People often overestimate their abilities. If you overestimate your ability and make questions too difficult, you can not answer at all. Questions that you can not answer at all are painful. It takes much time to answer all questions correctly. And you lose your motivation.

It is painful to repeat the test that is too difficult, and it is boring to repeat the test that is too easy. If you want to keep your motivation and learn happily, the difficulty level must be appropriate. Psychologist Mihaly Csikszentmihalyi said:

If the challenge is more than the skill, you feel uneasy.

If the challenge is less than the skill, you feel boring.

If the challenge is moderate, you enter a good mental state, called "flow," and you show high concentration to the challenge.

Csikszentmihalyi's flow theory

Horizontal axis: Skill

Vertical axis: Challenge

Areas from vertical to horizontal: Anxiety, Flow, Boredom

Using the spaced repetition method, the review interval of the easy question increases and the frequency of test decreases. So you do not need to mind that the question is too easy.

On the other hand, you need to be careful that the question is too difficult. When I tried spaced repetition for the first time with paper cards, I made this mistake. First I looked at the word list which sorted by difficulty level and chose a level that does not contain any words I know. To make matters worse, I made cards in alphabetic order. As a result, words with a similar appearance are shown altogether. It confuses my memory.

The confusing questions are frequently asked. As a result, every day I struggled with questions that I could not answer and lost motivation to continue.

On the other hand, the solution by Anki's author Damien Elmes is an automatic suspension. If you wrongly answer a question 8 times in the review, the question is automatically suspended and excluded from the study. In other words, too difficult cards disappear automatically.

Some people worry to lose the questions they made. The cards are excluded from the study but not deleted. You can edit the cards or cancel suspension at any time. Damien Elmes explained for those cards to do one of the following three.

If two or more cards are interfering, leave the other until you certainly remember the one.

Think whether it is worth remembering really, and delete it if it is not worth remembering.

Edit to make it easier to answer.

It is the similar concept as "rule 11: find and remove interference" in 20 rules to structure knowledge. The Anki automatically finds cards that are likely to interfere with each other and removes them from the study list.

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Footnote *28: Another good point is that Anki restricted to learn 20 new cards on a single day and to review 100 cards. This restriction is a concrete example of "cutting task by volume" seen in (2.3.2) Timeboxing. Studying too much is difficult to keep motivated because you can not see the goal. However, with the limited number of cards, you can see the goal. You can complete the task in about 1 hour.