Critical mass describes the point at which the spread-rate of an innovation or idea “tips” from being in a state of equilibrium to a state of rapid, exponential growth.

In this definition there are three independent stages:

- Equilibrium
- Critical mass
- Exponential growth

In his eponymous book, Malcolm Gladwell refers to the moment of critical mass as the tipping point. He provides a passage which illustrates these three stages beautifully.

“The best way to understand the Tipping Point is to imagine a hypothetical outbreak of the flu. Suppose, for example, that one summer 1,000 tourists come to Manhattan from Canada carrying an untreatable strain of twenty-four-hour virus.”

### Equilibrium

“This strain of flu has a 2 percent infection rate, which is to say that one out of every 50 people who come into close contact with someone carrying it catches the bug himself. Let’s say that 50 is also exactly the number of people the average Manhattanite — in the course of riding the subways and mingling with colleagues at work — comes into contact with every day. What we have, then, is a disease in equilibrium. Those 1,000 Canadian tourists pass on the virus to 1,000 new people on the day they arrive. And the next day those 1,000 newly infected people pass on the virus to another 1,000 people, just as the original 1,000 tourists who started the epidemic are returning to health. With those getting sick and those getting well so perfectly in balance, the flu chugs along at a steady but unspectacular clip through the test of summer and fall.”

### Tipping point

“But then comes the Christmas season. The subways and buses get more crowded with tourists and shoppers, and instead of running into an even 50 people a day, the average Manhattanite now has close contact with, say, 55 people a day. All of a sudden, the equilibrium is disrupted.”

### Exponential growth

“The 1,000 flu carriers now run into 55,000 people a day and at a 2 percent infection rate, that translates into 1,100 cases the following day. Those 1,100, in turn, are now passing on their virus to 55,000 people as well, so that by day three there are 1,210 Manhattanites with the flu and by day four 1,331 and by the end of the week there are nearly 2,000, and so on up, in an exponential spiral until Manhattan has a full-blow flu epidemic on its hands by Christmas Day.”

### To sum up

“Tipping points are moments of great sensitivity. Changes made right at the Tippong Point can have enormous consequences. Our Canadian flu became an epidemic when the number of New Yorkers running into a flu carrier jumped from 50 to 55 a day. But if the same small change happened in the opposite direction, if the number had dropped from 50 to 45, that change would have pushed the number of victims down to 478 within a week, and within a few weeks more at that rate, the Canadian flu would have vanished from Manhattan entirely. Cutting the number exposed from 70 to 65, or 65 to 60 or 60 to 55 would not have been sufficient to end the epidemic. But a change right at the Tipping Point, from 50-45, would.”

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