“I’m not offended by all the dumb blonde jokes because I know I’m not dumb…and I also know that I’m not blonde.”  — Dolly Parton

See someone in glasses? They’re smart, right? See someone, especially a woman, with blonde hair? They’re dumb, right? Blonde and wears glasses? Now there’s a conundrum.

These are ridiculous assumptions, and we all know it. Yet, they’re assumptions that people make all the time without thinking.

Psychologists have a name for this: “the halo effect”. The halo effect is the tendency for impressions in one area to influence one’s opinion or feelings in other, unrelated areas.

The Halo Effect and Safety

Safety is important. To help us be safe, we often develop safety checklists. For instance, vehicle safety inspections have a checklist that includes brakes, lights, steering, windshield wipers, turn signals, tires, and seat belts. There shouldn’t be anything there to argue with.

Different states have different checklists, however. Some also include the horn, window tinting, rearview mirrors, alignment, or fluid leaks. Important? Sure. But as important as brakes, tires, and seat belts? Probably not, or they would be on every state’s inspection checklist. In those states where they are included, though, they had better be in good shape or the car won’t pass the inspection. Inspections are pass/fail.

The result is that everything on the checklist is considered just as important as everything else. Because some things on the checklist are very important, the halo effect makes everything on the list seem very important.


This halo effect also makes all PPE seem equally important. Consider the routine PPE most of us wear in an industrial or process environment: steel-toed shoes, safety glasses, and a hardhat. Are they equally important in every situation? No.

I knew a safety director who mounted a personnel campaign against the routine requirement for a hard hat. Steel-toed shoes? Yes, because almost all work poses the possibility of dropping something on your toes. Safety glasses? In the plant, yes; debris could come from anywhere. But hard hats? He argued that hard hats were only necessary in open multi-story structures where things could be dropped from above, or in congested areas where equipment and piping were obstacles to be run into. Things were not likely to fall on one’s head if the only thing above was the ceiling or the sky.

He lost that argument. Too many people would have to “run back and grab my hardhat”. Unlike hearing protection, hardhats aren’t disposable, and the easiest place to carry them is on one’s head. So, hardhats remained part of the routine PPE at that facility.

But his point was valid. Including an “also” in the mix doesn’t make it is just as important.


Double-block-and-bleed (DBB) valves are one of my favorite safety features. They provide the redundancy of double block valves in an emergency when the need is to reduce flow to a negligible amount. And the bleed valve, normally closed but opened between the two closed block valves, provides the positive isolation necessary over an extended period when even a negligible flow is too much.

The challenge comes when calculating the average probability of failure on demand (PFDAVG) for a safety instrumented function (SIF) that uses a set of DBB valves. What is the purpose of the SIF? To immediately stop flow, or at least reduce it to a negligible amount, upon the onset of the emergency. What matters during the emergency? The two block valves. If either one closes, flow is stopped. This is a 1oo2 architecture, which is the most reliable of the four common architectures (1oo1, 1oo2, 2oo2, and 2oo3). How does the bleed valve contribute during the emergency? It doesn’t. Whether it opens or stays closed, preventing the unsafe condition from escalating only depends on at least one of the block valves closing.

Because it’s there, though, we tend to also include the bleed valve as one of the final control elements of the SIF. I get it. We want that bleed valve open over an extended period if the block valves are closed. But in the emergency, the bleed valve is just an “also” and doesn’t belong in the PFDAVG calculations.

There is a downside to including the bleed valve in the SIF. We not only worry about PFDAVG, but also spurious trips. The spurious trip of a bleed valve allows whatever is in that line to be vented at full pressure, which is rarely a safe condition.

Don’t Be Seduced By an “Also”

There are going to be many occasions where we “also” do something for safety. The fact that they are included does not make them as important as the main features, and we shouldn’t let the halo effect beguile us. In a DBB valve arrangement, the SIF description should say “Close the block valves.” If a review of the hazards associated with a spurious trip of the bleed valve indicates that it’s all right, then go ahead and add, “Also open the bleed valve.” But don’t include the bleed valve in the PFDAVG calculations.

In her comment, Dolly Parton added, “I also know I’m not blonde.” It got a laugh, but the point that really matters was when she said, “I know I’m not dumb.”

Don’t lose sight of what matters.