The errors are mainly due to the bias in the presentation. A random error is just as likely to be an overestimate as an underestimate, whereas bias is a systematic error that consistently overestimates or underestimates the true length. Most people who compare the lengths overestimate the length of BC, with its outward-pointing arrow, because of the optical illusion caused by the direction of the arrowheads. They are less likely to make this error when the arrows are replaced by perpendicular lines.
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