Fitts (1954) 根據訊息理論提出費茲定律來描述速度與準度的消長，強調影響瞄準動作主要的兩個變數是(1)目標距離(2)目標寬度，由這兩個變數比例組成的難度指數 (ID) log_2^(2A/w) 與動作時間存在著正線性關係，即難度指數愈高花費時間愈長。然而構成難度指數的變數，動作距離與目標寬度如何各自影響動作時間，則尚未見系統性的探討。本實驗選取9種寬度和距離總共81種試作組合用等效試作的概念探討在不同距離、寬度值對動作時間變異的影響。實驗結果發現寬度小使難度增加，花費更多時間。寬度大時減少難度，但影響程度非對稱性的。距離短時會增加試作難度，距離長時除了最小的兩個目標寬度外，長距離與其餘的目標寬度組合都會減少試作難度，表示寬度小在快速瞄準動作中佔有額外的影響力。因此，目標寬度和動作距離在中間值時較符合費茲定律的描述，在極端值時較沒有線性趨勢。透過本實驗可了解到參數漸增過程動作時間的變化，提供了分析時間變異的參考。

The effect of target width and target distance in aimed movement has been described clearly in Index of difficulty (ID: log_2^(2A/w) ) equation ( Fitts, 1954 ). The positive correlation of movement time and ID is robust and widely used in human-computer interface design. Purpose: How distance and target width affects movement time independently is seldom discussed. Method: In this article, we used 9 target widths and 9 distances to compose 81 levels of aiming task to observe the effect of target width and distance to movement time by introducing an effective assumption approach. Results: The results showed that both target width and distance did not affect movement time linearly like ID described. Small distances and target widths tended to increase movement time, and large distances and target widths decreased movement time. Movement time tended toward a nonlinear relation with both parameters in extreme values. Accuracy demand decreased when target width exceeded critical point. Long distance provided room for acceleration to high velocity and made the movement more efficient. The linear Fitts' Law relation happened in middle range of target width and distance. Conclusion: In summary, these two parameters behaved differently when increased gradually. Effective assumption approach offers more information about the source of change in movement time than traditional statistical analyses, and provides another point of view describing movement time variability.

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