Two Canadian academics, Sholom Glouberman and Brenda Zimmerman,  draw an important distinction between complicatedness and complexity.
Complicated systems are things like hi-tech machines, for example, an airliner or a computer. Such systems are not simple and often have intricate designs, sometimes involving millions of components. They are, however, determinant. If you understand how all the components relate and interact you can usually predict how the system will behave.
Complicated systems are described by adjectives such as linearity, certainty and predictability. They are deterministic, involving simple causality. Outcomes of such systems are usually those for which the systems was designed or intended.
In the management or development of complicated systems role and task descriptions are tightly defined. Knowledge about what to do and how to do it is provided by system experts or operating manuals and cascaded by top-down management structures. A firm focus is maintained on clearly defined objectives. Decisions are taken by considering clearly delimited options and making the best available choice.
Complex systems, on the other hand, involve mutual causality or interaction. They work in non-linear ways. Outcomes are emergent and adaptive to changes in the system and its environment. Outcomes are difficult to predict. They involve considerable uncertainty.
In complex systems the tight structures that are characteristic of complicated systems are usually not found. Rather than roles and tasks being closely defined, it is necessary to build and adapt relationships, which can remain ambiguous and ‘fuzzy’. Choices of action are often not clear, so there is an emphasis on ‘sense-making’ and interpretation of events and issues. Direction cannot be imposed from the top and decisions have to be based around emergent collective understandings of what works best and how.
Gokce Sargut and Rita Gunther McGrath  state that:
“ … the main difference between complicated and complex systems is that with the former, one can usually predict outcomes by knowing the starting conditions. In a complex system, the same starting conditions can produce different outcomes, depending on interactions of the elements in the system.”
A good example of a complex system is a prison. Prisoners and staff interact in a number of different and unpredictable ways at different times. They may forecast each other’s behaviour and act accordingly. Changes, such as a change of regime, may result in outcomes that are difficult to predict. There are competing agendas and antagonism and tension exist in various relationships. Sometimes these remain suppressed, often for long periods. Alliances between groups may be formed and then dissolved. Nobody knows exactly what is going on or why. It is possible that one small event results in a sudden and unexpected change. Something seemingly inconsequential can result in a riot - http://www.bbc.co.uk/news/uk-england-sussex-20553604
Complicated systems and problems can be described by diagrams or blueprints. Usually system experts have the relevant knowledge and experience to solve a problem. Operating manuals and procedural frameworks are usually sufficient to achieve safe and effective outcomes.
With complex systems and problems, however, there is no blueprint. No two situations are alike and everything has to be determined on a case-by-case basis. It is simply not possible to impose ‘by-the-book’ solutions. Indeed to do so can result in disastrous consequences. The solution may appear to be well designed, but the system itself is unpredictable and the impact may be very different from that intended.
Much of the history of child protection in Britain and elsewhere, including the history of trying to reform child protection systems and institutions, points to a widespread failure to recognise that child protection involves complex systems. Treating the child protection system as complicated, rather than complex, results in mechanistic solutions to problems that are inappropriate and often dysfunctional.
The complexity of child protection can be seen at the level of a case. Workers from different agencies and professions try to provide a service to a child and her family, but they all have slightly different perspectives and priorities. The child does not know whom to trust and the parents may be systematically misleading some workers or trying to create tensions and disagreements between different agencies or workers. There is variable quality of information. Some things that appear to be true at one time appear to be false at another. Tensions and disagreements occur between various professionals and practitioners involved in the case. Agencies may develop different agendas or priorities with respect to the case. Workers struggle to make sense of what is happening and what is true. The choices facing agencies may not be clear. Decisions can only be made as groups move towards some sort of consensus about how to ensure the child’s safety and meet her or his needs.
The complexity of child protection can also be seen at the levels of management and policy. “Working together’ involves different agencies coming to shared understandings and defining a common set of tasks. Professionals from different groups have to adjust to the practices and cultures of those from other backgrounds. Often working practices involve complex processes that have to be adapted to individual children’s needs, so that nobody fully understands the processes or can predict exactly how they will operate. Telling people to follow a particular policy or to adopt a particular procedure often does not result in the results intended.
Attempts to improve practice by introducing procedures or structured assessment instruments or computer systems can result in ‘work-a-rounds’ and token compliance. Targets and performance indicators may result in displaced activity. Myths that there is a single ‘right’ approach may lead to putative reconstructions of practice manifesting themselves in entries to case-notes or verbal reports to meetings. Rather than describing what has happened the worker is constrained to repackage reality; to say what should have happened rather than what actually did. Organisational discourse thus can become implicitly normative rather than descriptive and, as a result, the truth about operations becomes an unspoken secret.
The management of complicated systems can be quite directive. Experts who understand the system’s design are in a good position to tell others what the effects of certain interventions will be.
Managing complex systems, however, involves adapting to changes, rather than imposing them. Conflicts and tensions are to be expected as natural, not abnormal. The manager has to deal routinely with situations or events which appear to be unique. Everybody should be engaged in constant learning and adaptation. Reforms and designs that are borrowed from complicated systems and which are imposed top-down will often be completely ineffective in complex systems and may even be dangerous.
 Glouberman, S. and Zimmerman, B. Complicated and Complex Systems: What Would Successful Reform of Medicare Look Like? Commission on the Future of Health Care in Canada, Discussion Paper No. 8, July 2002. http://publications.gc.ca/collections/Collection/CP32-79-8-2002E.pdf
 Sargut, G. and McGrath, R. G. “Learning to Live with Complexity” Harvard Business Review Sep 01, 2011.