2.4.1 A ``trade-off'' analysis of cognitive models

2.4.2 ``Literal models'' promise more than they can deliver

2.4.3 They are not yet of practical use to designers

2.4.4 Why formal models are not very useful

2.4.5 ``Design and evaluation techniques'' are not used anyway

2.4.6 Failure to appreciate variety of possible solutions

2.4.7 Problems of validation without theoretical foundations

2.4.8 Ease of formal analysis suggests simplicity of the system

2.4.9 The expert is a learner at the edges