"One of the most famous examples of virality is YouTube. Before the site became the huge destination it is today, you would most likely find a YouTube video embedded on news site or personal blog. You'd watch it, and at the end, you'd be invited to email the video to people you know, and you'd also be given code to embed the video on your own site."

"true viral growth is incredibly rare. It took me a while to appreciate this: very few products have sustained a viral factor over 1 for any meaningful period of time."

"for a consumer internet product, a sustainable viral factor of 0.15 to 0.25 is good, 0.4 is great, and around 0.7 is outstanding."

"When the viral factor is less than one, it can interpreted as the amplification factor a = 1/(1-v). To calculate the total number of users, multiply the number of users acquired through non-viral channels by the amplification factor"

"And you can always feel product/market fit when it is happening. The customers are buying the product just as fast as you can make it — or usage is growing just as fast as you can add more servers. Money from customers is piling up in your company checking account."

"Ellis had found a leading indicator: just ask users “how would you feel if you could no longer use the product?” and measure the percent who answer “very disappointed.”"

"Ellis found that the magic number was 40%."

"We identified users who recently experienced the core of our product, following Ellis’ recommendation to focus on those who used the product at least twice in the last two weeks."

"We then emailed these users a link to a Typeform survey asking the following four questions:"

"If you instead use the ""very disappointed"" group of survey respondents as a lens to narrow the market, the data can speak for itself — and you may even uncover different markets where your product resonates very strongly."

"With this more segmented view of our data, the numbers shifted. By segmenting down to the very disappointed group that loved our product most, our product/market fit score jumped by 10%."

"I turned to Julie Supan’s high-expectation customer framework as a tool to do just that. Supan notes that the high-expectation customer (HXC) isn’t an all encompassing persona, but rather the most discerning person within your target demographic."

"you can either build something a large number of people want a small amount, or something a small number of people want a large amount. Choose the latter. Not all ideas of that type are good startup ideas, but nearly all good startup ideas are of that type."

"After throwing the responses into a word cloud, some common themes emerged"

"Politely disregard those who would not be disappointed without your product. They are so far from loving you that they are essentially a lost cause."

"From analyzing our third survey question, we knew that happy Superhuman users enjoyed speed as their main benefit, so we used this as a filter for the somewhat disappointed group"

"If you only double down on what users love, your product/market fit score won’t increase. If you only address what holds users back, your competition will likely overtake you."

"we got to work, starting with the lowest hanging fruit of low cost, high impact work so we could deliver improvements immediately."

"To increase your product/market fit score, spend half your time doubling down on what users already love and the other half on addressing what’s holding others back."

"We were careful to ensure that we didn’t survey users more than once, so as to not throw off the 40% benchmark."

"the product/market fit score is something that we’re going to continue to track. I think it's always useful for startups to look at this metric, because as you grow you'll encounter different kinds of users."

"Investors advising early-stage teams should avoid pushing for growth ahead of product/market fit. As an industry, we all know that this ends in disaster, yet the pressure for premature growth is still all too common. Startups need time and space to find their fit and launch the right way."

"Early adopters are more forgiving, and will enjoy your product's primary benefit despite its inevitable shortcomings. But as you push beyond this group, users become much more demanding, requiring feature parity with their current products. Your product/market fit score may well drop as a result."

"For founders, achieving product/market fit is an obsession from day one."

"when users spontaneously tell other people to use your product"

"what if you could measure product/market fit?"

"just ask users “how would you feel if you could no longer use the product?” and measure the percent who answer “very disappointed.”"

"731 Slack users in a 2015 open research project. 51% of these users responded that they would be very disappointed without Slack,"

"2. What type of people do you think would most benefit from Superhuman?"

"1. How would you feel if you could no longer use Superhuman?"

"3. What is the main benefit you receive from Superhuman?"

"4. How can we improve Superhuman for you?"

"FROM BENCHMARK TO ENGINE: THE FOUR-STEP MANUAL FOR OPTIMIZING PRODUCT/MARKET FIT"

"1) Segment to find your supporters and paint a picture of your high-expectation customers."

"To start, we grouped the survey responses by their answer to the first question (“How would you feel if you could no longer use Superhuman?”):"

"I turned to Julie Supan’s high-expectation customer framework as a tool to do just that."

"the high-expectation customer (HXC) isn’t an all encompassing persona, but rather the most discerning person within your target demographic."

"We took only users who would be very disappointed without our product and analyzed their responses to the second question in our survey: “What type of people do you think would most benefit from Superhuman?”"

"It’s a commonly held view that tailoring the product too narrowly to a smaller target market means that growth will hit a ceiling — but I don’t think that’s the case."

"you can either build something a large number of people want a small amount, or something a small number of people want a large amount. Choose the latter."

"2) Analyze feedback to convert on-the-fence users into fanatics."

"we decided to politely pass over the feedback from users who would not be disappointed if they could no longer use the product."

"With a clear understanding of our main benefit and the missing features, all we had to do was funnel these insights back into how we were building Superhuman."

"Implementing this segmented feedback would help the somewhat disappointed users get off the fence and move into the territory of enthusiastic advocates."

"3) Build your roadmap by doubling down on what users love and addressing what holds others back."

"For the first half of the roadmap, doubling down on what people love, we had to intuit the impact."

"4) Repeat the process and make the product/market fit score the most important metric."

"If your business has strong network effects (think Uber or Airbnb), then the core benefit will keep getting better as you grow."

"Investors advising early-stage teams should avoid pushing for growth ahead of product/market fit."

"And when you finally hit the product/market fit score you’re targeting, my advice is to push the pedal all the way down and grow as fast as you can."