Strategies in unbounded number line estimation? Evidence from eye-tracking

For bounded number line estimation, recent studies indicated influences of proportion-based strategies as documented by eye-tracking data. In the current study, we investigated solution strategies in bounded and unbounded number line estimation by directly comparing participants' estimation performance as well as their corresponding eye-fixation behaviour. For bounded number line estimation, increased numbers of fixations at and around reference points (i.e. start, middle and endpoint) confirmed the prominent use of proportion-based strategies. In contrast, in unbounded number line estimation, the number of fixations on the number line decreased continuously with increasing magnitude of the target number. Additionally, we observed that in bounded and unbounded number line estimation participants' first fixation on the number line was a valid predictor of the location of the target number. In sum, these data corroborate the idea that unbounded number line estimation is less influenced by proportion-based estimation strategies not directly related to numerical estimations.