Coronavirus: The Swiss Cheese Strategy

How Any Country Can Learn to Dance and Stop the Coronavirus

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In this article, you’ll learn:

  • How the US and the EU failed to control the virus, and how comparable countries succeeded.
  • How you can make sense of all the necessary measures with one simple idea.
  • Why the West’s testing and contact tracing is largely useless — and what they can do about it.
  • The questions that journalists and the People must ask politicians to keep them accountable.
  • How you can stop the virus in your own community, without the need of your government.

And much more! Alright, here we go.

You’d imagine eight months into the pandemic most countries would know exactly what to do to stop the coronavirus. And yet here we are.

You’ve probably seen charts like this one by now. The detail few add is the green line at the bottom, with Asia-Pacific countries. Let’s zoom in.

This is a myth-busting graph. Japan, Taiwan, China, Hong Kong, Singapore, South Korea, New Zealand, Australia, Mongolia, Thailand or Vietnam all followed different variants of the Hammer (heavy lockdowns when there’s an uncontrolled outbreak and they don’t know what to do) and the Dance (a series of intelligent measures to keep infections low), yet all have been successful.

This list includes all types of countries: democratic, authoritarian, continental, islandic, freedom-loving, Anglo-Saxon, developing, developed… They prove any country can succeed. And they’re not the only ones: From the Caribbean to Uruguay, Canada’s Atlantic Provinces or several African countries, many countries controlled the epidemic.

Meanwhile, most Western countries didn’t pay attention, suffered massive outbreaks, applied heavy Hammers to stop them, but never learned how to dance. When the summer recess ended, they were not prepared for the back to school season and its new wave of cases. As the winter progresses, it will only get worse.

This is what has happened in Europe over the last nine months. Source: Tomas Pueyo commentary and annotations on map from innouveau

Today, the Western world has another chance. Joe Biden is the new president of the United States, and will want to show a different approach to the virus. In Europe, multiple countries have woken up from their midsummer night’s dream and started and restricting movement again, including Ireland, UK, France, Belgium, Spain, Italy, Germany, and even Sweden.

For the countries that want to control the virus: How should they dance?

How to Dance

To keep the coronavirus at bay in your community, you have four layers of defence:

  1. Stop as many infections from coming in as possible
  2. For those that make it in, minimize the people they meet
  3. When they do meet people, minimize the likelihood that they will infect somebody else
  4. When they do infect somebody else, identify that infection quickly and neutralize it

None of these defences is perfect. But, together, they can stop enough cases to reduce the transmission rate R to below 1 and keep the virus out.

Let’s look at each one of them.

1. The Fence: Keep Infections out

It’s common sense that if at the border you don’t stop people coming from infested areas, some will bring the virus in and cause outbreaks.

I call Fences the measures to stop the virus at the border. There are three types:

  • Walls: Ban travelers from infected areas.
  • Quarantines: Allow people to come in but isolate them for 4 to 14 days.
  • Checkpoints: Test people at the border.

Countries can apply each Fence with different travelers: the higher the infection likelihood, the stronger the Fence.

Walls are very expensive, so they should be used sparingly. Checkpoints can be quite cheap, but won’t catch all cases. Quarantines are probably the sweet spot: a test, a filled form telling the authorities where the traveler will be, enforcement to make sure the quarantine is respected, and a big fine for those who don’t. With rapid tests on the day of arrival and after four days of quarantine, you will likely stop most of the infections.

As cases go down, countries — or states — with very few infections can open up to each other, creating “green bubbles” with no fences.

Some people look at studies about border closures and say: “They don’t work! They only delay the inevitable!” That’s because these studies always look at these measures in absence of other measures. Of course they’re not sufficient! Fences alone can not stop the epidemic. They are only one layer of defence. Somebody will eventually pass through and seed an outbreak.

No Fence, no defence.

What these people miss is that they might not be sufficient, but they’re necessary: They don’t work standalone, but without using them, it’s impossible to stop the virus. Tell me a single country that has been able to keep the number of cases low without a strong Fence. They don’t exist. Eventually, they get overrun. That’s why Japan, Taiwan, China, Hong Kong, South Korea, New Zealand and Australia all have strong Fences.

Conversely, that’s one of the big failures of the European Union during the summer. After a very hard spring, with heavy lockdowns across the continent, European countries let their guards down and opened their borders to each other, trading seeds. Now we know that a majority of EU’s current cases came from Spain. Something similar happened between US states.

No Fence, no defence.

2. Social Distancing: Abstinence from Meeting Others

Some infected people will make it into your community. You can’t avoid it.

When they do, you want to prevent them from meeting other people. The more people remain in their social bubbles, the more the infections will be prevented from traveling and infecting people from other bubbles.

Measures that promote social distancing go from the least aggressive, such as limits to the size of crowds, all the way to total lockdowns: closures of the economy that require every family to remain in their bubble.

Other measures include curfews, limits to the number of people that businesses allow at the same time, staggered operation hours, bans of certain types of gatherings, business closures…

Total lockdowns are destructive. They ask people to stay home and close all the economy but for essential businesses. That’s what I called the Hammer back in March, when I exhorted governments to apply it to stop the virus.

That made sense back then, when the true extent of the epidemic was orders of magnitude higher than we knew because we didn’t test enough, when our healthcare systems were collapsing, when we had no idea how to dance to tackle the virus. The Hammer would stop the growth of the epidemic and give governments a few weeks to get their act together.

That’s not the case today. If a country is hit by a first wave, applies a Hammer, then has seven months to prepare to Dance, yet has a second wave, and then wants to apply a second Hammer… What makes citizens believe the government will learn better this time around? Why would they believe there won’t be a third wave, a third Hammer, and then a fourth? Why wouldn’t they believe the government is irresponsibly playing yoyo between lockdowns and outbreaks?

Lockdowns can be justified, but only for a few weeks, to get the new wave under control, and only when a country has a good plan to learn how to dance: the faster the decline, the shorter and less harmful the lockdown, and the quicker people can resume a coronavirus-free life. In fact, this is exactly how the WHO says we should use them. Otherwise, they should be avoided as much as possible.

Luckily, we’ve learned enough to know the types of social gatherings that must be avoided and those that are ok.

For example, shopping with masks, social distancing and hygiene seem to be mostly safe. So shops in general should not close. In fact, we’ve learned something key about the virus to determine what gatherings to avoid.

It turns out most events and most infections don’t matter. Many different studies have proven that 10% to 20% of cases account for 80% to 90% of transmissions. By eliminating superspreader events, you can dramatically reduce the epidemic.

This article from Science Magazine is great at visualizing how the virus spreads in clusters and how backward tracing works to bust clusters

So what should countries do?

  • Ban gatherings above a certain number. The fewer the number of active coronavirus cases, the bigger the crowds can be allowed.
  • Target places that are likely to cause super-spreader outbreaks, such as prisons, elderly care homes, rehab centers, universities, or food packing plants. Help them prevent outbreaks with much tighter measures.
  • And when there’s a lot of community spread, close bars, clubs and restaurants, and even private parties.
  • Avoid lockdowns. And if you have to close establishments, schools should be the last ones to go, especially childcare.

The economy is driven by people meeting. When people can’t meet, they consume less and the economy suffers. So limiting social gatherings is the most expensive layer of defence. It needs to be avoided as much as possible. But that can only be done if the other layers of defence are strong.

3. “Contrafection”: Reduce Contagiousness When People Meet

The next layer of defence is to reduce contagiousness when people do meet.

There’s no word for “reducing contagiousness”, so I’m going to use “contrafection”. The same way as contraception is contra-conception (reducing conception when people have sex), contrafection is contra-infection (reduction of infection when people meet).

The virus spreads in very specific conditions: When a lot of people gather in a closed room for a long time while they talk, sing, or even breathe. The Japanese call that the 3 Cs: closed spaces, crowded places, and close-contact settings. My version of it:

Avoid crowded, confined, close, clamorous communication.

To methodically reduce contagiousness, we need to understand how the virus behaves and break down the steps that the virus follows to spread.

What usually happens is that an infected person coughs, speaks, or even breathes particles out that form a cloud, which lingers in the air with the virus inside. When the air doesn’t circulate, and under the right conditions, the virus survives hanging in the air until other people catch these particles through their eyes, nose or mouth.

Source: El Pais — A room, a bar and a classroom: how the coronavirus is spread through the air. This article has great visualizations. Go read it.

So to stop the virus:

  1. Avoid talking when in the presence of people from another social bubble. Definitely avoid singing or shouting.
  2. Spend as little time as possible with people from another social bubble.
  3. Prevent the virus from leaving the mouth and nose.
  4. If it does, circulate the air to prevent the particles from reaching other people.
  5. Create a hostile environment for the virus, so it dies even if it hangs in the air.
  6. Make sure it doesn’t reach somebody else’s mouths, noses and eyes.

The first point is achieved by avoiding talking or shouting when in crowds or in a room with other people. People laugh at Japan’s ban on screaming in theme parks; haters gonna hate. The country, meanwhile, is enjoying theme parks.

The second point is achieved simply by spending less time together. When a meeting can take 10 minutes instead of an hour, shorten it. Short interactions are mostly safe. It’s the hours-long interactions with many other people that must be avoided.

The third is achieved with masks. At this point, everybody should be wearing them in public, especially indoors (outdoors when few people are close by and they don’t stay close to each other for long, it is not as important).

The devil is in the detail, as usual. N95 masks are ideal. Surgical masks are great. Hand-made masks with a couple of layers of different materials also work. But single-material ones, like a bandana, don’t. It’s not enough to wear masks. Wear the right ones. This comes from the New York Times article: Masks Work. Really. We’ll Show You How

The fourth point means we need to spend all the time we can with other people outdoors. The virus barely spreads there.

The coronavirus barely spreads outdoors.

That means nearly all activity outdoors should be allowed — except maybe things like concerts, sports audiences, or other big crowds. It also means that, if any business is able to move its activity outdoors, it should be able to open. Similarly, schools should strive to take classes outdoors.

If it’s impossible to carry out something outdoors and it must happen indoors, there’s another solution: Ventilate as much as possible. This is something most people forget, but it’s such an easy measure. It’s cheap to open the windows and use a fan to circulate air. And if you can install a good ventilation system, that’s great.


The fifth point is something most people never really discussed. The virus has a hard time spreading with the right temperature and humidity.

The virus dies at high temperatures. The higher, the better. 25 degrees Celsius (~75 fahrenheit) is good. 30 (~85 fahrenheit) is better. Humidity between 50% and 65% is ideal to kill the virus too

It’s as if the virus had evolved to survive in cold, humid environments.

Like a bat cave.

Aside from virus survival, our noses are able to filter viruses better with higher humidity and temperature. So if a social gathering must happen indoors, aside from having great ventilation, people should increase the temperature as much as they can and adjust the humidity to between 50% and 65%.

Finally, there are very few cases of infections through touching objects and surfaces (called “fomites”). Hygiene is one of the measures people have taken seriously. It’s good to keep washing hands and disinfecting them.

The sixth and final step is that people should also wear masks to protect themselves. Most people understand they can catch the virus through their nose and mouth. But most forget they can also catch it through the eyes.

It might be a good idea to wear glasses or goggles to protect them. In a Chinese city where 32% of the population wears glasses, only 6% of coronavirus patients wore them. Infections have been reported by doctors without eye protection. The eyes have receptors for the coronavirus. And eye symptoms are common in coronavirus patients. All this evidence suggests protecting the eyes could reduce contagiousness. For those who can afford eye protection, it’s a measure that can help and can’t hurt much.

Preventing social interaction with social distancing is expensive but quite easy. Conversely, reducing contagiousness is the opposite: cheap but hard. Governments should focus on doing this right.

The fourth layer of defence is the same: cheap but hard. Governments must learn to do it too.

4. Test — Trace — Isolate: Catch and Neutralize Infections

We’ve talked about preventing infections from coming into a community, avoiding meetings when they do, and avoiding contagions when there’s meetings. The last layer of defence is to identify infections when they do happen and neutralize them.

That’s what testing, contact tracing, isolations and quarantines do (in short, test — trace — isolate).

What seems like a pretty easy thing to do has been completely botched by most Western governments — and journalists have not asked the right questions to keep politicians accountable.

Everybody obsesses over testing. And that’s good. It’s important. It’s necessary. Positivity (share of tests that turn out positive) needs to be below 3–5%, and tests need to come back quickly, ideally in 24 hours.

But it’s not sufficient. Testing tells you who is infected, so you can isolate them. But you also need to quarantine their contacts. And you need to make sure isolations and quarantines are actually respected.

Unfortunately, most governments have not even hired enough contact tracers. A good rule of thumb for the number of necessary contact tracers is to two to five times the number of daily cases. A country with 5,000 daily cases should have between 10,000 and 25,000 contact tracers as an order of magnitude.

That number is not the most important one. You shouldn’t be measured by the working bodies you have, but by what they produce. You want these contact tracers to be efficient, and the best rule of thumb for that is to ask what percentage of contacts of infected people end up in quarantine. Every government should report that. That number should be at the very least 60%, ideally above 80% (Juneau et al. 2020. Disclaimer: I’m part of the “et al” here).

Infected people have an estimated number of contacts. What share of those is provably quarantined?

Contact tracers talk about rules like 90–90–90–90: reach 90% of infected people, gather 90% of their contacts, reach 90% of those contacts, and quarantine 90% of them. All of that, obviously within two days or less, otherwise the virus will have too much time to spread.

Unfortunately, in some places, contact tracers have a hard time achieving that for two reasons. First, people obstruct them. That should be penalized. It isn’t. Second, those who have to endure isolations or quarantines suffer from it and don’t get compensated. Countries that have done it well pay for a hotel, give food, money and medicine, and reach out frequently to check in.

Thankfully, it is not necessary to trace all contacts. Japan has been very successful in part because of a tactic called “cluster-busting”. Most people don’t infect many others. Instead, just focus on clusters. When you catch a case, figure out where it got infected (that process called “backwards tracing”), and when you find a cluster, chase everybody there. If the infection was not in a cluster, move on.

This might be a bit harder without contact tracing apps. These have been proven to not work very well, but they might have a very useful role in cluster-busting.

The issue with contact-tracing apps is that most people won’t download them unless they’re forced to. You need around 80% of a population to have such an app to achieve control, but countries have managed to get 40% penetration at most (in Iceland. Canada is 15%. Singapore 25%. Germany 22%. France 2.7%)

However, what most people have missed is that you don’t need 80% penetration in an entire country. You need 80% in a given community. For example, events that could become clusters.

Photo: Joe Allison, via Anchorage Daily News

If the only requirement to go to the gym, to work, to a bar, to a bus, family dinner… was to scan a QR code, people would do it. When scanning a QR code, an app can record who was where when. If it’s mandatory for the event — which is easy to enforce at the entrance — everybody can easily be tracked and if one person is infected, automatically everybody else is identified. Why Western countries don’t use this system as mandatory to enter all types of social events is beyond me.

But that’s not even the biggest failure of Western governments. The worst one is the lack of support and enforcement of isolations and quarantines.

Testing and contact tracing are the intelligence to know who needs to be isolated or quarantined. The intelligence without the action is worthless.

Countries like Taiwan, Vietnam or South Korea track your phone or a device to make sure you stay home, and give you fines of tens of thousands of dollars and jail time. They use hotels and government facilities to isolate people, and give them money, food and medicine to compensate them. Western democracies decided that this was too hard. So they don’t enforce them!

What happens next? In Norway, two in three people who were supposed to quarantine broke it. In the US, visitors somewhat respect quarantines in only one of 50 states: Hawaii, the only one that enforces them.

No enforcement or support means no quarantines or isolations. Imagine the stupidity of lockdowns in countries that don’t have a good test-trace-isolate program. It’s literally like saying:

It’s too hard to isolate just a few people, so we’ll isolate them all.”

Governments such as those of Spain and France had no problem enforcing lockdowns with millions of fines. How come we never hear about fines for isolations and quarantines? It’s because they don’t exist.

It’s time for governments to start enforcing them. It can be as easy as asking people to send a selfie from home within ten minutes of a random prompt, or sending the police to do random checks. Or as sophisticated as just copying what already works in other countries. They also need to offer food, money and medicine to those in quarantine, to keep them home. For the infected that can’t stay home, hotel rooms should be available for free. The government can get them at a pretty good rate right now. I believe there’s low occupancy.

All of that is the bread-and-butter of test-trace-isolate. But over the last few months, a new tool has emerged to change the game: early detection.

Many countries have had a very hard time with testing and contact tracing, the intelligence part. What if there were cheap ways to quickly identify most of the people who could be infected?

One is Sewage Testing. It turns out you can easily find the coronavirus in the sewage system, and track it back to the source home. Some university campuses have been using that successfully. It’s pretty inexpensive, and it can tell you days in advance whether a new infection or cluster is brewing.

Another one is trained dogs, which might have 100% accuracy within 10 seconds. That means sniffing many more people, much faster, for much cheaper.

Finally, there are the rapid tests: Tests that cost less than $5, can tell how contagious people are within a few minutes, and can be manufactured in the tens of millions.

Slovakia has realized how much of a game-changer this is and is testing all its population this way. With these tools, tens of millions of people can be tested every day. That means the intelligence part of test-trace-isolate can be done extremely quickly and even with little reliance on contact tracing. But most governments don’t realize it.

If everybody was free to go anywhere, but had to take a rapid test and scan a QR code before entering any community, we could immediately know everybody that is infected and isolate these people, leaving everybody else mind their business.

It’s like making test-trace-isolate a super efficient 4th layer of defence.

The Swiss Cheese Strategy

These are the four layers to stop the spread of the virus: Fences, Social Distancing, Contrafection, and Test-Trace-Isolate.

None of them is perfect. All have holes that let infections pass. But together they form an impenetrable defence. You can look at it like layers of Swiss Cheese:

Source: Tomas Pueyo, based on the Swiss Cheese model for safety incidents by James T. Reason. The concept has been circulating over the internet, with people like Ian Mackay tweeting about it. Others include this one from Jennifer Kwan, this one from Siouxsie Wiles and Toby Morris, the State of Guernsey, and many more.

An infection might be able to pass one layer, or even two. But if there are several, the odds that the infection goes through every layer undetected becomes minuscule.

Imagine, for example, that a country has a Fence that catches 80% of infections, no Social Distancing, Reduced Contagiousness that eliminates 95% of infections, and a test-trace-isolate that neutralizes 50% of infections. Together, these layers catch 99.5% of cases. If the transmission rate R is 3 (the number of people infected by a source), it will be reduced to 0.015! Every infected person only infects an additional 0.015 people, killing the epidemic within a few weeks.

Each one of these four main layers is made up of smaller layers. Each one of them is crucial: Their effect is multiplicative. Each time you add one of them, you slash infections. The stronger they are, the more they slash them.

And once you have defence layers, you can play with them: Pick the ones that work best for your community and that are cheapest.

Cheeses of the World

Every country’s strategy can be summarized in a combination of these defence layers.

For example, China used every layer early on. They closed borders, ordered lockdowns, mandated masks, and used a test-trace-isolate program that included electronic tracking of their citizens and mandatory, forced isolations and quarantines. Unsurprisingly, they beat it.

South Korea and Taiwan didn’t need all of that. They applied strong Fences, only very local and short-lived business closures for Social Distancing, masks for Contrafection, and world-class Test-Trace-Isolate programs. Put in another way, they relied very heavily on layers 1 and 4, and a bit on 3, so that they barely had to use layer 2, the most expensive one.

New Zealand and Australia were very heavy on layers 1 and 2. Because they’re sparsely-populated islands, they can keep most infections out with a Fence, and they took care of local infections with lockdowns. Even the state of Victoria within Australia had a Fence and a lockdown to control it. That reduced the need to perform really well in layers 3 and 4.

All these countries have near zero prevalence, which makes it much easier to control the virus and open up the country at the same time. If there’s an outbreak, it can be identified and neutralized quickly. That’s how life is nearly normal in Taiwan, South Korea, New Zealand or Australia. Trying to do the same with a low but ongoing level of cases is much harder.

It’s doable though. Japan has fluctuated for months between 0 and 10 active cases per 100,000 people. How have they done it? They’re very strong in Layer 1 with an aggressive Fence, Layer 3 with very widespread mask wearing and the 3 Cs, and Layer 4 with their cluster-busting approach. It’s doable.

The one layer you want to avoid is the most expensive one: Layer 2, social distancing. That’s some expensive holed cheese.

The economy moves when people meet. Stop that, and you stop the economy. The more communities develop layers 1, 3 and 4, the less they need to rely on the expensive layer 2.

Unfortunately, Western countries never learned to use these cheaper layers effectively, so they keep going back to the one they know, layer 2. And they keep destroying their economies instead of learning how to Dance with the other layers.

What Should You and Your Country Do?

Fractals: Your Community Can Do It

So far, I’ve talked about countries because they tend to have the authority to apply all four layers. But they’re not the only ones. You can do it too.

You can apply the same philosophy to any community: regions, cities, university campuses, elderly care homes, meat-packing plants, prisons, businesses, homes…

Let’s take university campuses for example. What’s the equivalent of Layer 1, a Fence? They can test all students on arrival and four days later, keeping them secluded in their rooms in the interim, bringing them food and water every day — and Netflix! Students can’t leave the campus perimeter. If they do, they need to quarantine again. Workers and visitors get tested every day when they enter, and can’t stay overnight.

For Layer 2, Social Distancing, they would reduce the size of the biggest classes and form “bubbles”, groups of students that hang out together and don’t hang out with anybody else. Parties should be banned.

For Layer 3, Contrafection, high-quality masks should be provided by the school and wearing them should be mandatory. All classes that can be taken outdoors should. Those indoors require goggles and all windows open. If the temperature doesn’t allow it, ventilation should be upgraded, and classrooms should be made hot and humid.

For Layer 4, Test — Trace — Isolate, the more people can do rapid tests every day, the better. Schools should buy and distribute rapid tests for daily consumption. Otherwise, they should use sewage testing.

Students and educators should have a contact tracing app. All gatherings should scan a QR code made for that event. A positive test should trigger an immediate isolation, which should be tracked with the app. Violation of the isolation should be treated harshly. All contacts of a positive case should be automatically notified thanks to the QR code, and people should self-quarantine. It should all be enforced through the app.

Applying this mindset is especially important to communities and events more likely to suffer superspreader events. If all prisons, elderly care homes, food packing plants, schools, universities, offices and big private events followed these guidelines, they could stop the virus in their community.

That, in turn, would contribute to stop it more broadly, since most cases are driven by superspreader events.

No need to wait for the government to do everything. We can all do our share.

Now that we know what we need to dance, the question becomes: Do countries have a will to do it?

Dancing Countries

If they do, the path forward is very clear: If they have an uncontrolled outbreak, apply a lockdown (layer 2) while they quickly ramp up their other layers:

Layer 1: Fences

  • They should announce a Fence that includes testing on arrival and soon after, with a quarantine in between.

Layer 2: Social Distancing

  • Mandate a maximum size of people gatherings (e.g., 30 like in Sweden). Adjust it depending on the local prevalence.
  • Try to avoid other types of social distancing.

Layer 3: Contrafection

  • Mandatory masks when close to other people or in the same room with others. Masks are not necessary in the outdoors if you only have your family close by.
  • Goggles highly recommended indoors.
  • Allow most gatherings below a certain threshold of people — if they’re outdoors and with a minimum distance between people.
  • Mandate that indoors gatherings must have enhanced ventilation, with HEPA filters, higher temperature, and the right humidity.

Layer 4: Test — Trace — Isolate

  • Get enough testing for a positivity below 3–5% for PCR.
  • Announce the strategy for early detection.
  • Get around 2 contract tracers per daily case.
  • Require the publication of contact tracing efficiency: What share of infected people’s contacts are quarantined within two days?
  • Give resources to support people during isolations and quarantines with money, food, water, medicine and entertainment.
  • Make isolations and quarantines mandatory, tracked, and enforced.
  • Fine heavily those who break isolations and quarantines. Communicate these fines

This should work for most countries. But what if countries don’t want to dance? Or if they can’t? Argentina, for example, has been under a lockdown for seven months and still hasn’t controlled the virus. Others, like the US, never really tried. What are their alternatives?

Should they try to mitigate the epidemic without completely suppressing the virus? Or just let the virus run its course? What about deaths? Chronic conditions? Treatments? Vaccines? How long are each one of these going to take? How does that affect different countries’ strategies?

If you’re interested in knowing the answer to this question, sign up to get the next installment and let me know.

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Italian (another)

If you want to translate this article into other languages, do it on a Medium post and leave me a private note here with your link. I’ll add it here. Or reach out on Twitter.

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Thank you Yaneer Bar-Yam, Mick Costigan, Torsten Cordes, Carl Juneau, Barthold Albrecht, Mike Mitzel, and Christina Mueller for your help while I was writing this article.

2 MSc in Engineering. Stanford MBA. Ex-Consultant. Creator of applications with >20M users. Currently leading a billion-dollar business @ Course Hero

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