[Sigia-l] Error Prevantion Heuristic (slightly OT)

[Richard_Dalton at VANGUARD.COM]

Marion Summerville writes:
> > Richard Dalton writes:
> > One of Jacob's much touted "10 Heuristics" is: "Even better than
> > good error messages is a careful design which prevents a
> > problem from occurring in the first place". Sometimes though I think 
this
> > can be taken too far ... for example, if a system's acceptable inputs 
are
> > within the range 10-500, should the system prevent a user from 
entering
> > 600, or should the system accept it and give feedback to the user 
saying
> > that the acceptable inputs are 10-500?
>
> How about providing the info up front (immediately in context;
> right at the point of data entry) that the acceptable values are 10-500?
> 
> If "prevent" means "pop up an alert when the user changes focus
> from the field" rather than "accept the submission and return an error
> message" then prevent is better (imo; based on no data but watching my
> mother curse at the screen of her iMac).

Apologies - assume that we're already providing the info up front, would 
you
have the system prevent entry of anything above 500 - lets say this was an 
ATM
so when the user pressed "6", then "0" then tried to press "0" again the 
system
would prevent entry of the final "0" because it would create a number 
greater than
500?

This is a slightly rhetorical question because I know you'd never do that 
- so I
guess my question is "why?" - what heuristic or principle of "good design" 
is
overriding the "prevent errors" one in this instance?

 - Richard Dalton