April 14, 2013

How Perceptual Blindness (or Inattention Blindness) affects software testing

Perceptual Blindness (also known as Inattention Blindness) is the phenomenon in which people simply do not see things that are right in front of their noses. It has best been demonstrated by the question below the video here. Watch the video first!

Do watch the video before continuing reading!

If I asked you before watching the video that you’re going to watch a short video in which a gorilla will saunter to the middle casually, almost everyone would say, “yes, of course I’d notice that!” You’d think that something so obvious could never go completely unnoticed. However, a Harvard University experiment demonstrated that a full half of the people who watched the video and counted the passes as per the instructions missed the gorilla. It was as though the gorilla was invisible. That is perceptual blindness.

We miss a lot of what is going on around us and we have absolutely no idea that we are missing so much.


You might be surprised by the fact that you could so completely miss something that is right in front of you, but it happens all the time. It happens because human beings can only focus on a small number of things in their visual stream. When we are intently focused on something, we do not have a way to “see” other things in the scene.

Fair enough, this happens, and it happens for a good evolutionary reason too; your brain doesn’t want to be overloaded so it only focuses on what you tell it to focus on. That’s a good enough mechanism to have in place if it prevents your brain from overloading all the time. The problem lies here; Perceptual Blindness can and does affect us in critical  and dangerous situations. For example, NASA conducted an experiment in a flight simulator. Commercial pilots were tested to see if they noticed distractions on a runway during simulated landings. The strange part come here – the trained pilots did not notice the distraction and landed directly on top of it 25% of the time, but untrained pilots who didn’t know what to expect of a typical landing, saw the distraction. And maybe therein lies our explanation.

How does Perceptual Blindness affect testing?

You miss things when you focus too much on some details. Perceptual Blindenss is the reason why car drivers sometimes hit pedestrians, motorcyclists and bicyclists and then say they never saw them. When you’re driving you sometimes focus very heavily on “cars” and can therefore become ‘blind’ to everything else that isn’t a car.

In the world of software testing, think, how many times have you been browsing through the software you’re testing and completely focused on looking at one specific thing, maybe a specific function, or perhaps a single type of bug?

Let’s take a simple case. You’re testing a website to make sure that the designs correspond to the wireframes and blueprints. So far so good. You will be considering how accurately the website reflects the wireframes and that the spacing is all fine, but did you take note of the URL structure? And did you notice the typos in the headers? Let’s combine these; did you notice the typos in the URL that point to yourwebstie.com and not yourwebsite.com?

When you concentrate on a particular functionality, are you thinking about issues such as branding, spelling or grammar issues, W3C accessibility requirements and load time?

It’s nearly impossible for your attention capacity to handle all of these areas simultaneously, and of course, as in the gorilla video above, you’ll be happy that you ‘counted the right number of passes’ but you might miss the more important things to be seen.

How can we mitigate the effects of Perceptual Blindness in software testing?

There are three ways to combat the effects of Perceptual Blindness in software testing;

–       Testing in pairs

–       Communicate better

–       Cross-functional teams

And in conclusion, I leave you with this excellent road safety video. Don’t worry, it’s not gory or bloody at all, in fact, it doesn’t even take place on a road. What it does do however, is hammer home just how easy it is to miss things you’re not looking for. Enjoy!


Share article