Qualifying quantitative risk

Let’s start with quantifying qualitative risk first.

Ages ago I was under pressure from some upper management to justify timelines, and I found a lot of advice about using risk as a tool not only to help managers see what they’re getting from the time spent developing a feature (i.e, less risk) but also to help focus what testing you’re doing. This was also coming hand in hand with a push to loosen up our very well defined test process, which came out of very similar pressure. I introduced the concept of a risk assessment matrix as a way of quantifying risk, and it turned out to be a vital tool for the team in planning our sprints.

Five by five

I can’t the original reference I used to base my version from, because if you simply google “risk assessment matrix” you’ll find dozens of links describing the basic concept. The basic concept is this:

Rate the impact (or consequence) of something going wrong on a scale of 1 to 5, with 1 being effectively unnoticeable 5 being catastrophic.  Rate the likelihood (or probability) of something bad happening from 1 to 5, with 1 being very unlikely and 5 being almost certain. Multiply those together and you get a number that represents how risky it is on a scale from 1 to 25.

a 5x5 multiplication table, with low numbers labelled minimal risk and the highest numbers labelled critical risk

How many ambiguities and room for hand waving can you spot already?

Risk is not objective

One of the biggest problems with a system like this is that there’s a lot of room for interpreting what these scales mean. The numbers 1 to 5 are completely arbitrary so we have to attach some meaning to them. Even the Wikipedia article on risk matrices eschews numbers entirely, using instead qualitative markers laid out in a 5×5 look-up table.

The hardest part of this for me and the team was dealing with the fact that neither impact nor probability are the same for everybody. For impact, I used three different scales to illustrate how different people might react based on impact:

To someone working in operations:

  1. Well that’s annoying
  2. This isn’t great but at least it’s manageable
  3. At least things are only broken internally
  4. People are definitely going to notice something is wrong
  5. Everything is on fire!

To our clients:

  1. It’s ok if it doesn’t work, we’re not using it
  2. It works for pretty much everything except…
  3. I guess it’ll do but let’s make it better
  4. This doesn’t really do what I wanted
  5. This isn’t at all what I asked for

And to us, the developers:

  1. Let’s call this a “nice-to-have” and fix it when there’s time
  2. We’ll put this on the roadmap
  3. We’ll bump whatever was next and put it in the next sprint
  4. We need to get someone on this right away
  5. We need to put everything on this right now

You could probably also frame these as performance impact, functional impact, and project impact. Later iterations adjusted the scales a bit and put in more concrete examples; anything that resulted in lost data for a client, for example, was likely to fall into the maximum impact bucket.

Interestingly, in a recent talk Angie Jones extended the basic idea of a 5×5 to include a bunch of other qualities as a way of deciding whether a test is worth automating. In her scheme, she uses “how quickly would this be fixed” as one dimension of the value of a test, whereas I’m folding that into the impact on the development team. I hadn’t seen other variations of the 5×5 matrix when coming up with these scales, and to me the most direct way of making a developer feel the impact of a bug was to ask whether they’d have to work overtime to fix it.

Probability was difficult in its own way as well. We eventually adopted a scale with each bucket mapping to a ballpark percentage chance of a bug being noticed, but even a qualitative scale from “rare” through to “certain” misses a lot of nuance. How do you compare something that will certainly be noticed by only one client to something that low chance of manifesting for every client? I can’t say we ever solidified a good solution to this, but we got used to whatever our de-facto scale was.

How testing factors in

We discussed the ratings we wanting to give each ticket on impact and probability of problems at the beginning of each sprint. These discussions would surface all kinds of potential bugs, known troublesome areas, unanswered questions, and ideas of what kind of testing needed to be done.

Inevitably, when somebody explained their reasoning for assigning a higher impact than someone else by raising a potential defect, someone else would say “oh, but that’s easy to test for.” This was great—everybody’s thinking about testing!—but it also created a tendency to downplay the risk. Since a lower risk item should do with less thorough testing, we might not plan to do the testing required to justify the low risk. Because of that, we added a caveat to our estimates: we estimated what the risk would be if we did no testing beyond, effectively, turning the thing on.

With that in mind, a risk of 1 could mean that one quick manual test would be enough to send it out the door. The rare time something was rated as high as 20 or 25, I would have a litany of reasons sourced from the team as to why we were nervous about it and what we needed to do to mitigate that. That number assigned to “risk” at the end of the day became a useful barometer for whether the amount of testing we planned to do was reasonable.

Beyond testing

Doing this kind of risk assessment had positive effects outside of calibrating our testing. The more integrated testing and development became, the more clear it was that management couldn’t just blame testing for long timelines on some of these features. I deliberately worked this into how I wanted the risk scale to be interpreted, so that it spoke to both design and testing:

Risk  Interpretation
1-4 Minimal: Can always improve later, just test the basics.
5-10 Moderate: Use a solution that works over in-depth studies, test realistic edge cases, and keep estimates lean.
12-16 Serious: Careful design, detailed testing on edges and corners, and detailed estimates on any extra testing beyond the norm.
20-25 Critical: In-depth studies, specialized testing, and conservative estimates.

These boundaries are always fuzzy, of course, and this whole thing has to be evaluated in context. Going back to Angie Jones’s talk, she uses four of these 5×5 grids to get a score out of 100 for whether a test should be automated, and the full range from 25-75 only answers that question with “possibly”. I really like how she uses this kind of system as a comparison against her “gut check”, and my team used this in much the same way.

The end result

Although I did all kinds of fun stuff with comparing these risk estimates against the story points  we put on them, the total time spent on the ticket, and whether we were spending a reasonable ratio of time on test to development, none of that ever saw practical use beyond “hmmm, that’s kind of interesting” or “yeah that ticket went nuts”. Even though I adopted this tool as a way of responding to pressure from management to justify timelines, they (thankfully) rarely ended up asking for these metrics either. Once a ticket was done and out the door, we rarely cared about what our original risk estimate was.

On the other side, however, I credit these (sometimes long) conversations with how smoothly the rest of our sprints would go; everybody not only had a good understanding of what exactly needed to be done and why, but we arrived at that understanding as a group. We quantified risk to put a number into a tracking system, but the qualitative understanding of what that number meant is where the value lay.

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