On Accuracy and Precision

A joint post from Matt and Ben

Believe it or not, we’ve been getting inspired by MP3′s lately, and not by turning on music in the office. Instead, we drew a little bit of inspiration from the way MP3 encoding works. From wikipedia:

“The compression works by reducing accuracy of certain parts of sound that are considered to be beyond the auditory resolution ability of most people. This method is commonly referred to as perceptual coding. It uses psychoacoustic models to discard or reduce precision of components less audible to human hearing, and then records the remaining information in an efficient manner.”

Very similarly, in online advertising there are signals that go “beyond the resolution of advertisers to action”. Rather than tackling the problem of clickstream analysis in the standard way, we’ve employed an MP3-like philosophy to storage. Instead of storing absolutely everything and counting it, we’ve employed a probabilistic, streaming approach to measurement. This lets us give clients real-time measurements of how many users and impressions a campaign has seen at excruciating levels of detail. The downside is that our reports tends to include numbers like “301M unique users last month” as opposed to “301,123,098 unique users last month”, but we believe that the benefits of this approach far outweigh the cost of limiting precision.

Give a little, get a lot

The precision of our approach does not depend on the size of the thing we’re counting. When we set our precision to +/-1%, we can tell the difference between 1000 and 990 as easily as we can tell the difference between 30 billion and 29.7 billion users. For example when we count the numbers of users a campaign reached in Wernersville, PA (Matt’s hometown) we can guarantee that we saw 1000 +/- 10 unique cookies, as well as saying the campaign reached 1 Billion +/- 10M unique cookies overall.

Our storage size is fixed once we choose our level of precision. This means that we can accurately predict the amount of storage needed and our system has no problem coping with increases in data volume and scales preposterously well. Just to reiterate, it takes exactly as much space to count the number of users you reach in Wernersville as it does to count the total number of users you reach in North America. Contrast this with sampling, where to maintain a fixed precision when capturing long-tail features (things that don’t show up a lot relative to the rest of the data-set, like Wernersville) you need to drastically increase the size of your storage.

The benefits of not having unexpected storage spikes, and scaling well are pretty obvious – fewer technical limits, fewer surprises, and lower costs for us, which directly translates to better value for our users and a more reliable product. A little bit of precision seems like a fair trade here.

The technique we chose supports set-operations. This lets us ask questions like, “how many unique users did I see from small towns in Pennsylvania today” and get an answer instantaneously by composing multiple data structures. Traditionally, the answers to questions like this have to be pre-computed, leaving you waiting for a long job to run every time you ask a question you haven’t prepared for. Fortunately, we can do these computations nearly instantaneously, so you can focus on digging into your data. You can try that small-town PA query again, but this time including Newton, MA (Ben’s hometown), and not worry that no one has prepared an answer.

Unfortunately, not all of these operations are subject to the same “nice” error bounds. However, we’ve put the time in to detect these errors, and make sure that the functionality our clients see degrades gracefully. And since our precision is tunable, we can always dial the precision up as necessary.

Getting insight from data

Combined with our awesome streaming architecture this allows us to stop thinking about storage infrastructure as the limiting factor in analytics, similar to the way MP3 compression allows you to fit more and more music on your phone or MP3-player. When you throw the ability to have ad-hoc queries execute nearly instantly into the mix, we have no regrets about getting a little bit lossy. We’ve already had our fair share of internal revelations, and enabled clients to have quite a few of their own, just because it’s now just so easy to work with our data.

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