Case: Fitts Law

One thing to consider when designing UI's is Fitts Law. Fitts Law is a complicated looking formula which describes the amount of time it takes a person to move a pointer to a target. For example pressing a button with your finger or clicking a button with a mouse.

"MT = a + b log₂(2A/W + c)

where

• MT is the movement time
• a and b are empirically determined constants, that are device dependent.
• c is a constant of 0, 0.5 or 1
• A is the distance (or amplitude) of movement from start to target center
• W is the width of the target, which corresponds to “accuracy” "

From http://www.cs.umd.edu/class/fall2002/cmsc838s/tichi/fitts.html

Put simply Fitts Law shows that it is easier to target larger objects which are closer to you. This may seem intuitive but it is an important consideration when designing.

What Fitts Law implies is that buttons used most frequesntly should be biggest, as these are easiest to target. Not only that but also that the most frequestly used buttons should be located in close vicinity to each other.

This may seem obvious but there are many example of designs where this is not the case. When designing for children this idea is even more important. In my opinion the most commonly used button on a hand held device must be 'thumbable' without risk of pressing other buttons.

Examples of good and bad button layouts:

Key : 1 - Main Function button
2 - Secondary Function button (Menu)
3, 4 - Scroll buttons

A Bad Button Layout.

This layout might be acceptable for a sleek looking device aimed at the majority of the population. Notice however that two key buttons (1 and 2) and set far apart and also that all buttons are the same size not distinguishing their importance.

A better layout.



Here the main buttons are emphesised by being made larger. The buttons are also grouped appropriately.

Our initial prototype follows this law very well. The scan button is large (like number 1 above) as it is the first button that will be used for each use case. Also the button needs to be held when scanning the word, so by making it big, we have made it easy for children to scan words. Our scroll wheel has been positioned away from the scan button, which ensures that the two are not mistaken. The scroll wheel position means that the child can hold the pen horizontally and read the text with ease.