← QCE Digital Solutions Unit 2

Error tolerance

An interface design quality where mistakes are prevented, reasonable input variations are accepted, and users can easily recover, such as with undo, confirmations and clear error messages.

Part of: QCE Digital Solutions Unit 2 — Data-Driven Problems and Solution Requirements — Module 81

Understanding Error tolerance in QCE Digital Solutions Unit 2

What it does: A drop-down list restricts the input to valid values, so the user cannot type a mistake. The message states the problem and the accepted range, so the user can recover quickly. A confirmation before a destructive action gives the user a chance to stop a slip before data is lost. This is a key part of error tolerance. Undo lets users reverse a mistake easily. A message that only reports the delete does not help the user get the file back. An error-tolerant interface accepts reasonable variations such as capital letters or extra spaces rather than rejecting them. Clear error messages in plain language help users recover, for example: Please enter your email, like name@example.com.

Common mistakes: Permanent deletion gives the user no way to recover from a mistake. An error-tolerant interface moves the file to a recycle bin first, so it can be restored: move file to recycle bin. The undo step must run when the user chooses "undo". Checking for "save" means the restore code never runs when undo is requested. It should be if choice = "undo" then.

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This module's key terms are also in a printable crossword (Modules 2–115) and word search (Modules 2–139).

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Learning
Duo
A student clicks Delete on a quiz result and the app asks 'Are you sure you want to delete this result?' before removing it. Which design idea is this?
Poor error tolerance: the extra question just slows the user downError tolerance: a confirmation before delete prevents an accidental mistake
Matches
if clicked(deleteButton) then deleteRecord()
↓
The record is deleted straight away with no confirmation, so one accidental click causes permanent data loss.
Applying
Meanings
Term
error tolerance
Meaning
An interface design quality where mistakes are prevented, reasonable input variations are accepted, and users can easily recover, such as with undo, confirmations and clear error messages.
Practice
print "Delete this file? (yes/no)"
Testing
Gaps
choice = "yes" then
Check Answer
Spots
1delete file permanently
2print "File moved. Restore?"

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Quick Questions

1. A student clicks Delete on a quiz result and the app asks 'Are you sure you want to delete this result?' before removing it. Which design idea is this?
A. Error tolerance: a confirmation before delete prevents an accidental mistake ✓
B. Poor error tolerance: the extra question just slows the user down
A confirmation before a destructive action gives the user a chance to stop a slip before data is lost. This is a key part of error tolerance.
2. A user deletes the wrong file by accident. Which feature best helps them recover?
A. A message saying 'Delete complete'
B. An Undo button that restores the file ✓
Undo lets users reverse a mistake easily. A message that only reports the delete does not help the user get the file back.
3. A form asks for a state. Which design better limits mistakes in input?
A. A blank text box where users type any text
B. A drop-down list with QLD, NSW, VIC and the other states ✓
Limiting input choices with a drop-down list prevents spelling errors and invalid entries, so mistakes are prevented before they happen.
4. Which is the clearer error message for a date typed in the wrong format?
A. Please enter the date as DD/MM/YYYY, for example 05/03/2025. ✓
B. Error 4021: invalid input.
A clear error message says what went wrong and how to fix it. A vague code leaves the user unsure how to recover.
5. A search box finds 'Brisbane' when the user types 'brisbane' or ' Brisbane ' with a space. What does this show?
A. The interface accepts reasonable variations in input ✓
B. The interface has no validation at all
An error-tolerant interface accepts reasonable variations such as capital letters or extra spaces rather than rejecting them.

Match the Pairs

confirm("Delete this file?") → Asks the user to confirm before a destructive action, preventing accidental deletion
undoStack.pop() → Reverses the most recent action so the user can recover from a mistake
Limiting input choices prevents invalid entries by letting the user pick only from allowed options → <select name="year"><option>11</option><option>12</option></select>
A clear error message tells the user what went wrong and how to fix it → print("Age must be a number from 12 to 18")
if clicked(deleteButton) then deleteRecord() → The record is deleted straight away with no confirmation, so one accidental click causes permanent data loss.
print("Error 402") → The error message is vague. It does not explain the problem or how the user can fix it.

Key Term

error tolerance
An interface design quality where mistakes are prevented, reasonable input variations are accepted, and users can easily recover, such as with undo, confirmations and clear error messages.

Fill in the Blanks

1. print "Delete this record? yes/no" input choice if choice = "yes" then
Hint: Type the keyword that starts a selection, so the delete only happens after the user confirms.
2. lastDeleted ← record delete record record ← lastDeleted
Hint: Type the variable that holds the saved copy, so the user can recover the deleted item (undo).
3. if age < 0 or age > 120 then print "Enter an age between 0 and 120" end if
Hint: Type the Boolean operator that makes the check true when the age falls outside either end of the range.
4. allowed ← ["Red", "Blue", "Green"] input house if house not in allowed then
Hint: Type the Boolean operator that reverses the test, so choices outside the permitted list are caught.
5. input answer answer ← lowercase(answer) if answer = "yes" then
Hint: Type the function name that changes the letter case of text so different typing variations are accepted.

Spot the Error

Question 1
A delete file permanently
B print "File moved. Restore?"
C input reply
D if reply = "Y" then
E restore file
Line A — Permanent deletion gives the user no way to recover from a mistake. An error-tolerant interface moves the file to a recycle bin first, so it can be restored: move file to recycle bin.
Question 2
A lastAction <- currentText
B input choice
C if choice = "save" then
D currentText <- lastAction
Line C — The undo step must run when the user chooses "undo". Checking for "save" means the restore code never runs when undo is requested. It should be if choice = "undo" then.
Question 3
A input age
B while age < 0 or age > 120
C print "Age must be 0-120"
D stop program
E end while
Line D — After showing the error message, the user should be able to recover by trying again. Stopping the program forces them to start over. The line should be input age so they can re-enter a valid value.
Question 4
A choices <- ["Red", "Blue"]
B input pick
C while pick not in choices
D choices <- pick
E end while
Line D — The loop should ask the user to choose again until the pick is in the list of allowed choices. Assigning pick to choices overwrites the list of valid options. It should be input pick.
Question 5
A input answer
B answer <- uppercase(answer)
C if answer = "yes" then
D delete file
Line B — An error-tolerant interface accepts reasonable variations such as Yes, YES and yes. Converting to uppercase means the comparison with "yes" never matches. It should use lowercase(answer).

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