# Viral Math: R-Naught and Zarrella’s Hierarchy of Contagiousness

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Over the past few years I’ve developed two models of contagious content: R0 (pronounced: R Naught) and Zarrella’s Hierarchy of Contagiousness. Because of the way these two systems work, I put a lot of emphasis on reach building activities (getting lots of followers, likes and views) and metrics (number of followers and likes). Based on the reaction I’ve seen to some of my recent work challenging the hegemony of “engaging in the conversation,” I’ve come to understand that I may not have fully explained why number of followers is such a key metric.

Before an individual can share a piece of your content, three things have to happen. First, they have to be exposed to it (following you on Twitter or a fan of your page on Facebook). Second, they have to actually become aware of that content (I follow something like 8,000 people on Twitter and don’t see anywhere near everything they Tweet). Third, and finally, they must be motivated by something in that content to share it. Together, these three elements make up my hierarchy of contagiousness.

The conversion rate at each part of the hierarchy can be expressed as a percentage, the number of people who will complete that part of the process. And the three rates taken together represent R0. Here, R0 stands for reproduction rate, a concept taken from the study of infectious diseases called epidemiology. It is the number of new cases of an infection that a single infection will cause. If I have a cold and I give that cold to two people and each of them give it to two more people, the R0 of that cold is two (or 200%). The R0 of most biological pathogens is above one, but I’ve never found an idea with a sustained R0 above one. Given a large enough population and a long enough time, the R0 of every idea falls below one and the idea stops spreading.

Below is a simple graphic explaining my viral math with my hierarchy of contagiousness, the accompanying formula and some example numbers alongside.

If you have 10,000 followers on Twitter and you Tweet to all of them, you are exposing 100% of them to your content. (The exposure rate is 100% for most platforms, but for some, like email, you can segment your audience and only send to a small percentage). Let us assume 1% of them actually read your Tweet (an educated guess at an awareness rate based on observed click through, ReTweet and reply rates). That means your Tweet has the chance to motivate 100 people to ReTweet it. Assuming it succeeds with 1% of those users (another educated guess), your content will get one ReTweet.

Obviously, these percentages are made up and vary wildly, but try plugging a few of your recent pieces of content in and do the math backwards to estimate your R0. If you have 20,000 followers and a Tweet of yours got 16 ReTweets, you have a 0.08% R0. You can then assume that for every 1,250 new followers you ad, you would have gotten one more ReTweet.

As marketers, we can optimize at each step of this process, by building reach (number of followers) for “exposure,” using tactics like contra-competitive timing and personalization for the “awareness” step, and learning about viral triggers for the last “motivation” stage. Awareness and motivation tend to be the trickiest to optimize for, but they’re certainly worth it. However, the easiest is reach. Get more followers or likes, and you’ll get more shares (and in turn, more followers and likes).

I’ve found that some users are more influential than others, and this is generally means they themselves have a larger reach. But there aren’t very many things you can do as a marketer to attract a huge number of highly followed influencers to your content beyond the same tactics that you would use to attract a huge number of “normal” users. And I’ve also found that some users (like lebolukewarm in this example) can have influence larger than their reach would suggest, but it’s impossible to target these one-off, contextually influential users. You can only increase your sheer follower numbers and thereby increase the likelihood that you have users like lebolukewarm following you.

The number of followers (or Facebook likes, or blog subscribers) you have is the best measure of your social media reach. It’s elegant and easy to compare to your competitors. There are other, more complex (and expensive) ways to measure reach, but they aren’t much more useful than follower count. Your goal with reach metrics is not to measure every single possible edge case, but rather to measure most of your reach and monitor trends in it. The question you need to answer is simple, are you building your reach, or losing it?

And don’t get fooled by fake, hypothetical straw men arguments. If someone tells you that they’d rather have 100 “highly engaged” followers than 10,000 less “engaged” followers, ignore them. In reality, if you have 10,000 followers you’ll have a decent number of “engaged” listeners. (Unless you’re doing something really shady to get those followers.) In the vast majority of real cases, someone with 10,000 followers has more reach and influence than someone with 100.