The second thing that I’d like to do today is to distinguish two important categories of errors
that are based on differences in what the user’s mental model is about what they believe.
The first category is a slip.
With a slip, you have the right model of how a system works, but you just accidentally do the wrong thing.
So, if I go to reach for one button and press another — just by a motor error — that would be a slip..
On the other hand, a mistake is when I do what I intend to do,
but I have the wrong model of what I ought to do.
So, if I’m driving, and I think that I ought to take this highway exit to get [to] where I need to go,
and I take it exactly as I intend to, but I was wrong in my belief, that would be a mistake.
And, as a designer, you’ll correct these two kinds of errors — or prevent them — in your design differently.
Slips you’ll most often try to prevent by improving the ergonomics
or visual design of the user interface —
spread things out so it’s less likely that you’ll hit the wrong thing;
make targets bigger.
With mistakes, on the other hand, what you’ll need to do is [to] provide better feedback,
or make clear what the options are.
So, limit the number of mistakes that you [could] make.
You’ll want to improve the user’s ability to perceive the affordances of your software:
Make it clear to them what is possible to do.
Here we have an interface that led to a lot of user errors.
This is a ballot from Palm Beach County, Florida during the 2000 presidential election in the US.
There were two major party candidates —
the Republican candidate George Bush and the Democratic candidate Al Gore.
Across the nation they were, overall, running neck-in-neck.
There were also eight other candidates, to each gathered a smaller fraction of the vote.
A user’s vote was recorded by a hole being punched out along the centreline of the ballot.
It appears that, due to bad user interface design,
people who intended to vote for one of the candidates, Al Gore,
instead, accidentally pressed the hole corresponding to a different candidate, Pat Buchanan.
While we’ll never know for sure, the data suggested that this is probably the case.
So, the people who vvoted in Palm Beach County using this ballot style,
about 0.85% of the votes were for Pat Buchanan.
However, people who voted absentee, using a different style of ballot,
had a much lower rate of votes for Pat Buchanan,
and the reason appears to be that this hole in the middle right here was ambiguous —
Yes, there’s an arrow pointing to it from the right, but it kind of lines up to the spot on the left.
So, it appears that for about 0.6% of voters,
they thought that that second hole corresponded to Al Gore rather than Pat Buchanan.
And the question for you is: Is this a slip? or is this a mistake?
These erroneous votes are the result of a mistake,
because voters performed the manual operation that they intended to perform —
punching that second hole —
however, they had the wrong mental model about what punching that second hole meant.
WIth better user interface design, it could have been clear
which of these was the hole that corresponded to a Democratic candidate versus the Reform candidate.
Another important lesson to learn from the butterfly ballot problem is that of consistency:
Whenever we reuse designs that are already successful, we are less likely to make accidental mistakes.
By contrast, as happens with a lot of voting systems,
when every county makes their own voting system
— or at least there’s broad diversity in the voting systems used —
it’s much more likely that usability bugs will crop up.
And so one way that we could fix this would be to have a nationwide standard voting system
where everybody votes using the same user interface.
One appealing option for a nationwide voting system would be to use electronic voting.
If we were to build a better user interface for voting, what would it be?
Well, given that we’re in a computer science class,
one natural suggestion to offer up would be electronic voting.
And electronic voting certainly has some very clear appeals:
For example, it is much easier to internationalize to many different languages;
You can have pictures of candidates to make things clear;
You can have a touch screen so that you have direct manipulation.
All of these are important and good advantages to electronic voting.
However, as David Dill in the Verified Voting Foundation point[s] out,
there’s one major problem with electronic voting:
How do you know that the machine recorded the vote that you intended?
And their proposed solution to this user interface problem is really clever:
What they proposed is that the machine print out a paper receipt of the vote that you cast;
however, you don’t get to take the receipt with you, because that would run the risk of vote-buying.
Instead, that receipt falls behind a glass or plastic clear pane,
and so you can see it being printed out, and you can see it go into a bin,
and it’s stored there for the purposes of recount.
That way, you can always manually verify the computer-generated tally.
The butterfly ballot costs problems because the representation was really confusing —
What lined up with what was hard to figure out.
Here’s an example that’s much better:
This is a seat user interface for an automobile that employs a “world in miniature” strategy.
It offers controls for manipulating parts of the seat,
and the interface for doing that is a miniature seat itself.
So, if you’d like to move the headrest back, you can move the miniature headrest back.
By having this clear mapping, users are much less likely to make errors.
So far, we’ve seen how direct manipulation enables users to behave with much more expertise
by leveraging familiar real-world metaphors.
This “directness in real-world” metaphor — like “to move a slider you move a slider” —
helps give users a good idea of how each object works and how to control it.
And also, the interface’s physical form discloses what functionality it provides.
So this is all great, right? Well, here’s the challenge:
The reason that we have technology and software as opposed to the real world that we used to have
is that we want to be able to do something new!
So, the reason that have a digital slider as opposed to a physical slider
is that, at least somewhere in the system, there’s some kind of new functionality being offered.
And so, as Jonathan Grudin points out,
if technology is providing an advantage — if there is this new functionality —
at some point, the correspondance to the real world has to break down.