People are in organisations most valuable and expensive resource, but they are the most difficult element of an organisation to manage.  People possess a variety of talents and they will react differently in different circumstances.

In fact, in many ways people are unpredictable. This means that, unlike machines, they are not interchangeable, which creates problems for organisations, e.g. a person may work well one day but not the next, or may cope well with pressures one day but fail to cope another day.