# Kahneman's add one / three exercise

In his book Thinking, Fast and Slow, Daniel Kahneman describes a mental exercise which he calls Add One:

To start, make up several strings of 4 digits, all different, and write each string on an index card. Place a blank card on top of the deck. The task you will perform is called Add-1. Here is how it goes:

Start beating a steady rhythm. Remove the blank card and read the four digits aloud. Wait for two beats, then report a string in which each of the original digits is incremented by 1. If the digits on the card are 5294, the correct response is 6305. Keeping the rhythm is important.

This description is a bit vague and I'm wondering how the exercise is supposed to work exactly. He does not mention to cover up the card before reporting the modified digits. This would allow to perform the transformations on-the-fly digit by digit. This does not sound challenging at all and does not fit to Kahneman's following statements of loading the working memory, in particular:

Few people can cope with more than four digits in the Add-1 task.

My assumption was that he means something like that:

• Beat 1: Uncover card
• Beat 2: Read aloud first digit
• Beat 3: Read aloud second digit
• Beat 4: Read aloud third digit
• Beat 5: Read aloud fourth digit
• Beat 6: Wait + cover card
• Beat 7: Wait
• Beat 8: Say first digit + 1
• Beat 9: Say second digit + 1
• Beat 10: Say third digit + 1
• Beat 11: Say fourth digit + 1

However, a quick Google search reveals a few implementations, for instance:

To my surprise both attempts use the dull version of reporting the transformed digits while seeing them.

My question: Is there a better description of the exercise which was originally used in the scientific studies?