Quality in Design

• ISO9001/2, CMM, etc., derive from process quality assurance approaches in manufacture– Produce standard, undifferentiated products– Physicality of a manufacturing process permits its

decomposition into repeatable steps

• But software need only be written where the product is unique...– Unique products cannot be generated by

'standardised' processes– Intellectual property is less easily sliced and diced

Design is Always Iterative

• Design is like a "game of chess"– Designer makes a move, system makes a

countermove

• Every design choice balances pre-existing 'forces'...– Often well understood in isolation, but not when

they co-exist and interfere with each other

• And unleashes new ones, often unanticipated

Reliability

• Functionality that is not tested contains bugs