SURVIVAL OF THE FATTEST

Three weeks ago, I had my annual health check-up. All fine, thank you, but it did spark a thought. What if I wrote about health? Then again, who am I kidding — I can never ditch testing. So, instead, let’s make this a different sort of health check: the FAT Test. Not cholesterol, not body mass index, but Factory Acceptance Testing.

Now, FAT may not be the most fashionable acronym in today’s agile-obsessed world, but it has not been consigned to the scrapheap either. Far from it. In fact, in some industries, it is still the heavyweight champion of quality gates. Think of it less as a fad diet and more as a slow-release superfood: not glamorous, but essential in the right context.

The Factory Acceptance Test is typically performed at the supplier’s site, long before a system makes its way into the customer’s production environment. As any seasoned tester will tell you, everything works perfectly on the developer’s machine — until the customer turns up to witness it. FAT is all about verification and validation, often with the client looking over your shoulder, clipboard in hand, ready to check that contractual promises line up with actual performance. Once the supplier has flexed their testing muscles and passed this stage, the project graduates to the next gym session: SAT.

Site Acceptance Testing (SAT) takes place in the customer’s environment. Here the supplier is still the trainer, but the customer is not just spectating, they are spotting you on the bench press. SAT validates that the system behaves as expected in situ, often including early elements of User Acceptance Testing (UAT). And if it’s a brand-new system? Then we move into the equivalent of a cautious warm-up jog: live beta testing. This is where a carefully chosen group of users put the system through its paces in production, but with training wheels on. It’s not quite a marathon yet, more like stretching before the big race, but it gives real-world feedback without risking a full-blown failure.

These stages, FAT, SAT, beta, may sound like an old-school workout routine, but they remain the backbone in industries where the cost of failure is eye-watering. Government organisations, aerospace, defence, healthcare, pharmaceuticals, and any life-critical or mission-critical environment rely on these structured quality gates. In such contexts, skipping FAT would be like skipping leg day: you might get away with it for a while, but eventually something will collapse under the weight.

At its heart, testing is risk mitigation. We are not in the business of predicting the future, but of preventing the worst. FAT is a tool that allows us to build trust early between vendor and client, between design and reality, between promise and delivery. And just like a GP’s check-up, it is better to catch small issues in a controlled environment than wait until the patient (or project) is in the emergency room.

Historically, FAT has had quite the evolutionary diet. Born in the mid-20th century as a manufacturing quality gate, ensuring turbines, generators, and production lines behaved themselves before being shipped.  It quickly bulked up in the 1970s and 80s with the rise of automation and embedded software. Suddenly, FAT was not just about mechanical kit but also about code, interfaces, and control systems. By the 1990s, enterprise IT had pulled up a chair, borrowing FAT for big-ticket projects like ERP rollouts and large-scale databases. And in today’s Agile and DevOps world, FAT has slimmed down into slicker forms such as System Integration Testing (SIT) and User Acceptance Testing (UAT). Yet in industries where compliance is king and documentation is non-negotiable; the old heavyweight still holds its place.

The survival of FAT lies in three things:

1. Catching costly defects early — the software equivalent of nipping high blood pressure in the bud before it leads to something more serious.
2. Building trust between vendor and client — because nothing calms a nervous customer like seeing their system work under their own watchful eyes.
3. Providing contractual sign-off — that all-important certificate of fitness without which no project manager sleeps soundly.

So yes, while leaner, more agile methods are the diet du jour, the FAT test remains a vital part of the testing food pyramid. It is proof that in software, as in biology, it is not always the fastest or the leanest that survive, but those who adapt and prove their fitness when it counts. After all, it really is the Survival of the FATtest.