I know there is a lot of literature out there on this topic. But I hope what I am putting up today can help you to control your project better.
I just want to add a few thoughts. I want to take up a very important statement made by another enthusiast which was “PM must work to foresee potential obstacles (constraints) to efficient progress and identify the party most able to remove this obstacle”
In general parlance this is Project Risks Management. It can be Risks or Constraints or Impediments, call it what you like. But the most important aspect here is about PREVENTION! My friend calls it “foresee”, which is really the point. Let me elaborate further before going into risks management –
When planning we create the WBS(Scope), Schedule & the Cost(Budget). After we do that we do the Risk management process & then, go back for a round of iteration. Why?
Risk management can change above three(& possibly even Quality, Resources & other areas). The final Baselines(Scope,Time,Cost) will reflect the impact of risks. But the important takeaway is that there is much less possibility of your STC baselines derailing now! Unforeseen events are less likely to cause delay / cost overrun now. In fact if a project is going out of control it maybe because of a less than desirable risk management effort!
Now what are we doing here. We identify the potential risks(Obstacles- term used in the reference above), analyse them for Probability & Impact(use any tool or simply create an Excel sheet Risk Register) & assign owners to plan & implement the pre-planned responses if & when they occur. This is a collaborative exercise & all risks are threshed out & planned for.There can be Opportunity Risks too which are also identified(they are positive risks & can reduce project cost &/time)
So what did we do? Any cost risk on the project is identified & the response planned for. So also with Time & Scope Risks. Even in a Lean framework the daily Stand Up meeting identifies the impediments for the day(for each trade) & a response plan is made ready.
So not only is cost control achieved but also control on the Schedule & Scope. Yes there may be some unknown risks, but by & large about 85% of events that can cause Cost over run or Schedule slippage or Scope changes can be foreseen if this effort is done well & throughout the project life, not just at the start. Maintaining an updated Risk Register is a great way to track & take action on risks!