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I've noticed that most people when using a real or a virtual calculator, they hit the Clear button multiple times when clearing the screen (even though hitting it once is enough), so I started wondering: is there a flaw in the calculator's design that compels most people to do that?
What makes them trust the button less than the other buttons? Is there some sort of feedback mechanism that's missing? Even if it's just a habit, how does one end up developing it?
(Older) pocket calculators sometimes have several “cancel” buttons (C, CE, etc.) Typically, the CE button would only clear the last entry but not interrupt the current computation. For example, if you press 10 + 1 CE, you would see a 0, but the calculator still expects a second operand for the addition (i.e. it still has "10 +" in memory). Often, there would be no obvious indication of the current state of the calculator (i.e. is it reset and waiting for a new operation or in the middle of one?) If you want to start anew without having to remember which button is which or how a specific calculator works, you can simply press a couple of times on all cancel buttons and that's an habit many people kept from that time.
Other answerers have provided great logical reasons for how these habits could come about, but I think it is simpler than that (plus, how often are any of us logical?).
Calculators obviously have a state, since they do multi-step operations, but they don't clearly show their state. In many calculators, if you see a zero on the screen, you have no idea if the calculator is in the middle of an operation or not. So we press Reset a bunch to make sure.
Plus, many of us have gotten burned by starting a new calculation while the calculator was still in another mode, and gotten a totally unpredicted result. So we press reset a bunch to be sure.
Some calculators are better at this. For example the Win7 calc shows you prior operands. Some smartphone calcs show operands in vertical lines, and clear the screen when you press reset. Perhaps an even better enhancement would be to show the background screen in white if the calculator is truly reset, and colored if it's in the middle of something.
But there will always be the tendency to push the button a bunch of times, because better safe than sorry! ;)
On some old calculators, the clear button had double duty:
I think this meant that people tended to press clear a few times, to ensure that everything was cleared. It is possible that three clicks would clear the memory as well.
This is one of those habits that people acquired early on in the use of technology, which has continued despite the change in the technology.
And it is an interesting UX question, because the reality that clicking it multiple times has no additional effect does not change the perception. There are other cases of this, I am sure, but we actually just need to understand that this will happen.
Edit:
There is also the sense of clicking and clicking while I make the decision to move onto something else. It is not then based on habit, but just an attempt to reflect what is going on internally, that I need to clear my mind before I move on.
Since there is no negative outcome to hitting the button multiple times, the question becomes really more of, "why wouldn't you hit it more than once?"
It would only take using a calculator with this behavior one time to create this behavior.
Think of it this way....
A person who has never used a calculator that works this way, one day uses a calculator that does. After some initial frustration, he learns that to clear he must hit the button twice.
He continues the rest of that day. Every time he hits the button only once his calculations are messed up (negative feedback) every time he hits it twice his calculation are correct (positive feedback)
The next day he uses a calculator that doesn't do this. However the behavior is already learned. But now whether he hits the button once or twice doesn't matter: both give positive feedback. Since both give positive feedback there is nothing to promote unlearning the behavior.
Now we continue on days or weeks later, he uses different calculators but doesn't really differentiate them. Hitting the button becomes like touch typing, a simple learned response.
When I type I don't think of the letters that I type or even how I move my fingers as I type. I just think of the words and brain an my fingers know how to make the letter appear on the screen. When I hit clear on a calculator I don't think "find the clear button, press the button x number of times" ... no I think "clear" and my fingers press clear (however many times they do.)
Now here is the really interesting bit. Say after awhile he does stop hitting the button twice. Ok well no problem on his normal calculator. However every time he picks up one of those other calculators, he's going to get negative feedback and the double tap behavior will again be reinforced.
Since he doesn't really differentiate these calculators when he picks them up, eventually his brain will determine that the cost in terms of effort of not hitting the button every time is greater than hitting the button unnecessarily. So... he just presses the button twice all the time.
I'm going to departure a bit from the current line of thought. Back in the typewritter days, I remember seeing my dad's accountant typing really fast on the machine. Whenever he paused for any reason, he kept pushing the "shift" button several times in a row.
He knew pressing that key didn't do anything, and that was exactly the reason he pushed that key... it didn't do anything but kept his fingers busy, or gave the impression that he was busy, or simply because it helped him to keep that rhythm, who knows...
My claim is that some people will press the CE several times just because they know it won't do anything, and that's it.
It is low-cost fidgety action (on user's part) combined with a device that offers low levels of feedback for that action. Such a combination makes you want to repeat that affirmative action.
I find myself using ctrl-c multiple times for the same piece of text. I would do that much less often if MS Word's clipboard manager were open or I had a sys-tray app that blinks every time I use ctrl-c.
As another example, I face conversations where I am disinterested and the other too enthusiastic. He may repeat certain phrases ("job security", "future", "career") to compensate for my disinterestedness. But if I repeat that phrase in front of him, I find that he moves on.
Test for my hypothesis: would a user repeatedly press that button if it gave a satisfying click, or the screen blinked once?
Slow feedback can cause this. It makes the user feel like the button is broken and needs to be pressed again or harder, which would work in the real world.
Did it work? Did it really work? Really?
...also, just general frustration. We're used to some button-presses being idempotent (as with an elevator): once pressed, additional presses are a no-op, so if people are bored and waiting (think: crosswalk buttons), or annoyed with the interface, they may hit it a bunch of times.
Repeated presses may also be used for emphasis--"Clear, dammit!"--or for efficacy--in conversation, we're used to having to work very hard to 'erase' a previous statement.
Finally, a button press takes less time than it takes for the user's mind to switch tasks, so while the mind is switching, just keep pressing 'clear' to keep the hands busy. Otherwise you're gaping at a blank screen and forget what you were doing.
These people, I have to admit myself included, have had experiences before with calculators that if they hit clear or 'C', and do a math expression, the result is incorrect. So in their mind, they hit the clear button several times to "totally clear" the contents before proceeding.
Have you also noticed that they will hit the 'C' and 'CE' buttons over and over again, not just 'C'? They are not sure which one does which, so they just slam on both of them until they feel satisfied.
Some of the credit-card sized, solar-powered calculators (without a battery backup), which were all the rage in the early 1990s, didn't always reset to the initial state completely when under a low-light condition, after pushing the C (clear all) button, and thus would behave unpredictably after such "incomplete reset" - I think some of the registers were not cleared due to very low power provided by the small solar panel. For this reason, it was customary to press the button multiple times - which prevented the problem.
Also, some models of the calculators had the buttons combined into one: CE/C - first push would erase the last entry, second would reset to initial state.
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Note that on simple calculators, both C and CE shows a zero after hitting it. For users who do not frequently use the calculator, it is counter-intuitive to "manually" recall what is the previous state. It is also often unclear which one is the previous state so these users end up having to start all over again. So pressing C and CE is both ensuring that the calculator really is starting from zero and for the user to take a deep sigh!
For users who frequently use calculators and are familiar with the C and CE buttons, or who knows the feature really well, it is more likely that they will only hit the button once. I would not imagine them hittin the CE more than once, but if they do it on the C button, they are probably not thinking of clearing the calculator but rather a fidgety action while they are deep in thought thinking about what to key in for the next calculation.
I'm going to answer this question with another question: why do people press the elevator button multiple times, some even if a light indicator turns on immediately after the first time? You see this often with old people. I think the answer is psychological.
Most people by now are pretty accustomed to the way electronics and digital circuitry works: things are usually either on or off. Pressing the button elevator multiple times won't make it come faster, resetting the calculated multiple times won't make it reset "harder". Still, this model of interaction with hardware relies on an assumptions that things work as expected. I believe most of us still have a lot of analog habits, though, and in the analog world you sometimes have to apply yourself a bit more to get stuff to work. We know that sometimes you have to jiggle a key so the lock works and sometimes you have to hit the fan so it stops making a noise.
It doesn't necessarily mean that things have to malfunction for us to give them an "analog" treatment. Sometimes we perceive things as just a bit more complicated than we'd like them to be, and in the trade-off between not being sure whether the elevator's button has registered the signal (which makes us feel worried about waiting like idiots for something that's not coming) and between just one more push of a button, the latter is often the more cost-effective choice.
So, to summarize, even digital things sometimes work in unexpected ways (due to malfunction or hidden complexity) and when faced with this uncertainty many people fall back to their analog habits of giving HW an extra nudge to make sure it knows what they want from it.
Though mono-causal thinking is often for questions like this and makes for the good reading, there are multiple reasons why people do this and the primary reason may vary from person to person.
The user is probably aware (judging from the feedback here at least) that the action is not necessary or useful, but they feel compelled to do so regardless due to a number of physiological or psychological compulsions such as brought on by a combination of the following:
Stress, Nervousness, Habit, and the desire to fidget; not helped by stimulants (caffeine mainly I hope)
Also consider: When people press clear, it is just before they type in another calculation so they are in thought and thus not paying particular attention to what their fingers are doing, exacerbating these effects.
It's a deep instinct that something is more assuredly "done" if the act of doing it is repeated. Why does a dog make little circles before it lays down? Why do people hit the crosswalk button over and over and over when deep down they know that it won't make the light change any faster?
We do receive positive feedback in many such cases. For example the more times I hammer a nail the more sure I can be that it's driven all the way in. The more times I chew a bite of food the less likely I am to choke on it and the easier my digestion will be. There are countless other examples of 'assurance of completion by repetition'. People are simply doing something that comes quite naturally to them: generalizing their experiences and applying them to new experiences. And as previous posters have noted, there is no negative feedback; there is no reason to not press the button repeatedly.
In the case of people who push the crosswalk button again and again, I usually attempt to be the negative feedback by poking fun at the irrational behavior. Your calculator, if you so chose, could do the same sort of thing. A single click would simply silently clear. A second click would produce a popup stating, "Yes. We have officially cleared the calculator now. One click will suffice. Thank you." People will be annoyed at the popup which will require another click to get rid of, and they will soon learn to click only once. Either that or they'll learn to use a less snarky calculator. ;-)
There are probably a few explanations.
And 2 reasons that are a bit more speculative:
The repetitive punching of the clear button is really caused by the user not knowing the difference between CE and C. CE clears the current entry, but C clears the entire operation. When most people punch the clear button multiple times, it means they want to clear the entire operation. But because CE and C both give the same feedback when they hit it, there's no way of knowing if the entire operation is cleared for someone who isn't sure what the difference between the two buttons are. So by pressing it multiple times, the user feels that the entire operation gets cleared, not just the current entry. Here's a great article that offers an innovative solution to the current calculator interface: http://uxmovement.com/thinking/why-calculators-need-a-better-user-interface/
My guess is most people have experienced accidentally re-using the result of the last calculation in the next calculation they're trying to make.
Once I was adding a sequence of numbers and got in the habit of hitting the plus key after each number, THEN looking at the next number. I got distracted, moved on to a new calculation and came back and started entering a new, unrelated number only to discover it was added in to the previous result by mistake.
This got me in the habit of pressing the C button frequently on in-fix calculators.
For what it's worth, I grew up with Hewlett-Packard RPN calculators, and when I use them, I virtually never clear the previous register contents since it's much less likely you'll accidentally use a previous calculation with post fix calculations.
So... I'm going for the "I just want to return the calculator to a known state" explanation.
Repetition expresses the user's uncertainty as to the system state. The user is uncertain as to whether Clear eliminates only the current entry, or resets the entire computation.
Calculators, especially one-line skeuomorphic ones, violate the following usability heuristics:
In contrast with handwritten computations and more advanced mathematical software, one-line calculators obscure all but the current numerical input, leading to greater uncertainty. The user's need for certainty is further driven by the tedious cost of "hunt and peck" input.
Therefore pressing Clear multiple times is a low-cost hedge against an opaque system.
Most people who do so (I am definitely not one of them, but have seen people doing it), would have had bad experiences at some point in time, for example, the previous numbers being used for the next calculation because the C key didn't clear it up. I've also seen people pressing Ctrl+C or Ctrl+S multiple times, and people even repeatedly right clicking their desktops and pressing refresh (Windows only).
There could be multiple reasons for these habitual actions (most people wouldn't think or even wouldn't know that they do this):
I think the simplest explanation and one in which I have no evidence, is our need to be as thorough as possible. We want to make sure something has been done properly. Not everyone will have this compulsion.
A similar example is if I lock my car using the remote fob, I still feel the need to double check the door handles to make sure its locked. I may even walk away from my car only to return and check again! I know, crazy.
There is a simple reason:
Pressing Ctrl+C or Ctrl+S is a double-key combination, so it has higher probability of mistyping. If I am typing at my fastest rate, then this is part of a streaming sequences of stuff I am doing. Pressing it more than once greatly reduces the probability of error, whereas if I had to stop and intentionally considered each Ctrl+C, this dramatically reduces my typing efficiency.
In other words, while a single, conscious single press is enough. In practice, a single press is not sufficient in practice to statistically produce the low error rate and high efficiency that results from inserting a few extra double presses.
For most of my computational life, I have used a graphing calculator. The significance of that is that I can see where I am and, when entering another calculation, I can consciously decide if I want to keep going or start a new calculation. (By including the Ans variable, usually some 2nd^Enter click.) When I use a regular calculator, I usually just use it for quick calculations. Sometimes I need to start over, and I hit one of the buttons, and sometimes it doesn't actually clear out. Frustrating. I have never had a reason to really understand the difference between C and CE on a calculator because it is just so easy to hit C and CE a couple of times when I use a basic calculator.
Your answer lies in the answer of the following question:
The reason to all these lies in the sub conscious. A guitarist is too used to playing on the guitar, and hence whenever his favorite tune plays, his mind connects it to the time he was strumming his guitar.
Similarly, simultaneous hand and leg movements is deeply stored in your subconscious. You reaffirm this notion everytime you walk. You will not realize this is happening, but your legs and hands move together. A slight change in that motion and suddenly you feel uncomfortable.
Now that we have understood how the subconscious works, lets explain the calculator button.
Whenever you use the calculator, you're either copying an equation form a paper, or calculating expenses that you had thought in your mind. So the user's mind is doing TWO things at the same time;
(1) thinking about the expenses or reading the equation from the paper, and
(2) typing the numbers in the calculator.
Since there are two things, the task that is more trivial and has less possibility of change is transferred to the sub conscious. The sub conscious has the task of pushing buttons on the calculator, and continuous instruction is transferred by the conscious brain.
When we stop, or an equation has finished computing, the conscious brain pivots into thinking about what the answer means, or whether it tallies, or how much savings he has left and what not. During this pivot, the subconscious' last instruction was the clear button, and it continues to press it until the conscious brain tells it to stop or press another button.
A similar thing happens, when you're into deep thought, and you don't know what you're looking at or if someone is calling you, because the subconscious nature of looking or hearing is overpowered by the thought in your head.
A similar habit can also be observed in Lifts where people press the CLOSE door button multiple times. The reason is very simple. Since the users don't get any feedback from the system about their action, they tend to ensure that they did it right. In the case of Lifts, they keep on pressing the CLOSE door button until it starts to close.
The similar happens in the case of Calculators. As many people have mentioned, calculator have C and CE. On pressing CE, users only get a part feedback (in older calculators) which sets screen to 0 which appears to be same as C. But on further calculations, the results appear to be erratic. These errors force the users to press CE multiple times. This behaviour eventually shifts to C because they get confused between the functions. The erratic calculations reinforce their mental model to ensure if everything has been set to 0.
