How a picture helps - copper in Devon
Sometimes a diagram only needs a sentence or two to explain its message. Take the diagram above. The horizontal axis is divided into decades, the vertical axis shows production of copper (ore and metal) in each decade for Devon and Cornwall, the south-western counties of the UK. What can you see? There were three decades (1830s, 1840s and 1850s) when production was steady. For the previous century, production had increased steadily. Then it dropped dramatically, so that the production in the 1890s was less than in the 1720s.
What happened? Did the many copper mines in Cornwall and Devon all run out of mineral at the same time? That would seem unlikely. If you were the owner of a copper mine in the 1860s, could you have foreseen the crash? For a time, the Devon Great Consols Mine was the biggest copper mine in the world. Its owners must have expected (forecast?) further profitable years. What we have here is actually the consequence of world affairs, and not simply local ones. The production of copper plummeted because vast new sources of copper were discovered in Chile, the USA, Australia and elsewhere, and it was cheaper to mine the element there.
I came across this diagram in "England's Landscape - The South West" (edited by Roger Kain). It illustrates at least two lessons for OR people. (1) The value of a simple diagram. (2) Forecasting is a difficult process, because there may be external factors which have not been important before.
There are other examples of dramatic changes to time series as a result of some global event which affects the status quo. Immigration from Eastern Europe following the collapse of communism there has had many effects on the population of Western Europe. Nearer to Exeter, the number of students from east and west Africa coming to the UK to study mining and other subjects dropped suddenly following the end of apartheid in South Africa; it became much cheaper to study in Cape Town than in Cornwall.