Except for third party materials and otherwise stated (see Creative Commons Attribution-NonCommercial-ShareAlike 4.0 Licence.), this content is made available under a
The material acknowledged below is Proprietary and used under licence (not subject to Creative Commons Licence). Grateful acknowledgement is made to the following sources for permission to reproduce material in this free course:
Figure 1: © The Open University and its licensors
Figure 2: © The Open University
Figure 3: © ismagilov/Getty Images
Figure 4: © The Open University
Every effort has been made to contact copyright owners. If any have been inadvertently overlooked, the publishers will be pleased to make the necessary arrangements at the first opportunity.