Planning (and why people suck at it)
When you look at it, planning makes a project go round. It defines the lifespan, control and scheduling of all activities. And when we're being honest, that's what projectmanagement (in it's technical terms) is all about.
Still, even planning is a subjective matter. After all, a planning is nothing more than the expectations of one person (or, when well thought out, that of several). What a planning basically says is: "We expect this activity to take X hours, using X resources, costing X <fillinpreferredcurrencyhere>".
There are various approaches to planning. Some of them are more reliable than others, but most of the time, you don't have the luxury, time or opportunity to choose. Let's take a look at some of the approaches available to us, shall we ?
The 'Easy Peasy!'-method
The Easy-Peasy-method is one of the trickiest forms of planning. When using this method, somebody on your team says 'Oh, I can do that! Easy! Just gimme an hour and it's done.'. As you have no reason to distrust your teammembers, you plan one hour (two, to be on the safe side) for the specified activity. In most cases, this might work out fine. However, keep in mind that the task at hand might not be so easy-peasy after all, and what you planned to take two hours might take two days. Deadlines woosh by, agreements are broken and Hell breaks loose. With added Hell-ness.
Is there a way to prevent this ? Of course there is. Ask questions. A lot. When somebody says 'Oh, I can do that! Easy!', then feel free to ask 'Really ? And how would you do it ? In small, specified steps ?'. Often, you will find out that people make assumptions that may or may not be valid. Try to locate these assumptions, test them and soon you'll discover that 'Easy Peasy' only exists in a world where the Tooth Fairy exists, politicians are honest and where everybody really wants to get along with everyone else.
The 'I Don't Want To Commit'-method
Some people seem to excel at not making any commitment. When confronted with the horrible question of 'When might this be ready ?', they summon a fog as thick as peasoup, scatter arguments like 'presuming that X does Y and not Z' and suddenly seem very anxious to visit the bathroom. Anything not to commit to any form of deadline.
This, I believe, is a good thing. It shows that whoever is doing the job is aware of the fact that in a project, things hardly ever go as planned. However, there is such a thing as too much caution. Basically, the easiest way to plan this way is to say 'Alrighty, let's take two weeks for it then', watch people object and up the planning by a few weeks. Make sure that all risks are well documented and remind people that a planning is always an estimate. Nobody's going to beat you to death when the team fails to meet a planning as long as you've documented risks and dependencies.
The 'We've done this before'-method
Now this is definitely a great method. As you may be aware, most organisations have a projectmanagement office that keeps track of all projects that have passed in the company. These records may contain Lessons Learned reports, old plannings, project plans, quality criteria, etc, etc. If you have the luxury of such an organisation, read the records. All of them. Use the collective wisdom in those documents, as it is very likely that everything that happened in those projects will also happen to yours. It's Murphy's Law, but with a rowdy attitude after a good night out in town. It doesn't like you and will definitely kick you when you're down.
Plannings based on these methods are often the most accurate. They're based on experience, past failures and success, and collective wisdom. The only downside is that very few organisations have such a projectmanagement office. Oh, surely, they have the people. It's all there on paper. But every time a new project starts, the wheel is re-invented and the smeg starts over again.
Conclusion
Plannings are always a subjective matter. Sickness, holidays, personal problems.. anything can make a planning completely derail and head for the gutter. Therefore, be sure to be flexible in plannings. Use common sense, try to think of all the possible scenarios and then, when your imagination has stopped summoning complete disasters.. use the average. Document risks. Ask questions. And for Pete's sake (if Pete is your teammember), don't use plannings as a weapon. A wet newspaper is even more effective than that.