All project work is random work. There are three core random variables on all projects, shown below. There are sub-variables as well as all the ...ilities involved in project work, but let's start with the major three.
Fixing, 1, 2, or all 3 of these random variables does NOT make the randomness go away.
These variables are random and all variables on projects are random because of uncertainty. This uncertainty (as mentioned on many other blogs) comes from two sources. Aleatory uncertainty that is the underlying natural randomness of all project activities. This is called irreducible uncertainty. It can't be reduced. Nothing you can do will reduce it. It's there and will always be there. This is a statistical process. The only way to work in the presence of irreducible uncertainty is to have margin. Cost margin, schedule margin, technical margin.
The second is epistemic uncertainty. This is uncertainty that is event based. It's there but can be handled in some ways. Those ways can include buying two of everything in case one breaks, having redundancy in other forms - a backup site for the data center, testing, prototypes, and other activities that provide a Plan B when the probability that something will go wrong becomes true and that thing that went wrong is no longer a probability but has turned into an Issue.
So Here's the Real Problem
When we hear, we don't need to estimate, I can fix time and budget, that doesn't make the randomness go away. It just sets an upper bound on what you CAN spend and when you HAVE TO BE DONE. Those uncertainties that create the randomness are still there. Then fixed time and fixed budget plans, leave open the technical randomness as well. The time and budget are still random inside the constraints set by the project.
There's no getting around this. No matter how often someone says you can. Those someones were asleep in the engineering probability and statistic class. Here's the classic engineering course we were all forced to take as physics grad-student Probability and Statistics. †
This is basic probability and statistics of project work. The probability that something will turn out unfavorable is created by epistemic uncertainty. The statistical variances of everyday life are created by aleatory uncertainty.
Ignoring these uncertainties means it's going to turn out bad for those paying for your work
You need margin to protect from irreducible uncertainty. You need specific actions to protect from reducible uncertainty. So you can in fact fix the cost and schedule IF AND ONLY IF (IFF) you have margin and risk buy down plans. When someone says we've fixed the duration and the budget. two things come to mind.
- Budget is NOT cost
- budget is not real dollars
- Budget is a target for the cost. The cost is a random variable
- Your spend rate - real dollars - is a random variable.
- No cost margin, you're going to be over budget before you start, IF you plan on meeting the customer's needs.
- You can no meet the customer's need and just stop when you run out of money.
- This is basic Managerial Finance
- Duration is cost, since time is money where I live
- A fixed duration is a target completion time
- Both uncertainties - aleatory and epistemic are present in the duration cost random variable
A third notion is the killer notion
When you fix time and cost, have sufficient risk buy down activities to reduce the epistemic uncertainty that creates the probability of something going wrong to an acceptable level, and have sufficient margin to cover the expected overruns in duration, you still have the technical reducible and irreducible uncertainties that the things you building won't work, won't be what the customer wants, will cause other issues - these are called externalities in the economics of software development, and other unknowns, possibly unknowable at the beginning of the project.
When you fix time and or budget, and don't have protections for reducible and irreducible, you're going to be late and over budget and you have willfully ignored those outcomes. Oh and by the way, there is a probability your little gadget is not likely to meet the needs of those paying you either.
These immutable condition (aleatory and epistemic uncertainty) are completely ignored in agile development. Agile provides rapid feed back to the risk management processes of software development. But agile is NOT a risk management process in and of itself. That's a topic for another time.
If you think you have no uncertainties - reducible or irreducible, and have fixed the budget and duration and maybe even the outcomes. You're likely on a de minimis project. Good luck with that.
† We had to take a few courses outside our major, and this was another. Classical Electrodynamics. This was an engineering course. We had a foundation of electrodynamics from the physics point of view. In that view everything can be solved through Maxwell's equations. A simple set of partial differential equation describing how electromagnetism works. When asked to give a talk on antenna theory in the engineering course, a friend (I was too afraid at that time) went to the chalk (yes no white boards) one wrote done maxwell's equations for the reciprocity theorem of antennas in free space. The Professor at the back of the told him (Steve) to sit down. We're engineers not physicists we want to know HOW things work not WHY things work