If you drive to work in the morning, the time taken will vary depending on the whims of the daily traffic. That's known as common cause variation. If one morning the traffic grinds to a halt, due to a burst water main up ahead, your drive to work might take much longer. That's known as special cause variation.
It's important to understand the difference when embarking on process improvement. If you react to a special cause, when your process is already stable, you can make conditions worse and cause instability. Just look at the stock market for a good example. This has come to be known as tinkering and is to be avoided at all costs.
Don't let this happen to your organisation. Establish a ‘tinker’ alarm to alert you when anyone is thinking of making a random improvement or pulling a few levers, in response to a sudden change in a metric, just to see what will happen, and make sure that any proposal is based on sound fact and method and not on wishful thinking.