Hello, post-completion error

Imagine you’re in a foreign country, say, Mexico, and you have run out of pesos. You go to the nearest cash machine, open your wallet and… your card is not there. Not ideal, isn’t it?

After spending a silly amount of pesos on food, clothes (poncho!) and gifts for family and friends, I run out of money. I went to the cash machine and noticed that my card was not in the wallet. At that point I was using the wallet actively for 2 days while doing lots of touristy things, and it was only now that I noticed that the card was missing. After going through all my things methodically a few times I confirmed the card was indeed lost.

The last place I used it was a cash machine at the INAOE campus where I was attending the Winter School on Ubiquitous Computing. After telling my colleagues a few times that this was the last time I remembered using my card, it finally struck me: what if it was the post-completion error? What if that cash machine was one of those that give you the money first? What if I just took the cash and simply left the card there? After all, everyone makes errors.

I went back to the machine with my colleague’s card (“here’s my PIN, I trust you” O_O) to withdraw some money for her and decided to take notes. To get the cash, I had to complete 10 steps:

  1. Enter PIN
  2. Confirm the type of the account you want to use (debit, savings, credit card?)
  3. Select the amount
  4. You have to pay tax. Agree/Disagree
  5. Select an amount to donate to some charity (or decline)
  6. Do you want the receipt? Yes/No
  7. Get cash
  8. Get the receipt
  9. Do you want anything else? Yes/No
  10. Get the card

No wonder I forgot my card! I don’t even remember seeing the screen with Step 9.

Post-completion error

Post-completion error is an error where a user misses the steps of a task that are not directly related to the goal and have to be completed after the goal has been reached, e.g. leaving the original in a photocopier after all photocopying is done. As users are focused on their goal, as soon as their reach it, the task is mentally marked as done. In my case, the goal was to get money. I was also distracted and ill, which increases chances of making an error. Another issue here is the number of steps needed before the goal is reached and their relevance: as the user wants to quickly withdraw money, additional screens are seen as annoying and distracting, which increases the chances of just grabbing the cash (“Finally!”) and leaving.

This experience illustrates nicely why the design of cash machines has been changed and most (if not all) UK ATMs give the card back first: the best way to avoid the post-completion error is to ensure that there are no important steps that need to be completed after the goal is reached.

If the whole process cannot be redesigned, the system should allow users to recover from the error. In this case, the cash machine swallows cards left by forgetful users to prevent them from being stolen. Unfortunately, the bank destroys all cards they find inside the machine…

~Falka, 14 January 14