Summary: here are two different screens of an incoming call on iPhone. The one uses the traditional buttons to accept and decline; the other requires users to drag the slider to accept or press the Sleep/Wake key twice to decline. This design can demonstrate a design philosophy of Poka Yoke.
I like iOS with its elegance and stability. Nevertheless, on the first day I used iPhone, I got extremely awkward when it rang loudly on a conference but I didn't know how to hang it up. It only told me to drag the slider to answer the call, but showed nothing about how to decline it! Therefore, after the conference, I did an experiment to see how to decline it. However, I found the screen of incoming call changed. There was no slider but two buttons and I could decline the call by tapping the Decline button.
This means iPhone does have two different screens and actions of incoming calls. But why? Are they applied for different scenarios? Or is it just a bug in design? To find the answer, I head for the guide of iPhone on its official website. In the section of Phone in Apps, I found the page of Manage incoming calls, which introduced the way to answer or decline a call and it mentions:
So the simple button screen displays when a call coming and your phone is awakened while the slider screen shows up when iPhone is locked. But why? Why apple uses two different screens for incoming calls by the status of the phone? I searched on the apple official website but didn't find the answer.
So I made a guess, that this design is a way to prevent accidental touch. When your phone is locked, it’s probably in the pants pocket, and can be touched by your leg. I often encounter this situation: I put my phone awaken in the pocket and when I take it out after 10 minutes, it's still awaken, and the contents on the screen changed. Therefore, if the traditional buttons are used in the locked mode, the call can be easily accepted or declined by an inadvertent touch and this can lead to terrible results for users. However, the slider needs users to drag it to a certain place, which can't be completed by normal accidental touch. Sleep/Wake button on the side of a phone is also less likely to be pressed. ( On previous models of iPhone, this button was on the top of the phone and could be easily accidentally pressed. ) With a slider, the possibility of a call accidentally accepted or declined decreased. But I guess it'd better if the user can be told of how to decline a call in the slider screen.
This design can lead us to have a glimpse at one principle commonly used in design: Poka Yoke (ポカヨケ), which means "inadvertent error prevention". It was firstly used in claims that good design should take the responsibility of preventing mistakes happen and eliminate unwanted results by preventing, correcting, or drawing attention to human errors as they occur. Though we can't say that the inadvertent touch in the calling case should not be blamed on the user, the design of adding additional control conditions to prevent unwanted results is one of the examples of Poka Yoke.
What shouldn't escape our notice is that Poka Yoke is not equal to fault toleration. The former pays attention to prevent mistakes from happening while the latter tries to remedy and decrease the influence of a mistake.
I was inspired by the work of Shigeo Shingo, who first invented Poka Yoke and draw principles and methods in industrial manufacture to realize it. To make Poka Yoke applied to the design of Internet product, I replanted some strategies and found examples to support them.
1. Exclusive Strategy
That is, exclude the possibility of making certain mistakes by controlling the environment and setting rules.
For example, in the process of sign-in, many apps require the user to fill the phone number. When the user taps the blank for phone number, the keyboard will bounce up. However, in most apps, the keyboard here only support typing numbers and the user can't switch to the Alphabetical keyboard. They also only permit the user to input 11 digits (standard digits of mobile phone number in China) to take the next action. These rules excludes the possibility that the user miss, add or mistakenly inputs letters. However, they still can't prevent the user from inputting wrong numbers.
2. Difficult Strategy
This strategy increases the difficulty or adds control mechanisms of the completion of an action to make mistakes less likely to happen inadvertently and draw the user's attention when a possible mistake is happening. It can also help the user to notice the result of a dangerous action. The slider mode of answering calls on iPhone is one example of this strategy in the prevention of accident errors.
Another example is the confirmation of important actions, which is commonly used when the user wants to delete something.
3. Automatic Strategy
The Automatic Strategy uses previous inputs or information from the user to automatically judge the values of some parameters that need to match with the previous inputs or information. The user can also adjust these values if necessary. It can reduce the input load of the user, on the one hand, and on the other hand, prevent the mismatch caused by the user's mistakes.
The example of this strategy is when we transfer money on the payment apps like Alipay or WeChat Payment, they require us to fill both the card number and the bank of the other card. The card number must match the bank. After inputting the card number, we can find the value of the bank blank is automatically selected.
Another example is many apps provide the memory of information. They can save the inputted information and automatically filled when the user needs it again,
4. Isolated Strategy
Dangerous actions should be separated from other common actions to avoid being touched mistakenly. The classification of information is also helpful to users' recognition of the interface and make them subconsciously divide these dangerous actions from the normal.
5. Warning Strategy
Using special colors like red or notice to draw the user's attention on this danger of such action in order to prevent mistakes. This is also common in the design of Delete and often combined with the Isolated Strategy.
Above are the all the strategies I have replanted. These strategies can be combined together and should be used cleverly according to the situation. However, these strategies are only hypothetical because I haven't conducted experiments to confirm the effectiveness of these strategies. This is common in many articles I've read. I am looking for chances to prove them with statistics rather than pure logical analysis.
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