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http://ritter.ist.psu.edu/papers/ritters01.pdf

This document describes the shape of the learning curve, and how it's universal over most tasks. It also gives some very exact mathematical functions with witch one could calculate a curve for a certain task.

https://www.youtube.com/watch?v=ea6UuRTjkKs

This video explains the difference between 'difficult' and 'punishing' (or tedious) tasks,  and formulates examples of well-executed difficult tasks. It also sets up some guidelines to follow if you want to set up difficult tasks yourself.

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Defining the All-Important Difficulty Curve
This article gives a brief overview of our topic and some examples of games and the different (difficulty) curves they use. The examples are written in a clear manner and show the effect of these different uses.

ExtraCredits on Pacing in Video Games

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How can the teaching community learn from video games?

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How can the teaching community learn from video games?

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In the video game industry, there's been a lot of research done on how to manage interest curves and difficulty, in order to make entertaining games. However, most teachers don't know much about games and how they work, and often say they don't have a place in the classroom. We want to see if we can use the research that's already been done in this field to help teachers in the classroom.

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ExtraCredits on designing good reward systems

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ExtraCredits on designing good reward systems

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https://www.youtube.com/watch?v=h86g-XgUCA8

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ExtraCredits on making difficult things fun

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ExtraCredits on making difficult things fun

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https://www.youtube.com/watch?v=ea6UuRTjkKs

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https://www.youtube.com/watch?v=5LScL4CWe5E&t=1s

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ExtraCredits on the difficulty curve in games

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ExtraCredits on the difficulty curve in games

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https://www.youtube.com/watch?v=YsCmyIsqscc

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http://pascal-luban.blogspot.com/2009/11/difficulty-curve-is-there-such-thing-as.html

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http://blog.rpgmakerweb.com/tips-and-tricks/the-difficulty-curve-and-rpgs/

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https://onlinelibrary.wiley.com/doi/full/10.1111/j.1467-8578.2008.00402.x

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http://ritter.ist.psu.edu/papers/ritters01.pdf

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https://www.quora.com/Why-do-people-like-puzzles

https://www.puzzlemuseum.com/faqs/whypuzzle.htm