# Using machine learning to rank search results (part 1)

A large catalog of products can be daunting for users. Providing a very fine grained filtering of search results can be counter-productive: it leads them from information overload to lack of choice.

On e-commerce sites, this results in poor conversion—users leaving the site without checking out.

The key is obviously to provide relevance and choice, which is much more complicated than it sounds, as different users may have very different tastes.

This describes how I explored a machine learning, neural networks based solution to relevance ranking.

Disclaimer: if you’re taking this seriously, take scientific advice. I’ve been a scientist in another life, but what follows is really an engineering, get-it-done approach to the problem.

Let’s start with some background on the website I’m building this for.

HouseTrip is a holiday rental site; our products are flats and houses that guests can rent for a period of time, in many (mostly European) cities. Our users (guests) come from many different countries (again, mostly European), and can be business travellers, couples, groups of friends, or families.

Let’s assume I’m planning a trip to Paris with my wife and kid for the Autumn half-term holiday. When I land on the site, I’m asked to enter a destination, dates, and the size of my party, all of which I diligently enter.

The search results page (SRP) shows me a nice map and a number of properties. It also suggests I refine my search by prompting me to open a filtering panel:

Wow. Now, this is powerful. But 29 filters? And a total of 1,170 available properties? I’m not quite a millenial, but I am lazy nonetheless, and I feel bored already. I’d really like this thing to just offer me a home I’d like.

After all, in the modern Web, I’m tracked and profiled everywhere, so I’m entitled to a customised, tailored experience, right? Right?

Unfortunately… it’s not that simple. The website does know a few things about me: the place I want to go to, when, and the fact I’ll be travelling with two other people. It might also know I’m using it in English language, from a Swedish IP address (Ok, corner case here, I’m behind a VPN), and possibly that I’ve visited before.

But that’s really it.

It does not know that the other people are my wife and kid (therefore, I need just 2 bedrooms, not 3); or that I really need my Wifi fix; or whether I’m price sensitive.

How can the app make an educated guess about the properties I would like with so little information? Well, it does know about other people who have booked in the past, some of which may be similar to me in some way—and it could leverage that knowledge to tailor results for my benefit (and, well, the company’s).

##### A simplistic ranking engine

A first baby step towards making search results relevant to me is to try making them more relevant to visitors in general.

The very first “ranking engine” at HouseTrip was very simple. We would show products higher in the list if they’ve been booked more often—that is, sort by descending number of past purchases.

After all, the feedback from our users was that those products “work”: they were purchased, so they’re certainly relevant. Two main issues arose over time:

• sellers (hosts) would game the system by listing multiple product (properties) as a single entry, to boost their ranking;
• new sellers and new products wouldn’t stand a fighting chance to ever sell;
• and of course, this in no way guaranteed that users would see results that mattered to them.

The next step up, which we introduced in early 2012, was to rank results according to a heuristic: our measure (as experts of the domain) of what a “good” product was, in general, for users.

The general idea is to combine quantitative information about a given property in a weighted linear formula, the output being the ranking score.

We determined a set of quantitative, measurable attributes of a property, which we proved to correlate to the likelihood of it getting booked. We name $$p$$ the vector of normalized attributes:

This includes, for instance, the number of photos the host provided; the length of its description; or the average time its host took to confirm a booking.

We then assigned each a weight, and defined the score as the weighted average:

The weights were initially defined very informally, as a combination of what we thought was important, and how well a particular attribute correlated with the odds of purchase (as measured in historical data).

Importantly, we’d remove from the formula any attribute that could not be measured (for instance, the time-to-confirm for new hosts), which is equivalent to assuming a newly listed product is “average” for those attributes. This ensured fairness.

On introduction, this new ranking mechanism gave us a 10-15% boost in conversion (measured through split testing). Implicitly, this means that users were getting more relevant results on average.

In the following 18 months, we iterated on the secret recipe, adding and removing attributes and fiddling with the weights to gain further improvements—still in a very unscientific manner, but it “worked”.

The optimisation we performed can actually be considered a manual (and error-prone) implementation of a gradient descent in the space of possible weights.

We initially thought we’d try to generalise this an implement an on-line genetic algorithm to learn the weights automatically and adapt to changes in user behaviour… which sounds exciting, but has a fundamental issue:

We’d still not be taking the specific user searching for a product into account.

##### Formulating the ranking problem

Let’s take a step back and formulate the problem we’re trying to solve.

We want to show users relevant products first. “Relevant” in this context means “likely to engage the user”. In e-commerce lingo, engagement means the user will spend their hard-earned cash on your product.

So this is really a sorting problem. This reduces to a comparison problem: if you can order 2 products, there’s a number of well-known algorithms that can sort an arbitrary-length list of products.

So what we’re aiming for is a black box that looks like this:

where $$u$$ is a vector of normalised attributes of the user, and $$p_k$$ the attributes of the two compared properties. The operator $$\succ_u$$, informally means “more relevant than, for user $$u$$”; defining it clearly will be part of the challenge.

The good news is that we don’t have to be very successful in designing $$\phi$$: we just need to do better than a random sort order, and better than the linear score $$S$$ does.

In other words, our target is to beat this function (which ignores $$u$$):

that is, finding a $$\phi$$ that is more “accurate” than $$\phi_0$$ (we will define accuracy in detail later on).

The less-good news, however, is that whatever the black box $$\phi$$ ends up being, it will have a number of data-related challenges to face:

• there is little known information about the user. In our context, we only know about their chosen locale, the destination they searched for, the dates at which they want to travel, and the size of their party. We don’t know, for instance, about their gender or age group, which we know (from market research) has an important influence on their product preferences.
• some of the information we have is continuous (e.g. the price of the property); some is discontinuous or discrete (e.g. the party size, number of photos), or even non-numerical (the locale, trip dates).
• the data has inconsistencies: 4 people travelling could be two couples, a group of friends, or a family with two children, and have very different expectations.
• the data is noisy: the behaviour of individual humans is not very predictable and has a hefty dose of randomness.
• there are known non-linear relationships between inputs ($$u$$, $$p_k$$) and the value of $$\phi$$: for instance, we know for a fact that price sensitivity in some destination is inverted for some locales, or some periods in the year.
##### Obtaining data to model relevance

Coming up with a good model for $$\phi$$ requires three things:

• a “ground truth” dataset, i.e. a list of example instances of $$u,p_1,p_2$$ where the value of $$\phi$$ is known, and the space of possible values for $$[u,p_1,p_2]$$ is well covered;
• an optimization algorithm that generates a candidate based on “training” data;
• a metric to determine how “good” a candidate model is.

Finding what to learn on, i.e. our dataset, is actually tricky. In our case, we’re lucky enough to have harvested raw user behavioural information (using KissMetrics’s anonymised data dumps, and imported into KMDB). This contains one entry per user and per key page of our transaction funnel (per page, if you will).

Which point in the user journey should we learn on? Checkouts (bookings) sounds like an obvious place to find “positive” training events: if $$u$$ has booked $$p$$, we’d want $$\phi(u,q,p)$$ to be 1 for most values of $$q$$. But we wouldn’t be able to select good examples for $$q$$, because those would be the answer to “which are the properties the user chose not to book?” Sure, they probably viewed the listing page for other properties, but we’d need to be reasonably sure they chose not to book because of the property itself, not for some other reason (e.g. they had made their choice already, and were just window shopping).

Ultimately, we chose our focal point as the point of enquiry (this is where users ask the host to confirm availability, which is a preliminary step towards booking). Think of it as a shortlist: a user browses several properties and opts to enquire on a handful. Informally, we believe this to be a good point because the “positive” and “negative” events are as meaningful to the user.

We can now define our relevance operator more clearly:

• $$p_1 \succ_u p_2$$ if $$u$$ enquired about $$p_1$$ but not $$p_2$$, and vice-versa;
• $$p_1 =_u p_2$$ if $$u$$ enquired about both $$p_1$$ and $$p_2$$, or neither.

Our dataset is therefore:

• for all users $$u$$ having enquired on a given day,
• for all properties in $$P=\{p_1,\ldots p_n\}$$ that were enquired on by $$u$$ that day, or visited in the 6 hours before the last enquiry,
• the set of tuples $$[u,p_i,p_j,\phi]$$, where $$p_i,p_j$$ covers all combinations of an enquired and non-enquired property in $$P$$, and $$\phi$$ is 1 if $$p_j$$ was enquired but not $$p_i$$, and -1 in the opposite case.

Phew, that’s quite a mouthful. Let’s make this a bit more visual. Out dataset is going to be a long list of this kind of data:

user info left property info right property info $$\phi$$
0.12 0.34 0.56 0.70 0.81 0.82 0.93 0.52 0.43 0.39 -1
0.40 0.27 0.70 0.90 0.07 0.06 0.06 0.95 0.76 0.38 1
0.13 0.51 0.80 0.95 0.73 0.52 0.32 0.47 0.47 0.61 -1

(not the real data)

A number of tricks are involved:

• attributes are normalized from 0 to 1, with an attempt to “spread” the possible values as much as possible. For data that’s distributed in a bell curve, that typically meant scaling so that the 5th percentile is 0 and the 95th is 1. As we’ll see in part 2, transforming the data (often using a logarithmic scale instead of linear) is a form of kernel trick that helps machine learning algorithm by linearising the input.
• category information (e.g. locale) is converted into numerical information. We currently serve in 6 languages, which means instead of a “locale” column in the dataset, we’ll have 6 columns, and for each user exactly one will have the value 1, and the others 0.
• replacing missing inputs with the median value for other entries. This only works well when the distribution is reasonably well known, and the relative number of missing inputs is low (experimentally, over 10% seems to start hindering learning).

We import all this into Redis, using a custom library called pythia (bonus points if you get the clunky allusion). Hopefully we’ll clean it up enough to open source it.

At this point we feel like we have a solid set of data: roughly 1,500,000 entries per month, or just under 40 million entries points total. Let’s run a little sanity check: is this enough data to find a good model of $$\phi$$?

Let’s get an order of magnitude of how big the space problem is. For instance, each of the locale dimensions has 2 possible values, so the resolution is 2. We’ve observed it’s very rare to have parties of more than 8 people, so let’s say the resolution of that dimension is 8. A property can appear to have low, average, or many photos (compared to its number of bedrooms), so let’s say the dimension is 3. The price wan be significantly below, a bit below, a bit above, or significantly above the local market, so let’s call that 4.

dimension resolution
locale 2^6
party size 7
trip length 5

photos (p1) 3
price (p1) 4

photos (p2) 3
price (p2) 4

If you multiply the apparent resolutions of all dimensions for our problem, this tells you the number of “cells” in the n-dimensional grid of the attribute space: in our case, that’s roughly $$10^{14}$$.

This means we’re looking at one data point every $$10^{14} / 40\cdot 10^6 ≈ 2.5\cdot 10^6$$ cell, also known as “not a lot”. Plus the data’s not just sparse, it’s likely to not uniformly cover the problem space either.

Practically this doesn’t necessarily mean we won’t be able to model, but we shouldn’t expect miracles.

We should, however, start putting this to the test.

##### Artificial neural networks

I’ve done my best to be algorithm agnostic so far, but the title and summary gave away the approach anyway, so no surprises here: we’re going to use artificial neural networks (ANNs) to fit a model to our data.

If you’re never heard of them, an Artificial neural network (ANN) is a function which maps a tuple of numbers to another (i.e. in $$\mathbb{R}^n\to\mathbb{R}^p$$), can model arbitrarily complex data sets with arbitrary precision, and has known techniques to fit the model to the data. It’s also an example of biomimetism as it takes inspiration from the workings of the brain.

This deck of slides from the University of Kaiserslautern is a pretty decent intro to ANNs. It’s conveniently free if you don’t have the books. And also, it’s from a place that’s pretty close to Ramstein Airfield, which I found funny because of the other Rammstein (which is indeed named after the place!).

If you’re a developer or engineer, you’ll probably also want to have read one of these two books, although I’ll do my best to stay legible:

If your background is web applications and e-commerce, the green one is probably an easier start. If you’re an engineer with a strong CS background, go for the red one. Both are excellent, as is (mostly) the norm with O’Reilly.

While ANNs are not most state-of-the-art modeling method out there, it’s still pretty popular, and importantly has been around for a while, which means:

• even non-experts will have some familiarity with the concept, unless they drank their way through college;
• there are good libraries floating around; we’ll be using FANN, which has good Ruby bindings.

Now that’s out of the way and you’re familiar with ANNs, we’re specifically going to adapt (or take inspiration from) Tizano Papini’s SortNet approach to learning-to-pairwise-rank with neural networks.

Remember the naive, linearly-weighted ranking method I mentioned a few pages above? It can be re-imagined as a trivial ANN, with no hidden layers, and where the output node has a linear activation function:

For comparison, the net we want to train will look like the following, with a number of inputs for the user and the pair of properties we want to compare:

You’ve probably noted there are two outputs, but we only want one value (+1 or -1). The reason we’ll train this way is that experimentally, it seems ANNs used for classification work better with one node per category.

We’ll simply map between the two by training with:

and conversely, when apply a trained network to a control example:

In English, we expect our net to have a high $$o_1$$ when the property $$p_1$$ should rank higher, and vice-versa.

Training the ANN consists in presenting training examples where the “truth” is known (i.e. the values of the outputs are known), and running an optimization algorithm to determine the weights of the connection between functions. Typical algorithms fall in the backpropagation category.

Very fortunately, we don’t really need to go much deeper, as the FANN library will do all the heavy lifting for us.

##### Evaluating performance

To keep experimentation realistic, we’ll use 1 month of data for training, and the following 2 weeks for control. This isn’t exactly by the book (you’d normally take one set, and randomly pick training and control examples), but the point of all this is to have predictive power, i.e. use past observations to predict future behaviour.

The last piece of the puzzle is for us to measure how well (or poorly) our trained networks perform.

Ours is a classification problem: for a given input $$[u,p_1,p_2]$$, we classify “negative” entries where $$p_1\succ_u p_2$$, and “positive” those where $$p_1\prec_u p_2$$. The traditional way to evaluate performance of a classifier is to produce a confusion matrix; and to get a single figure for performance, reporting on accuracy (the proportion of “correct” predictions).

To get a sense of whether this would work at all, we train a network with our 28 inputs (10 for the user, 9+9 for the properties) using the cascade training algorithm and a target of 28 hidden neurons (completely random guessing that last figure).

Training, as mentioned above, is done on 1 month of user behaviour data. We filter out outliers, e.g. users making way too many enquiries (more than 8), users making too few enquiries (1 or 2), keeping only users who have viewed at least as many non-enquired properties as they’ve enquired (ie. offer as many positive as negative events), and so on. We then generate all possible pair of properties for each user, and the desired output ($$[1,0]$$ or $$[0,1]$$). We also make sure there are exactly as many positive than negative samples (otherwise things tend to fail badly).

We generate a control set in the same fashion, on the 1 month of data following our training set.

After training, and after running our confusion matrix script on the control set, we obtain:

 true positive 31.9% true negative 23.8% false positive 26.2% false negative 18.2%

which means our accuracy on the very first attempt is 55.7%: our predictor would be accurate slightly over half the time; which means that a property a user would enquire about would be ranked above a property they wouldn’t slightly over half the time.

That doesn’t sound too fantastic, but for a machine learning engine on human data, it’s actually quite good! Remember we only need to beat random sorting and our original, simpler ranking.

Running the original, linear scoring-based ranking ($$\phi_0$$) on our control data yields a surprising result: its accuracy is 44.5%, which means it performs worse than random (we hadn’t tweaked it in a while, and usage patterns do evolve).

So even without further optimisation, our neural network would be a winner!

A handful of performance facts (on a Core i7 2.5GHz machine running OS X):

• importing 2 months of data from various databases in to Redis takes roughly 30 minutes, and consumes 700MB per month of data.
• exporting datasets to train or control on takes ~5 minutes per month.
• training the 28-input ANN using the cascade algorithm takes about 2 hours (note that this algorithm also learns how many nodes are needed, and what their response function should be; not just the vertex weights).
• the 28-node ANN I mentioned above can make about 120,000 predictions per second, which means in the worst case (used as a comparator in a $$O(n^2)$$ sorting algorithm) it could sort a set of 350 properties in a second. That’s not going to cut it: a modern search engine should not take more than 500ms to provide results, and ranking is only part of the problem.
##### Beyond the proof-of-concept

At this point we’ve proven the approach could be viable, although significant effort is still required.

Eventually this will need to be automated, relatively unsupervised, and fast. When using ANNs, part of the difficulty (some would say magic) is to pick:

• the correct inputs (and transformations thereof)
• the number of hidden layers
• the number of nodes
• their activation functions (sigmoid, linear, gaussian?)
• the duration of training (number of “epochs”)
• the training error function.

Our target is to achieve 60%+ accuracy, and be able to sort even large sets of properties (2000+) in a few tens of milliseconds.

In part 2 (coming soon), we’ll explore several of this points: expect lots of graphs and data!

I hope this article will inspire some engineers to read on, and possibly apply advanced techniques to building apps that work even better for consumers!

Occam’s razor, really: explore the simplest solution first. I suppose SVM could be another candidate technique (although it’s not good at modelling non-linear relationships). After all, this is an engineering exercise: delivering a solution with reasonable effort and performance. Long story short, I knew from experience that ANNs could possibly help here, and ruby-fann was available.