Occam's Razor, KISS and the Duck Test sound playful, but together they are a reliable way to find the real cause of a problem fast.
The principle of parsimony, also known as Occam’s Razor, says the simplest explanation that fits the facts is usually the correct one. In design and business it pairs with KISS (Keep It Simple, Stupid) and the Duck Test (if it looks, swims and quacks like a duck, it probably is one). When a system worked in state A and broke after you added B, B is almost always the cause. Start simple, and ask what changed.
Key takeaways
- Principle of parsimony = Occam's Razor: the simplest explanation is usually right.
- The Duck Test: if it looks, swims and quacks like a duck, it probably is one.
- KISS (Keep It Simple, Stupid) is the UX-friendly version of the same idea.
- Overthinking creates problems; simple explanations solve them.
- When something breaks after a change, the change is the likely cause; ask what changed.
What is the principle of parsimony?
The principle of parsimony, from the Latin lex parsimoniae, advocates for solutions built with the fewest possible elements.
Definition
In plain terms: when faced with a problem, the simplest explanation is often the correct one. Searching Wikipedia for ‘principle of parsimony’ redirects you to Occam’s Razor, because they are the same idea.
The Duck Test and KISS
The Duck Test: if it looks like a duck, swims like a duck and quacks like a duck, it probably is a duck. KISS, Keep It Simple, Stupid, is a core UX principle and a hyped-up version of the same wisdom. Creativity and bold ideas drive progress, but the trouble starts when we overthink solutions. That is where Occam’s Razor steps in.
Parsimony in decisions: a worked example
Faced with a large set of affected websites, the obvious suspects (content, authority, penalties) each explained some cases but not all. A colleague’s simple question, if one thing is about content, maybe the other is about something else, eliminated a variable. Investigating further revealed a technical issue on one site, then similar issues across many others. If it looks like a duck, you probably have a duck: the root cause was mostly technical, and once identified, fixing it was straightforward. When uncertain, the parsimonious move is to just ask the people involved: what did you change, add or remove in the past year?
The takeaway
The solution is usually in front of you. If a system was in state A and working, then you introduced B and it broke, B is the likely culprit. When you cannot see what changed, reflect: grab your rubber duck, and ask what you did yesterday, last week, last month or last year. In short: embrace Occam’s Razor, trust the Duck Test, and Keep It Simple, Stupid.
The principle that, among competing explanations, the simplest one that fits the facts is usually correct. It is also called the principle of parsimony.
A rule of thumb: if something looks, swims and quacks like a duck, you can reasonably conclude it is a duck. It helps identify things by their obvious traits.
Keep It Simple, Stupid. A core UX principle: favor simple solutions over clever, overcomplicated ones.
Start with the simplest explanation. If a system broke after a change, suspect the change first, and ask what was modified recently.
Yes. They are two names for the same idea: prefer the explanation with the fewest assumptions.
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