The first year I was at Facebook we always knew what to build. Most of us were recent college graduates like our users, so building for ourselves was a reasonable proxy. That era ended quickly as the product expanded globally. Data became our best tool to understand the products we built and how people use them. But data can mislead us. Forewarned is forearmed, so I want to raise awareness of some of the pitfalls.


In the wake of the News Feed launch people were surprised we were willing to cannibalize our page view numbers. Which was funny because in all the myriad of discussions we had it never came up once; we just didn’t care about that metric. We wanted people engaging and posting more content and as it turns out, that’s exactly what happened. Activity doubled almost overnight and never regressed.

I wonder what product we would have built instead if page views had been our main metric.

Focalism is a cognitive bias that causes people to place too much importance on one element of a decision rather than considering all the data. People buying cars tend to focus on the odometer and year of the car rather than maintenance records or a mechanical inspection. People building products tend to focus on a small number of metrics when what makes a good or bad product may be more holistic.

These are hard problems but we need to make sure we don’t let one rogue metric dominate the discussion. Balancing many metrics that potentially compete seems like a pretty good way to avoid myopic optimization. Deltoid is one example of a tool designed to help unwind that complexity.

Hard to Measure Success, Easy to Measure Failure

In the beginning looking through a photo album required an entire page load for each new photo. We developed a new feature called photostream which allowed people to browse photos much more quickly without reloading the page. But as that meant the ad wouldn’t change it would negatively impact advertising revenue. It was a hard debate to resolve because the hit to page views and ad revenue was measurable while the benefit of the unlaunched feature was not. But we launched it because we had a strong sense that the value we were creating was greater than what we were losing.

There are a lot of situations where the cost of something is clear and easy to measure but the benefit is difficult to measure (or vice versa). In such situations, it can be pretty easy to let the data you do have outweigh the data that you don’t have, but that doesn’t mean it is actually the right course of action. Focalism is particularly difficult to avoid when there is little else to focus on.

Fortunately, this is one we actually have done pretty well on, at least at a strategic level. Consider our relentless focus on building user value (hard to measure) first and advertiser value (easier to measure) second. We believe the former subsumes the latter in the long run. I think having a leader who is willing to push teams beyond their local optima is crucial.

Pareto Efficiency

When our site is faster our users spend more time on it overall. But certain features can also cause our users to spend more time. How do we handle the case where an engaging feature trades off against site speed? If we insist that no metric ever decreases then we may be at a stalemate — the feature can not expand without slowing the site down and the site can’t speed up without limiting the feature.

When optimizing a global system with nonlinear interactions, some of the subsystems are likely in suboptimal states. Each individual product, taken on its own, might be better with purpose built controls. In contrast, the best thing for the user, and Facebook, may be to use standard controls that are visually consistent across all products and more performant.

In economics, a system of allocating resources that can not be changed without making the allocation worse for at least one individual is said to be Pareto Optimal. Sometimes we have to take leaps and accept that some things will get worse in order to find a better overall balance of value.

Complex Data

When developing Messenger we were lucky to have a user research team that identified the dramatically different modalities of how people interact with messaging products. Some people read everything and aggressively delete messages, others just skim subjects and use search to navigate. If you looked at data for average inbox size or number of read messages you would find numbers that accurately described neither group, and if you built towards that you would be building a solution that nobody likes.

It can be compelling to make data out to be simpler than it is. Normal curves are so easy to understand and widely applicable that we can easily assume all data fits such a model. The reality is that there are many cases where data is multimodal or distributions are heavily weighted. Even for data that appears to fit simple models, we can be deceived. Consider Simpson’s Paradox where a trend observed in two different sample groups is reversed when the groups are combined.

Vigilance is the only known solution to this issue.

Analysis Paralysis

Sometimes data just doesn’t give us the answers. While we must be cautious about letting our intuition misguide us, we must be willing to admit when it is the best we have. There is a point where the marginal value of information is below the value of immediate action. Being willing to launch quickly and iterate is crucial here because it is the basis for future data.

I think this affects us a lot more than we recognize at Facebook. I have personally been a part of countless discussions that debated the merit of a feature for longer than it would have taken to build. We get caught up on hard to agree upon questions like “how good is it?” as opposed to just building something and then asking “how can we make it better?” We need to remember that we settle arguments here with data one way or another, and if there is no existing data the best way to get some is to build something. Launching gives us an opportunity to be data driven.

At the same time, if you’re going to move ahead with no data you have to own what comes. You can’t be surprised later if there’s something you could have known with a little more due diligence.


Bias is a pretty obvious thing to be aware of when dealing with data but it bears mention. There is a really long list of these for those who are interested, but here are a few favorites:

Selection Bias - When we weigh what Facebook employees think of a product as representative of our users. Novelty Effect - The tendency for metrics to initially improve when new technology is deployed but not because of interest in the technology itself but rather the novelty of it. Priming - We go to great lengths to avoid this but it may still sneak in. If I hear a product is bad before I use it I’m likely to react more negatively. If I know a friend of mine worked on it I may be more likely to think it is good.


Make no mistake, the only way to make progress as an organization in the long term is to be data driven. I raise these pitfalls not to deter us but rather to ensure we have the humility to avoid falling into the trap of False Precision. We should be cautious about believing we understand things better than we do. All of our data, our analysis, and our understanding thereof are limited. Often, they are the best tool we have and we should use them but we shouldn’t delude ourselves in believing that we can quantify and understand everything that is going on. We should always make a best effort when it comes to drawing on data, but accept that sometimes there are no easy answers when charting new territories.

(first written in 2010, updated for 2020)