How long should an onscreen message last?

["Chris Knight" <cknight -at- attcanada -dot- ca>]

Hello all.

I've been a contract tech writer for 17 years, and worked with some of the
best User Interface and Human Factors people, but I have a question that's
never come up before: How long should an onscreen message last?
I am pretty much the UI designer on this project, so *I* get to say.

Here are some factors to consider:
The product is a security system (a computer appliance, really) that gets
installed at an entrance to a building.
Know that "Find a faded printed directory, press 3 or 4 numbers on a
telephone keypad ...... "Hi!" "Hi; it's me!" "I'll buzz you in." ...
Bannnnnnnngggggggggggt Open door. Go in building." feeling?
Well, these guys have a *computer*, with a *touch screen*, that you put your
best digit onto to dial "the party to whom you wish to be connected". Cool,
what?

So if the person "dials" a number not in the (on-screen) directory, or if
the system is set to ignore requests (e.g. for this particular suite, or,
for *any* suite at this hour), a message is displayed (the bottom line of
the screen is reserved for messages) saying "ACCESS DENIED", which lasts 1.5
seconds before being replaced by "Idle", awaiting the next person's best
digit pressing the screen to call his mother.

The time is definitely too short (Some people do not notice the message at
all, which leaves them puzzled as to why nothing is happening. Others catch
that there's a message ... "What was that??? Poof, it's gone! And now it
says 'Idle'. Why isn't the phone connecting to the party to whom I wish to
be connected???").

But how long should it be?
What's a good length of time to last so that no one has the message
disappear just as she notices it?

An important complication is: Wait *too* long -- longer than the person
whose access request has been denied (gotta love the passive voice for stuff
like this) -- and the "Access Denied" message could be found by the *next*
person that wants to get in, who might see the message and be confused, and
not try to call Aunt Molly. We have no way of knowing how soon that next
person will be along, but we don't want her to be confused, so we can't let
the message persist *too long* either.
How long is long enough, then, AND, how long would be too long?

Here's my hypothesis:
First, better not wait longer than 10 seconds to dismiss the "ACCESS DENIED"
message.
But I'm thinking *5 seconds* should be long enough, while the disappointed
party (who couldn't connect with that other party) stares at the screen, and
slowly, scratching his head, and without vandalizing the system, leaves the
front of the panel ... for the next person. No more than 10 seconds, though.

I will test and report.
Comments?
Anyone have any relevant research?


Chris
_____________________________________
Christopher Knight, Technical Communication Architect
http://members.attcanada.ca/~cknight/
E-mail: cknight -at- attcanada -dot- ca
Phone: 604 877 0074
_____________________________________
"Laughter is the representative of tragedy, when tragedy is away."
Wyndham Lewis



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