Why Drucker said “what everyone knows is usually wrong”

Questioning all assertions can be a valuable tool in making business and management decisions

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With so much information being dispensed about Covid-19 confronting us around the world, it is inevitable that many predictions given by tired, frustrated experts who are doing dangerous but lifesaving work daily, sometimes are more negative than need be. Yet there is reason to believe that whatever everyone seems to know, especially under stress, is frequently incorrect.

That was one particular remark that I clearly remember Peter Drucker*, considered to be the father of modern management, making again and again in the classroom. Yet, only after I wrote and explained this in several of my books about Drucker did it appear anywhere and I have never seen it among his published work. This was the seeming contractor statement that “what everyone knows is usually wrong”.

Drucker was right

Maybe through repetition I finally began to think more deeply about what his words really meant. This strange statement is not only true, it can be immensely valuable to remember in every business and management decision, and many other areas of human endeavor. It means that it is wise to always question all assertions no matter where they originate or how many others agree. This is true despite a majority of people seeming to “know” a “fact” to be true and that can be safely assumed without analysis or questioning. So-called “common knowledge” should always be suspect. Evidence must be sought and examined closely because in a surprisingly high percentage of cases, the information given, especially under stress, will turn out to be inaccurate, true only under certain conditions, true only to a limited extent, or it may have been true in the past, before a new factor that made it incorrect was added.

This can lead us to overlook valuable innovations and to reject some ideas without investigation. I now consider this simple statement critically important among Drucker’s work and accurate in just about everything, such as management, politics, economics and our great fears in the Covid-19 pandemic and what we face in this environment. Many times, if we ignore these “facts that everyone knows” we can introduce remarkable innovations which seem impossible at first, but change everything. Through this mindset we learned to fly and eventually venture into space, though such impossibilities may have started with beliefs alone.

Is what everyone knows true or false?

Some “facts” once thought by everyone to be true we laugh at today. “The world is flat” or “the earth is the center of the universe” are typical examples. Doubt some of these things in past centuries and you could be sent to prison or even burned as a witch. The Ancient Greeks believed that everything was made up of only four elements: earth, air, fire and water. I do not think that you got imprisoned for believing otherwise, but you were at the very least considered ignorant.

In modern times we learned that many of these views were mistaken. When I took chemistry in high school, I learned that a periodic table of elements had been formulated by Russian chemist and inventor Dmitri Mendeleev, and that it had been established that there were exactly 93 elements which were arranged by atomic mass. You got an “A” if you could name them all. Had we proposed that there could be more, I am certain that we would have been immediately corrected by our teachers. In the words of Richard Rodgers and Oscar Hammerstein in their song in Oklahoma, “things have gone about as far as they could go”. Today, there are 118 elements – or so “everybody knows”. I am uncertain since the numbers are changing: when I checked this in 2008, there were 102. And they forgot to tell us in high school that Mendeleev had documented only 63 elements, the other 30 had not been discovered in his time.

We see a lot of Arthur Conan Doyle’s famous detective Sherlock Holmes on TV and in movies. Everyone knows Sherlock’s most famous utterance was a sentence consisting of only four words, “elementary, my dear Watson”. The detective would respond with these words to Dr. Watson’s surprise at an unexpected deduction made by Holmes. Maybe everyone knows this, but everyone is wrong.

Holmes did not utter these immortal words in a single instance in anything ever written by Doyle, either in the four published novels or in the 56 short stories about the adventures of Sherlock Holmes. Where did people ever come up with something so universally believed, yet incorrect? It came from English actor Basil Rathbone who played the part of Sherlock Holmes in Hollywood movies in the 1940s that responded with the famous sentence. These words fit the character of Holmes perfectly and although they do not emanate from Doyle’s creation, they became a known fact about Holmes that everyone knew because of the movies.

Analysis before belief

Many years ago, I was involved in the selection of one of two designs for a new aircraft proposed by two different companies for the US Air Force. The companies were The Boeing Aircraft Company and McDonnell-Douglas Aircraft Company. Those who know the aerospace industry also know that the former company eventually acquired the latter, but this has nothing to do with my story. Both proposed modifying one of their standard airline designs, already in production and in use.

Periodically we would meet with each aircraft company’s design team to access progress on each company’s proposals.

On one occasion, one representative said: “You can save $10m for each aircraft produced if you would allow us to deviate on the size of the escape hatch by two inches. That would be the standard size of the hatch for current airliners. They successfully passed all FAA tests.” I promised to look into his request and it could save lot of money, which is something the US Air Force was always interested in.

Tracking down the origin

The source of this requirement was the engineer who had put this requirement into the proposal package listing aircraft design specifications. Where did he get it and what was his source for the requirement?

I contacted the engineer responsible for the proposed specification, only to be ignored. “We cannot do it,” he told me. “This requirement comes directly from our aircraft design handbook with specifications that we must follow for all new transport aircraft.”

This meant that my source had another source. This other source was the design handbook. Not only did it require standard requirements, but “everybody knew” that because of its reliability, these dimensions were the correct ones for the escape hatch that we were required to use.

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Was the source valid?

Both reliability and validity are concepts that come from testing. The validity of a test tells us how well the test measures what it is supposed to measure. It is a judgment based on evidence about the appropriateness of inferences drawn from test scores. However, we are not looking at test scores here, we are looking for sources. So where did this particular specification in the aircraft design handbook come from? Knowing that source could help me decide whether this specification was valid for the aircraft we now wanted to build, and we still had not located the original source for this information.

I knew that every specification in the aircraft design handbook was referenced as to where it came from and what it was based on, which was usually original testing. I asked the engineer to do the necessary research to find out on what tests this design specification was based on. Surprise, surprise – this specification was based on an aircraft test done with propeller-driven aircraft almost 30 years earlier, which traveled at about 120 miles (193.1km) per hour. The one we were developing traveled at about 500 miles (804.7km) per hour and in this instance, the design specification was not valid. We gave it to one of our aeronautical designers who advised us to forget what everyone knew in this case – the two inches at the air speeds we were anticipating for an emergency escape would make little difference. We took his recommendation and saved millions of dollars without compromising safety.

A lot of what we learned and are learning about Covid-19 is true, including the effectiveness of “social separation”. This virus is very dangerous and we would be foolish not to pull out all the stops in combating it. However, we are fighting now using modern technology and innovative medical, political and economic thinking, and this is being done sometimes at great sacrifice and risk by the best brainpower available. We will ultimately be successful, but we have yet to determine the length of time that social separation must be maintained. Forever is a highly unlikely answer and not immensely helpful. The longer the better is an assumptive answer whose only guarantee is to ensure the economy will lie dormant. All this is being tested now, but Drucker’s warning has not changed: what everyone knows is usually wrong.

Be safe and healthy and also live long and prosper.

*Syndicated and adapted from

A Class with Drucker by William A. Cohen (AMACOM, 2008)

Drucker’s Way to the Top by William A. Cohen (LID, 2019)