to measure comparison speed and accuracy. We present results from two online experiments with 178 human subjects who compared pairs of strings to determine whether the two strings were identical. The strings varied by length (5 characters to small paragraphs), difference (matching versus making a random change or a hard-to-detect one), alignment (vertical versus horizontal), and presentation (Overlay versus Adjacent). Our experiments test whether, by encoding and memory tasks into perceptual tasks, overlay formats can offer advantages in speed and accuracy. Results show that overlay offers a substantially faster method, with improved accuracy for long strings. Nevertheless, mean error rates were at least 4 percent, indicating that even the improved approach may be insufficiently accurate for many verification tasks.

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