We have come to the end of the week, but before we finish, we have one more measurement to look at. The characteristics of a virus mean that we need to consider time and population when we define a “user”.

Prevalence

(Missed the first six lessons? Start here.)

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Yesterday we learned about incidence, which is the number of new cases. People that weren’t users before.

That tells us how fast the virus is spreading and how effective it is.

Prevalence is the measurement of how many people are infected, right now, or at some time in the past.

That tells use how much exposure there is. How much power the virus has to survive.

Some viral things get popular over night, and the following week they are gone as fast as they came in.

Some viral things grow slowly, but the bigger they get, the more unstoppable they become. 

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Normally in UX, we will measure how many people become registered users, or paying users, which is called conversion.

We also measure how many people stop being users, which is called “churn”.

And we often measure “active users” which is the number of people who use our stuff every day or month.

Prevalence is sort of like all of those ideas in one number. And of course, you want it to be as high as possible, because that means more people are using your shit.

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How often do users use your design?

In the third lesson of this series, we learned that the more people that can be infected by each user, the faster the virus will spread. That was called K-Factor.

Prevalence will increase when your K-Factor increases, and vice versa, because it creates more simultaneous users, regardless of other factors.

If people can use your design really often, like email, or if your app naturally shares content with lots of people each time, like Twitter, then your K-Factor will be much higher.

A design that you only use once, or only once a year, like AirBnB, will spread much more slowly.

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Duration also changes everything.

Another major thing to consider is: how long does it take to use your viral thing?

If it’s just a photo or an article, probably not very long. If it’s Facebook, you might be there for 8 hours at a time, and you might be an active user for years.

Things that are fast to use (they have a short Viral Cycle, remember?) spread much faster than things that take time to use or learn.

BUT…

Things that a user can use for a long time have much more value. They last and stay popular over time. They have higher prevalence numbers.

You will never get millions of people to look at a photo for hours, so the prevalence numbers will be lower (and incidence rate will be higher).

But you might get millions of people to look at Facebook for hours. But more commitment means more work to get new users (lower incidence rate, higher prevalence).

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For a more technical explanation of Prevalence, go to Wikipedia.

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And that’s it! Great work this week!

This week you have learned several major factors in what makes a product or content travel quickly through the internet.

Virality is not a simple topic. There are many moving parts. If you picked up any new ideas or strategies this week, you are ahead of the curve.

When you’re working on your next project, especially if it’s something social, make sure you create a short viral cycle, make it easy to transmit to lots of people every time it’s used, make it easy for people to personalize it, connect it with behavior that already feels good, and measure the incidence and prevalence to analyze your success.

Good luck!

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Questions? Ask me on Twitter.