The Ninety-Ninety Rule and Overcoming Unplanned Work

By Stefan Auvache

"The first 90 percent of the code accounts for the first 90 percent of the development time. The remaining 10 percent of the code accounts for the other 90 percent of the development time."
- Tom Cargill

In software engineering, there is a principle jokingly called the Ninety-Ninety Rule—once you code the first 90%, you need to code the second 90%.

“The second 90%” refers to all of the stuff that you didn’t anticipate needing to do at the outset of the project. Software development projects always have unforeseen complications and extra tasks attached to them. The Ninety-Ninety Rule is a reminder for anyone taking on a new project or setting an ambitious deadline. It means that unplanned work is lurking in the shadows, waiting for the right moment to strike. And you’d better be ready when it does.

Unplanned Work

Unplanned work is exactly what it sounds like, and it is a universal experience.

It is the leaky pipe you find while cleaning the garage. It is the frozen windshield on a cold morning that makes you late for work. It is the urgent request, the last-minute meeting, and the angry customer demanding that you drop everything and fix their problem. All of the firefighting and damage control and unaccounted-for stuff that prevents us from doing what we already have planned falls into the category of unplanned work.

In business, unplanned work wreaks havoc. When project managers don’t take interruptions and extra tasks into consideration, they set unrealistic deadlines and assign unknown amounts of work to already overextended teams. When progress inevitably stalls, executives and other leadership often assume that employees are lazy or incompetent. People get fired, teams shrink, and even less work gets done.

A similar pattern arises in daily life. We are all the project managers for our own lives, and when we don’t account for unplanned work, we sabotage ourselves. We take on too many commitments, underestimate the time they’ll require, and end up having to choose which obligations to complete and which to abandon.

How to Handle Unplanned Work Like the Pros

Thankfully, professional project managers have been dealing with this forever. They have learned how to manage uncertainty, mitigate risk, and deal with unforeseen problems. Here are some tips from professionals on how to plan for unplanned work.

Gain Visibility

In Gene Kim’s novel The Phoenix Project, a new leadership team is charged with saving a floundering IT department. They struggle at first because they don’t have a clear view of all ongoing work. Deadlines are constantly missed and no one understands why. Their first action is to map out all of the projects, responsibilities, and other work assigned to the IT department. Doing so provides insight into the workload, bottlenecks, and process issues of the department. Only then is the team able to fix problems.

The first thing that a good project manager focuses on is visibility—an understanding of how much work is already present. If you don’t know how much work there is to do on a project, you can’t accurately set deadlines. On a larger scale, if you don’t know how many projects and responsibilities you already have, you can’t confidently decide if you have the bandwidth to accept more work.

To gain visibility, do your best to identify all of the work assigned to you—tasks, meetings, appointments, etc.—and how much time each of those things will take you to complete.

Double Your Timeline

Once you have an idea of how long it will take you to finish a project, double your estimate.

I’m dead serious about that. Project managers often double or triple their estimations for how long it will take to complete a project.

We underestimate how much time it will take to finish something because, believe it or not, our brains are wired for optimism. Psychologists Daniel Kahneman and Amos Tversky found that we build our timelines around best-case scenarios, not realistic expectations. This is called the planning fallacy—the tendency to believe that something will be different or better than in the past, even when history has repeatedly proven otherwise.

In the beginning of a project, an optimistic timeline feels motivating. But as problems arise, the schedule becomes unrealistic. Doubling your estimate is a way to account for the invisible, unplanned work hidden in every project.

Plan slack into your schedule. When something goes wrong, you can take extra time to fix it without falling behind schedule. If things run smoothly and you finish all of your work early, you will be ahead of schedule.

Learn to Triage

The best project managers understand that not all tasks are created equal.

When unplanned work shows up, don’t automatically treat it as an urgent task that needs your immediate attention. Instead, triage.

In hospitals, doctors triage to decide which patients need immediate care. A patient in cardiac arrest is treated before a patient with a dislocated thumb. The same logic applies to your projects and priorities. Just like a doctor prioritizes patients, you can identify urgent, essential work and separate it from the stuff that can be done later (or doesn’t need to be done at all).

Don’t give the same attention to every piece of unplanned work. Triage to avoid wasting time doing unimportant things.

Respect the Ninety-Ninety Rule

I run into unplanned work every single day.

As a software engineer, it shows up when a “quick fix” turns into a massive undertaking or an urgent issue pulls me away from whatever project I am working on. There’s always something unexpected that requires my attention. In my personal life, unplanned work looks more like a sick kid before a fully-booked weekend or some yardwork that turns into two trips to Home Depot.

Running into surprise tasks and detours during a project isn’t a sign of failure—it’s the nature of doing work. That’s okay, provided you prepare properly to handle it.

Don’t overcommit. Don’t try to do everything at once. Don’t panic. Do your best to understand how much work is on your plate, plan in some slack for the unexpected, and triage appropriately. You will gain control over your schedule and your productivity will increase.

Respect the Ninety-Ninety Rule. Plan for unplanned work.

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