Parkinson’s Law – work expands to fill the time available for its completion
This means that if you reserve a set amount of time to do something then even if you don’t need that amount of time (psychologically speaking) the task grow in size and complexity until it fills the space you have allocated, often you will find yourself running out of time and even over running. This is why you never plan contingency into tasks if you do, you will use it its human nature. So when your planning plan like a realistic optimist, keep a supply of contingency some where close, tell people its there but tell them they have to go through a form of beauty competition to get it. The beauty competition will be enough to keep people focused and trying ot get things done on time, but knowing there is contingency will encourage people to be braver on there estimates.