The Hidden Assumptions That Can Sabotage Your Next Project

I’m renovating my house at the moment, and the time has come to install the new oven. Very exciting! It’s a nice, big, fancy oven, replacing a 25-year-old model that wasn’t even top of the range when it was new. I love cooking, and I love eating even more, so I’m very excited about this oven. Then, a limiting assumption hit my plans hard. Every house has a limit for how much electricity (measured in amps) it can draw from the grid. In my house, it’s 63 amps. For those who prefer kilowatts (kW), that’s 14kW. Except my new oven, when running at full capacity, will draw 18.4kW. So, if I ever use it at full tilt, it will blow the fuse to the house—even if nothing else is running.

What other limiting assumptions do we make, continue to forget about, and eventually have come back to bite us in our projects?

Other types of limiting assumptions

The limiting assumptions we tend to overlook are generally related to capacity (including throughput), lifespan, or availability.

For example, when installing a new ERP system for a government department of 500 employees, we assume that the system, which is licensed for up to 1,000 users, will be sufficient—until the department merges with another, and now the ERP system needs to be replaced almost immediately. Or, when building a new customer service AI system, we assume that a three-year payback period means it’s a good return on investment—until ChatGPT comes along, disrupting the industry, and our AI system looks outdated and needs replacing quickly. Similarly, when building a data warehouse solution that requires a four-hour overnight window for processing, we assume that since it’s being used in only one country, four hours of downtime is plenty—until the company rolls out the system globally, and now there is no longer a four-hour window.

Of course, there are many other assumptions, but we tend to be better at identifying those related to cost, quality, scope, technology, and the environment. It’s the ones we’re not good at identifying that we need to watch out for.

How to identify a limiting assumption

Every decision we make comes with assumptions attached.

To identify them, we can ask ourselves three simple questions:

1. What is the capacity limit that would make this decision a bad decision?

2. What is the lifespan limit that would make this decision a bad decision?

3. What is the availability limit that would make this decision a bad decision?

Had I asked these questions when purchasing my oven, it might have gone like this: I use 6kW at peak on a normal day. If I add 18.4kW to that, does my house have more than 24kW capacity? The oven comes with a five-year warranty; what percentage of these ovens break down beyond repair after that period? How long will spare parts be available?

Just by asking three simple questions, I could have identified limiting assumptions that never crossed my mind at the time of the decision.

How to challenge or validate a limiting assumption

Once we’ve identified our limiting assumptions, we need to test whether they’re acceptable or likely to cause problems.

First, make sure the assumptions are documented. Second, put those documented assumptions in front of someone with the authority to approve the decision in light of the assumptions. Third (this is where project managers add the most value), consider whether the decision-maker has (i) access to all the information needed to validate the assumptions and (ii) the authority to make the final call. If they don’t, escalate it to a higher level to be on the safe side. For instance, your ERP system sponsor may not know that the department is about to merge, invalidating the capacity assumption.

Respectfully insisting on access to the right information and decision-making authority can be the difference between success and being bitten by a limiting assumption.

As for the oven, I fear the journey is only just starting. The cables laid in the street provide each house with 63 amps. To get more power to my house, I’ll need to dig up the street, lay a larger cable, and connect that extra capacity to my house—all of which I have to pay for. I already have solar panels and batteries installed, so maybe it’s time to investigate wind power too...