在觸控螢幕大小有限的行動裝置上進行圖示或連結的點擊，對於使用者來說容易造成困擾－尤其當操作畫面中的目標排列過於擁擠或是探討不同年齡層的使用者表現時。目前多數探討「使用者喜好的適當目標大小」的文獻，都藉由「點擊預先決定好尺寸的目標」來決定結果；因為使用數量有限、且無法連續調整大小的目標來作測試，結論通常建議年長者須使用較大的目標。拜科技之賜，使用者現今可在多點觸控螢幕上，透過掐捏手勢進行連續的目標縮放；因此，本研究也運用多點觸控科技，探討符合兩種不同操作要求之目標尺寸大小。一是兼顧速度與準確性的目標尺寸（最佳目標尺寸，MATS）：目標尺寸不僅大到足以正確點擊，且小到避免不必要的縮放操作時間；二是以準確性為主的目標尺寸（舒適目標尺寸，MCTS）：目標尺寸大到令使用者有信心，絕不會發生錯誤點擊的狀況。本研究收集20位年輕人（平均年齡26.3歲）及年長者（平均年齡66.7歲）分別在具備5吋、7吋及10吋觸控螢幕的行動裝置上，在四種顯示密度條件下，透過縮放及點擊任務所得出的主觀最佳目標尺寸及舒適目標尺寸；同時，以極限目標尺寸（LCTS，目標小於極限目標尺寸會讓使用者的點擊成功率不到一半）作為衡量使用者能力的指標。研究結果顯示，年輕人及年長者在最佳目標尺寸上有相似的表現（年輕人為7.80公釐或114.46弧分，年長者則為7.04公釐或83.09弧分），但在舒適目標尺寸上則不同：年輕人平均為12.43公釐（181.30弧分），年長者平均為9公釐（106.57弧分）；另外，使用者可負荷的極限目標尺寸平均為3.09公釐（45.27弧分）。根據研究結果，本研究所得出的最佳目標尺寸平均值（7.42公釐或98.76弧分）與主流科技公司（例如Apple Inc.及Microsoft Corporation）提出的設計準則相符，並建議可將舒適目標尺寸應用於螢幕空間足夠的大型觸控裝置上。本研究亦發現在10吋裝置上需要最小的極限目標尺寸及最大的最佳目標尺寸和舒適目標尺寸，推測除了使用者能力之外也有被心理層面影響。另外在操作較不熟悉的7吋觸控裝置時，不僅需要最大的極限目標尺寸，亦產生最低點擊正確率及需要最長完成任務時間等。另一方面，顯示密度的提高將導致最佳目標尺寸增加，除了引發年長者在速度與準確性兩方面做權衡取捨，也會有較高的生心理工作負荷發生。此外，年齡方面的結果與過往研究不同，年長者在最佳目標尺寸及極限目標尺寸的表現並不低於年應人。最後，本研究在全面考量生心理層面後所得出的最佳目標尺寸、舒適目標尺寸及極限目標尺寸亦提出於行動裝置遊戲上之按鍵應用。

Operating mobile devices with limited sizes of touchscreen to click on icons or links was easily to cause difficulty to users, especially the specified target size on different settings of layout or user performance between different age groups. Most of the existing literature focused on appropriate target sizes in clicking tasks by testing user performance with targets of prearranged sizes. Due to test for target sizes in limited number of alternatives and discrete scale, in general, larger targets were proposed for aged users. Thanks to innovation in technology, users now could manipulate target sizes on a multi-touch screen by zooming in/out with a pinch gesture continuously. Therefore, this study investigated two target sizes for different operational tasks by the multi-touch technology. First, a speed-accuracy balanced target size (the Most Acceptable Target Size, MATS), i.e. a target that is not only big enough to get accuracy guarantee in clicking but also small enough to avoid unnecessary operation in zooming; Second, an accuracy-oriented target size (the Most Comfortable Target Size, MCTS), i.e. a target that is be enlarging enough for the user to ensure accurate clicking absolutely. In this study, MATS and MCTS were determined subjectively by young (age on average were 26.3) and elder (age on average were 66.7) users and recorded simultaneously in a zoom-and-click task on 5-, 7- and 10-inch touch-screen mobile devices with four levels of display sparsity. Least Capable Target Size (LCTS, the target just big enough for the user to click with above-chance accuracy) was defined to measure the user performance to represent the indicative of users’ capability. Results showed that young and elder users had similar performance on MATS (7.80 millimeters or 114.46 minutes of arc for youngsters and 7.04 millimeters or 83.09 minutes of arc for elders), but different on MCTS for youngsters (12.43 millimeters or 181.30 minutes of arc on average) and elders (9 millimeters or 106.57 minutes of arc on average). In addition, users could afford 3.09 millimeters (45.27 minutes of arc) on LCTS. According to the results, MATS on average (7.42 millimeters or 98.76 minutes of arc) was corresponded with the user interface guideline proposed by mainstream technology companies (Apple Inc. and Microsoft Corporation), and MCTS was suggested to use on a large touchscreen with more space for information display. 10-inch device required the smallest LCTS for successful clicking, but the largest MATS and MCTS from users’ ability and effect of psychology. Unfamiliar 7-inch device was found that not only required the largest LCTS but also caused the lowest clicking accuracy and longest task completion time. On the other hand, increase of display density would cause MATS increased, and speed-accuracy trade-offs were possibly elicited for elders to adapt, in the meantime, causing the higher psychophysiological workload. Besides, the result of age was different from the previous studies that performance of elder users was not worse than young users on MATS and LCTS. Finally, An application for different definitions of button size for mobile games by MATS, MCTS and LCTS determined in the experiment was proposed, and these target sizes in the current study were the ones had been comprehensively considered about the psychophysiological aspects.

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