Max apparently didn’t notice Heather’s tone, because he continued without skipping a beat. “Tell me what causes a machine to malfunction”.

“That’s obvious. Something went wrong with the machine.”

“So how do you fix it?”

“Well, you just do.” Heather replied.

“Let’s knock off for today”, Max suggested. “In the meantime, think about those two questions”.

Seth was hot and tired after spending the whole Saturday in Max’s back yard. “Mom, can you turn down the thermostat, it’s really hot in here!”

The thermostat was set at 74, so Heather set it to 68. She didn’t like spending the extra money for electricity, but she wanted Seth comfortable.

“Mom, please turn down the thermostat, it’s still boiling in here”. It was a half hour later, and Seth still had the same complaint. What in the devil was wrong with the thermostat? Turning it all the way down to 50, she noticed the fan didn’t come on.

She phoned the landlord, and asked him to fix it. The landlord, a very nice man named Terry Melbourne, guided Heather, over the phone, to the breaker box in the closet, and asked her to verify that the breaker marked “air conditioner” was flipped to the left. It wasn’t -- it was flipped to the right. He instructed her to flip it to the left, and when she did so, she heard the fan go on. “That should fix the problem”, Terry said.

But it didn’t. An hour later, the air conditioning went off. Heather checked the box, and once again, the air conditioner breaker had flipped to the right again. She called Terry, a little bit irritated.

“Hang on, I’ll be there in a few minutes.”, said Terry. True to his word, he arrived 20 minutes later with a bright flashlight, a multimeter, and some tools. Turning on his flashlight, he flipped every breaker to the right except the air conditioning breaker, and then took Heather on a tour of the house.

“Look for anything on. Any light, any sound, let me know.” he said. They didn’t have to look long. On the living room desk, Heather’s computer lit the room brightly. Yesterday Heather had moved the computer to the living room after rearranging the apartment. Terry uttered a vile oath, and then apologized.

“Some fool electrician wired this place wrong”, Terry said.

“Your air conditioner is supposed to be on its own circuit breaker, but somebody wired this socket, you know, the one your computer’s plugged into, into the air conditioner circuit breaker. For now, find another outlet for your computer. In the next week or so I’ll have someone wire this correctly. Don’t worry, there’s no danger, it’s just an inconvenience that I’ll have fixed as soon as possible”.

An hour later Terry called her, explaining that his records showed that Janblowski Electric had installed that outlet two years ago. He’d get them to fix the problem, no charge, tomorrow.

Heather went to bed, but she couldn’t sleep.

She had assumed the problem was the thermostat, but in fact it was the breaker. No setting of the thermostat could get the air conditioner to turn on, but as soon as she flipped the breaker, the air conditioner worked. She had guessed at the wrong cause.

Hey, just like Wintekeller had wrongly guessed it was the water pump, and then the radiator. Maybe she was as incompetent as Wintekeller. That put a new spin on things. Except that she was not an air condition repairman, but Wintekeller was a car repairman.

But it wasn’t really the breaker, was it. The breaker was flipping only because her computer was on the same breaker as the air conditioning, overloading the circuit. The circuit breaker was doing what it’s supposed to, flipping on excess current.

Terry had fixed the problem by moving her computer. But, she really wanted her computer where she had put it. Terry had fixed the symptom, but his solution was a great inconvenience. Luckily, within a week he’d have it fixed the right way. Heather finally fell asleep and dreamed of a stack of boxes. On awaking, all she could remember was some sort of stack. She thought of this while she and Seth biked to his school, and then as she rode back. At home again, getting dressed for her waitress job, she suddenly grabbed a pen and paper and drew a stack of boxes. Staring at it for several minutes, she finally wrote in the boxes:

The air conditioner didn’t work because the circuit breaker was flipped.

The circuit breaker was flipped because the computer drew extra current through it.

The computer drew extra current through it because the computer outlet was wired to the Air Conditioner circuit breaker.

The computer outlet was wired to the air conditioner circuit breaker because Janblowski Electric messed up when wiring it.

The air conditioner failed because Janblowski Electric had messed up. But the failure went through a chain of causes.

Heather Gets a Lesson

It was dark by the time Heather’s restaurant shift ended, so she couldn’t work on the car. She called Max nonetheless:

“Hi Max, it’s me, Heather. I know why a machine malfunctions. It malfunctions because a person made a mistake. And you fix it by undoing that mistake.”

“Interesting Heather. How did you come to those conclusions?”

Heather told the entire air conditioner story, from the moment Seth asked her to turn down the thermostat until the time Terry told her about Janblowski Electric. There was silence on Max’s end, and then he spoke.

“Interesting Heather. Tell me this. What if the circuit breaker blew and kept blowing because a rat ate through the insulation on the wiring, shorting it? Would that be a person making a mistake?”

“Well Max, I guess the human error would be allowing rats in the building, and you’d fix it by getting rid of the rats.”

“Would getting rid of the rats cause the wires to grow new insulation?”

“Of course not. Part of undoing the human mistake is undoing the damage it caused. I guess just like there can be a chain of causes, there can be a chain of damage.”

“Do you have a diary, Heather?”

That was an awfully personal question. Yes, Heather had kept a diary since she was 14, and every couple years she still wrote in it, but she’d not told anyone, not even Jake or Seth.

“Why do you ask that, Max?”

“Because I want you to write down that last answer, and remember it forever”.

“OK Max, I’ll write it down on my computer, in a file where it will never get lost. Part of undoing the human mistake is undoing the damage it caused. I guess just like there can be a chain of causes, there can be a chain of damage. I guess I answered your questions, right Max?”

“In a way you were right, Heather, but not in a very practical way. Do you really need to fix the human mistake to get the machine running? Wouldn’t fixing the wiring fix the air conditioner, regardless of whether you knew Janblowski did it or not?”

“Yes, but if you neglect the human cause, the problem will just happen again.”

“Very good Heather! Write that down too. But let me ask you this: Isn’t your first duty to fix the machine, and then later prevent future occurrence?”

“Yes, I guess so, but what are you driving at?”

“Heather, you mentioned a chain of causes, and at some point the chain transitions to human error, whether such human error is obvious like Janblowski Electric, or much more subtle. Would you agree that to fix the machine, you need only follow the chain to the last technical link?”

“Sure Max, if you don’t care about preventing future occurrence.”

“Exactly. You fix the machine with the last technical link, and then if possible prevent future occurrence by tracing the cause chain through human error. But remember, nobody pays a car mechanic to prevent future occurrence. They pay for a fix, and they want it as cheap as possible. Sometimes the human error is obvious, like the lady who failed to change her oil for fifty thousand miles. But sometimes it’s subtle. What was your error taking care of your car? You maintained it. Your only error was bringing it to Wintekeller. In summary, you first fix the problem by correcting the last technical link on the chain, and then if possible prevent future occurrence by tracing the human cause chain. By the way, there’s a name for the last technical link on the chain -- it’s called the root cause. Remember that. Another thing. Remember the ’chain of damages’ you described? That’s called ’consequential damage’.”

“Max, you’re a genius. I simply find the root cause, fix it, fix any consequential damage, and I’m done. If Wintekeller had spent any time trying to find the root cause, I’d have a perfectly running car today!”

“That’s true Heather, but let me leave you with this one question: How do you find the root cause? See you tomorrow Heather.”

“Bye Max.”

Terry’s Explanation

Heather hung up the phone and dialed Terry. “Hi Terry, thanks for fixing my air conditioner.”

“Da nada!”, laughed Terry.

“Terry, could you please tell me how you figured out the root cause of the problem?”

“Heather, you saw exactly how I figured it out. I turned off all the other breakers, saw what remained on, and unplugged it.”

“But how did you know to do that?”

“Do you want to become an air conditioning mechanic Heather?”

“No, but maybe a car mechanic.”, said Heather, for the first time voicing an ambition that had slowly crept into her life.

“I can see the relation with a car’s air conditioner, but what does your apartment’s air conditioner have to do with a car’s engine, brakes, or transmission?”

“Well”, said Heather, “doesn’t a car mechanic need to find the root cause to fix a car?”

“Yes”, Terry replied cautiously.

“That’s just it Terry, I don’t know how to find the root cause and you do.”

“But Heather, it’s different every time. It depends on the machine, on the surroundings, on the problem.”

“OK Terry, but you must have gone through some mental process to figure it out.”

“Not really. When an air conditioner repeatedly flips the circuit breaker, you always look for other stuff on the same wire.”

In spite of his protestations to the contrary, Terry was beginning to get interested in where Heather was going with this.

“OK, but what if you just can’t find any other stuff?”

“Then the problem’s in the air conditioner, and you call an AC tech.”

Silence filled the phone line as Heather tried to put that information in a place where it would fit. “In other words,” she said slowly, “you looked for other stuff on that circuit breaker to see whether the problem was in the air conditioner itself.”

“Exactly Heather! The breaker is flipping. Logically, the breaker can be flipped for only four reasons:

1. The air conditioner’s bad
2. Something else is connected to the air conditioner’s circuit breaker
3. The wiring in the wall is bad
4. The circuit breaker itself is bad

Terry continued. “By far the easiest thing to test for is other appliances connected to the same breaker. That’s a five minute test that doesn’t even require a screwdriver. So I did that. Otherwise, I’d have called in an air conditioning tech.”

“So what you did, Terry...” Heather paused to assemble her thoughts. “What you did is figure out all the alternatives, and then perform a test to choose between them.”

“I guess you could say that.” said Terry.

As she helped Seth with his homework, Heather went over it in her mind. Finding the root cause is a matter of choosing between alternatives using technical tests.

As Heather slept that night, she dreamed she was a mechanic fixing a really difficult problem in a car’s power steering. Recurring through the dream was a design. For some reason she knew that design was vitally important. Within the dream, Heather willed herself awake, went to her desk, and with the dream fading, sketched the design. She then went back to sleep.

Fixing Seth’s breakfast the next morning she saw her sketch. She wondered why she had thought it so important. It looked like this:



Heather folded it, put it in her pocket, and rushed to cycle Seth to school and then get to Max’s house. She was anxious to tell Max what she’d discovered.

What Max Had to Say

“Max, you find the root cause by choosing between alternatives, using tests.”, Heather gushed. She went on to tell the story of Terry’s finding the root cause last night.

“Very good Heather.”, replied Max. “But how do you find all the alternatives? If you count every nut and bold, a car has tens of thousands of parts. You can’t even count the alternatives, let alone choose between them!”

“Well Max, Terry didn’t deal with nuts and bolts. His alternatives were: The air conditioner, something connected to the air conditioner’s circuit, the wiring in the wall, or the circuit breaker.”

“Fair enough Heather, but let’s say the problem was in the air conditioner itself, and you had to fix it. What then?”

Heather pondered for a moment, then spoke slowly. “I guess I’d need to choose between alternatives in the air conditioner -- what -- the compressor or the electronics?”

“Well yeah, plus the outside unit, the pipes between them, and the freon itself.” Max replied.

Heather was on a roll. “And if it were in the outside unit, is the problem that the fan’s not spinning, or the heat transfer part doesn’t transfer, or...”

A broad smile of wonder spread across Heather’s face. “Oh my gosh, I understand!”.

“What do you understand, Heather?”

Heather brought last night’s drawing out of her pocket and gave it to Max. “I dreamed of this design last night. Does it mean anything to you?”

Now Max was the one to smile. “By George, I think she’s got it!”

They spent the next couple hours discussing repeatedly testing among alternatives. Max said that this process was called “divide and conquer” by some, and “half splitting” by others. Ideally, half-splitting rules out half the system as a container for the root cause, then ruling out half the remainder, then half that remainder, on and on until the root cause is found. In reality, the splits wouldn’t necessarily be exactly half.

The sun was red in the west -- it was time for Heather to go. As Heather walked out the door, Max asked one last question. “Heather, when do you start half splitting?”

Heather looked like he’d asked who was buried in Grant’s tomb. “The instant you begin the repair.”, she said incredulously.

“Not true”, smiled Max. “Think about it and we’ll talk tomorrow.”