易用度

易用度（粵音：ji6 jung6 dou6 | 英文：usability）喺工程學上係指一個系統（一部機械或者一隻電腦軟件）有幾能夠俾用家用佢有效率噉達到目的。

定義

「

原版英文："... the extent to which a product can be used by specified users to achieve specified goals with effectiveness, efficiency and satisfaction in a specified context of use."
粵文翻譯：（易用度係）一件產品有幾能夠俾特定嘅用家用嚟達到特定（件產品設計嚟達到嘅）目的，而且喺特定嘅情境下達到目的上嚟有效果、有效率同埋能夠令用家滿意。

」

喺最簡化嗰種情況下，易用度可以用學習曲線嘅概念嚟想像。學習曲線係心理學同相關領域上用嚟模擬學習現象嘅一種數學模型：想像家陣有個人一路係噉做工作 $T$ 做一段時間，正常嚟講，隨住時間過去，佢喺工作 $T$ 上嘅表現正常嚟講會愈嚟愈好（做得愈嚟愈熟手），並且去到某一個點因為受試者技術去到最高點而變平；如果用 Y 軸做嗰個人喺工作 $T$ 上嘅表現、X 軸做時間，就會畫出一條 Y 值隨時間上升嘅線，呢條就係嗰嗰個人喺工作 $T$ 上嘅學習曲線，即係好似^[2]：

y=C_{1}x^{b}

；

當中 $y$ 係喺工作 $T$ 上嘅表現（一個理論上會隨住學習而數值下降嘅指標）； $x$ 係經驗（或者時間）； $C_{1}$ 係指第一吓工作嘅表現；而 $b$ 呢個參數（ $-1<b<0$ ）會反映個工人嘅學習率。而家假想 $T$ 係「使用一隻軟件」－如果一隻軟件對用家嚟講容易學識點用，噉用家喺使用隻軟件上嘅學習曲線應該會比較快到達平嘅最高點^[3]^[4]^{:p. 2}。

處理器模型

易用度分析嘅其中一個最重要部份係考慮用家所做嘅認知過程：人類處理器模型（human processor model）係易用度分析上成日用嘅一個理論模型，主要攞嚟估計一位用家做某一樣認知作業（例如係撳某一個掣）要用幾耐嘅時間；根據呢個模型，人喺做認知作業嗰陣會經過一柞基本步驟，包括咗

用感官感知外界資訊並且理解自己周圍發生緊啲乜嘢事（perceptual subsystem）、
用工作記憶同長期記憶等嘅記憶功能整合手上嘅資訊（cognitive subsystem）同埋
用資訊整合得出嘅結果控制身體做出適當嘅郁動（motor subsystem）；

即係好似下圖噉^[5]：

人類處理器模型喺易用度分析上相當有用。例：設計者家陣想設計網購網站，佢哋用心理實驗嘅方法知道咗，用家對眼望勻嗮成個網頁平均要用 230 毫秒，理解得到嗮望到嘅係啲乜就平均要用 300 毫秒，做決策（「買唔買」同埋「要買邊樣嘢」）就平均要用咁多咁多毫秒，而腦部傳訊號落手嘅肌肉令手做出「撳掣」嘅動作就平均要用咁多咁多毫秒... 等等；將呢啲數值加埋之後，班設計者就可以估計到用家平均會喺個網頁上嘥大約幾耐嘅時間，而呢個數值可以攞嚟做易用度嘅指標之一－易用嘅電腦嘢係應該能夠俾用家快速達成目的嘅^[5]^[6]。

評估

喺實際應用上，人機互動專家會用各種方法評估一個系統嘅易用度，而呢啲評估方法有得分做兩大種^[7]^[8]：

易用度測試：指真係搵用家（受試者）返嚟試吓用個系統；喺呢個測試過程當中，設計者會要求啲受試者自由噉用吓，跟住又會叫佢哋試吓用某啲特定嘅功能嚟達到某啲里程碑；喺呢個測試過程當中，設計者會用觀察受試者嘅行為，又會某啲方法量度一啲反映易用度嘅指標，「指標」包括「受試者用咗幾多時間嚟達到里程碑」以及「有幾多百分比嘅受試者成功達到里程碑」呀噉－如果個系統易用，應該會出現「多數受試者都能夠喺短時間之內搞掂」嘅情況；而且設計者喺觀察受試者嘅行為嘅過程裏面，仲能夠得到「用家犯錯嗰陣傾向犯乜嘢錯」嘅資訊，而呢樣資訊都幫到設計者度吓個系統嘅設計要點樣執^[9]。可以睇埋心理實驗。
易用度檢測：指靠易用度專家用客觀方法親自做評估；例如認知走查法係一種成日用嘅易用度檢測做法，當一個設計者用認知走查法嗰陣，佢首先要列出嗮「如果用家想要用隻軟件達到某樣效果，需要經過邊啲步驟？」，例如評估緊隻繪圖軟件，列出如果用家想要增加幅圖嘅光度需要先後撳邊個掣；然後 foreach 步驟，設計者都會問幾條問題：
1. 用家能唔能夠輕易睇得出要撳邊個掣？
2. 用家能唔能夠理解隻軟件對嗰個功能嘅描述？
... 等等。個設計者會一路做以上嘅嘢，做到評估嗮隻軟件嘅各種功能為止^[10]^[11]。

睇埋

文獻

Mazumder, F. K., & Das, U. K. (2014). Usability guidelines for usable user interface (PDF). International Journal of Research in Engineering and Technology, 3(9), 79-82.

引述

[1]
T. I. S. Organisation, "Ergonomic Requirements for Office Work with Visual Display Terminals, ISO 9241-11", (1998).
[2]
Fioretti, G. (2007). The organizational learning curve. European Journal of Operational Research, 177(3), 1375-1384.
[3]
De Laveaga, A., Wadman, M. C., Wirth, L., & Hallbeck, M. S. (2012). Ergonomics of novices and experts during simulated endotracheal intubation. Work, 41(Supplement 1), 4692-4698.
[4]
Kainda, R., Flechais, I., & Roscoe, A. W. (2010, February). Security and usability: Analysis and evaluation (PDF). In 2010 International Conference on Availability, Reliability and Security (pp. 275-282). IEEE.
[5]
Card, S.K; Moran, T. P; and Newell, A. The Model Human Processor: An Engineering Model of Human Performance. In K. R. Boff, L. Kaufman, & J. P. Thomas (Eds.), Handbook of Perception and Human Performance. Vol. 2: Cognitive Processes and Performance, 1986, pages 1-35.
[6]
Liu, Yili; Feyen, Robert; and Tsimhoni, Omer. Queueing Network-Model Human Processor(QN-MHP): A Computational Architecture for Multitask Performance in Human-Machine Systems. ACM Transactions on Computer-Human Interaction. Volume 13, Number 1, March 2006, pages 37-70.
[7]
Lewis, J. R. (2006). Usability testing. Handbook of human factors and ergonomics, 12, e30.
[8]
Bastien, J. C. (2010). Usability testing: a review of some methodological and technical aspects of the method (PDF). International journal of medical informatics, 79(4), e18-e23.
[9]
Dumas, J. S., Dumas, J. S., & Redish, J. (1999). A practical guide to usability testing. Intellect books.
[10]
Blackmon, M. H., Polson, P. G., Kitajima, M., & Lewis, C. (2002, April). Cognitive walkthrough for the web (PDF). In Proceedings of the SIGCHI conference on human factors in computing systems (pp. 463-470).
[11]
Spencer, R. (2000, April). The streamlined cognitive walkthrough method, working around social constraints encountered in a software development company (PDF). In Proceedings of the SIGCHI conference on Human Factors in Computing Systems (pp. 353-359).

[1] [1]
T. I. S. Organisation, "Ergonomic Requirements for Office Work with Visual Display Terminals, ISO 9241-11", (1998).

[2] [2]
Fioretti, G. (2007). The organizational learning curve. European Journal of Operational Research, 177(3), 1375-1384.

[3] [3]
De Laveaga, A., Wadman, M. C., Wirth, L., & Hallbeck, M. S. (2012). Ergonomics of novices and experts during simulated endotracheal intubation. Work, 41(Supplement 1), 4692-4698.

[4] [4]
Kainda, R., Flechais, I., & Roscoe, A. W. (2010, February). Security and usability: Analysis and evaluation (PDF). In 2010 International Conference on Availability, Reliability and Security (pp. 275-282). IEEE.

[cardetal1986-5] [5]
Card, S.K; Moran, T. P; and Newell, A. The Model Human Processor: An Engineering Model of Human Performance. In K. R. Boff, L. Kaufman, & J. P. Thomas (Eds.), Handbook of Perception and Human Performance. Vol. 2: Cognitive Processes and Performance, 1986, pages 1-35.

[6] [6]
Liu, Yili; Feyen, Robert; and Tsimhoni, Omer. Queueing Network-Model Human Processor(QN-MHP): A Computational Architecture for Multitask Performance in Human-Machine Systems. ACM Transactions on Computer-Human Interaction. Volume 13, Number 1, March 2006, pages 37-70.

[7] [7]
Lewis, J. R. (2006). Usability testing. Handbook of human factors and ergonomics, 12, e30.

[8] [8]
Bastien, J. C. (2010). Usability testing: a review of some methodological and technical aspects of the method (PDF). International journal of medical informatics, 79(4), e18-e23.

[9] [9]
Dumas, J. S., Dumas, J. S., & Redish, J. (1999). A practical guide to usability testing. Intellect books.

[10] [10]
Blackmon, M. H., Polson, P. G., Kitajima, M., & Lewis, C. (2002, April). Cognitive walkthrough for the web (PDF). In Proceedings of the SIGCHI conference on human factors in computing systems (pp. 463-470).

[11] [11]
Spencer, R. (2000, April). The streamlined cognitive walkthrough method, working around social constraints encountered in a software development company (PDF). In Proceedings of the SIGCHI conference on Human Factors in Computing Systems (pp. 353-359).

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