Ergonomics –The Study of Work

Origins in WWII As more controls

and devices were added to British combat planes

pilots made more errors,

endangering the missions

Affordances

Ergonomics studies the range of normal Human Abilities and

Limitations

Mappings & Affordances

• Mapping is the relationship between objects. Turn the steering wheel right the car goes right.

• Turn signals on a motorcycle: flip up, flip down – bad UI

Novice Users

– Confused– Willing to learn– Makes errors of understanding– Blames themselves

Intermediate Users

– Confident– Makes slips, not erros– Invokes “magical thinking”• Herbert Simon – Satisficing

Folk theories – eg: Turning the thermostat on high will heat the house faster– Timer theory – thermostat controls how long the device is on– Valve theory: like a hose – turn up the valve and increase the

rate of heat flow

The theory may be incorrect but it should be usable

Expert Users

– Confident– Explains and understands why things work– Able to use tools in innovative ways– Tends to blame the designer

Types of Errors

• Capture Error - where one frequent behaviour replaces another (drive to work instead of a friend’s)

• Descriptive Error – use the wrong object which has a similar use – throw spaghetti into the garbage instead of a pot

• Data Driven errors– Metathesis in typing– Entering an old phone # instead of new one– Slips of the tongue

Types of Errors Continue• Associative Activation Errors

– Pick up phone and saying come in– Answering office reply at home

• Loss of Activation Error– Start on one activity (going to the next room) and forgetting

why you went• Mode errors

– When different devices have different modes of operation and you use the wrong command

• Target errors – When you go for one control instead of the other

(maybe the ejection button shouldn’t be next to the ignition)

The Human Eye has Rods and Cones

Rods detect black and white and allow us to discern lines and edges – they are shape detectors

Cones allow us to detect colour

Distribution and Sensitivity of Cones

Visual Acuity of the Human Eye

• ½’ of arc to distinguish pixels• 18’ of arc to distinguish text(Assuming vision corrected to 20/20)

Always check the distance from the monitor and compare to monitor specs.

1’ (one minute) of arc = 1/60th of a degree

Visual Acuity

A Too Small To Read

This is large enough to read

18’ of arc

½’ of arc

Too close – one can see individual pixels

Good! Pixels blend together

Eye Hand Co-ordination

Fitt’s Law for Reaction Time

T = k0 + k1 log2(Distance/Size +.5)

There is a minimum reaction time.The Further the hand has to travel the longer it takes. The Smaller the object the Longer it takes.

Big objects close by are quickly and accurately clicked. Small objects far away take longer and are easily missed