Introduction
Preference elicitation [Goldsmith and Junker2009] is the task of interactively inferring preferences of users and it is a key component of personalized recommendation and decision support systems. The typical approach consists of asking the user to rank alternative solutions [Chajewska, Koller, and Parr2000, Boutilier et al.2006, Guo and Sanner2010, Viappiani and Boutilier2010] and use the resulting feedback to learn a (possibly approximately) consistent user utility function. These algorithms rely on a fixed pool of solutions from which to choose both candidates for feedback and final recommendations. However, when thinking of an interaction between a user and a salesman, one imagines a more active role by the user, who could suggest modifications to candidates. For instance, in a trip planning application, when commenting a candidate trip to New York, the user may reply: “I’d rather visit the MoMA than Central Park”. This is especially true when considering fully constructive scenarios [Teso, Passerini, and Viappiani2016], where the task is synthesizing entirely novel objects, like the furniture arrangement of an apartment or a novel recipe for vegan tiramisù. Coactive Learning [Shivaswamy and Joachims2012] is a recent interactive learning paradigm which allows the user to provide corrected versions of the candidates she is presented with.
While Coactive Learning approaches adapt the preference model based on userprovided option improvements, the set of features that the utility is defined by is assumed given and fixed. This is not always a realistic assumption. When faced with a complex decision, users may not be fully aware of their own quality criteria, especially in large, unfamiliar decision domains [Chen and Pu2012, Pu and Faltings2000]. Even more so in constructive settings, where the option catalogue is exponentially (possibly infinitely) large and generated onthefly. Crucially, the user may become aware of novel preference criteria, in a contextspecific fashion, while exploring the decision domain [Payne, Bettman, and Johnson1993, Slovic1995].
One way to tackle this problem is to enumerate all potential user criteria in advance, by combining a fixed set of features with one or more operators (e.g. multiplication or logical conjunction). This solution however has drawbacks. First, the number of feature combinations suffers from combinatorial explosion, making learning harder and more computationally demanding. Most importantly, entirely novel and unanticipated user criteria can not be added to the feature space.
Example critiquing (or conversational) recommendation systems [Tou et al.1982, McGinty and Reilly2011, Chen and Pu2012] provide an alternative solution. In this setting, preferences are stated in term of critiques to suggested configurations. Upon receiving one or more proposals, the user is free to reply with statements such as “this trip is too expensive” or “I dislike crowded places”. Critiques are integrated into the learner as auxiliary constraints or penalties [Faltings et al.2004]. Options presented at later iterations are chosen based on the collected feedback, focusing the search on more promising items. Example critiquing is explicitly designed to address the above difficulty: by being confronted with a set of concrete items, the user has a chance to realize that she cares about features that she was previously unaware of [Chen and Pu2012]. Unfortunately, typical conversational systems do not support numerical modelling of user preferences (e.g. weights), and often assume noiseless critiquing feedback.
In this paper we present a new algorithm, Coactive Critiquing (cc), that unifies coactive learning and example critiquing, harnessing the strengths of both strategies. Coactive Critiquing builds on the coactive learning framework by further allowing critique feedback. We view critiques as arbitrarily articulated explanations for the userprovided improvements, e.g. the user may explain her reason for suggesting the MoMA over Central Park by stating: “I prefer indoor activities during winter”. In this work, we assume that there is an interface between the algorithm and the user which translates the user’s critiques into (soft) constraints^{1}^{1}1For instance, it could be a simple form that allows the user to combine attribute values to form critiques. We are currently working on automated approaches based on NLP and rule mining.. Newly acquired constraints are included into the learning problem as additional features. We extend the regret bounds of shivaswamy2015coactive shivaswamy2015coactive to the more general case of growing feature spaces. Our empirical findings highlight the promise of Coactive Critiquing in a synthetic and a realistic preference elicitation problem, highlighting its ability in offering a reasonable tradeoff between the quality of the recommendations and the cognitive effort expected from the user.
In the next section we position our work within the related literature. In the Method section we motivate, detail and analyze our proposed method. We describe our empirical findings in the Empirical Evaluation section, and conclude with some final remarks.
Related Work
There is a large body of work on preference elicitation [Goldsmith and Junker2009]. Due to space restrictions, we focus on the techniques that are most closely related to our approach.
Coactive Learning (cl) is an interaction model for learning user preferences from observable behavior [Shivaswamy and Joachims2012], recently employed in learning to rank and online structured prediction tasks [Shivaswamy and Joachims2015, Sokolov, Riezler, and Cohen2015]
. For an overview of the method, see the next section. The underlying weight learning procedure can range from a simple perceptron
[Rosenblatt1958] to more specialized online learners [Shivaswamy and Joachims2015]. Further extensions include support for approximate inference [Goetschalckx, Fern, and Tadepalli2014] and multitask learning [Goetschalckx, Fern, and Tadepalli2015]. These extensions are orthogonal to our main contribution, and may prove useful when used in tandem. However, in this paper, we only consider the original formulation, for simplicity. Our approach inherits several perks from cl, including a theoretical characterization of the average regret [Shivaswamy and Joachims2015] and native support for constructive tasks. The main difference between the two methods, which is also our main contribution, is that in cc the feature space grows dynamically through critiquing interaction. cl instead works with a static feature space, and is therefore incapable of handling users with varying preference criteria.The concept of critiquing interaction originated in interactive recommender and decision support systems [Chen and Pu2012, McGinty and Reilly2011, Tou et al.1982]. Critiquing systems invite the user to critique the suggested configurations, thus supporting the exploration and understanding of the decision domain. Collected critiques play the role of constraints (or penalties) in filtering the available options, allowing the search to focus on the more promising candidates. Our approach is most closely related to userinitiated critiquing protocols, where at each iteration the user articulates one or more critiques [Chen and Pu2012]. In cc
critiques are elicited at specific iterations only, selected by a heuristic balancing cognitive cost and expressivity of the acquired feature space (as discussed in the Methods section). Few critiquing recommenders model the user preferences numerically. In contrast,
cc associates weights to both basic and acquired features (i.e. critiques). One exception is the method of zhang2006comparative zhang2006comparative, which employs a learned linear utility model. The user chooses an option from a pool ofhighest utility options. In this context, critiques are simple textual descriptions of the advantages of each suggestion over the reference option. The estimated utility is updated through a multiplicative update based on the user’s pick.
cc instead uses the (userinitiated) critiques to improve the expressivity of the feature space. Other critiquing recommenders that include an adaptive component are concerned with developing effective query selection strategies, e.g. [Viappiani, Pu, and Faltings2007] and [Viappiani and Boutilier2009].Method
We first introduce some notation. We indicate column vectors
in bold and vector concatenation as . The usual dot product is denoted and the Euclidean norm as. Later on we will compute dot products between vectors of different lengths. In this case, the shorter vector is implicitly zero padded to the bottom to match the length of the longer one.
Coactive Learning.
We consider a preference learning setting with coactive feedback; is the set of feasible item configurations ; these are represented by an dimensional feature vector . We assume that the feature vector length is bounded, for some constant . The attractiveness, or subjective quality, of a configuration is measured by its utility, which we assume [Keeney and Raiffa1976] to be expressible as a linear function of the features . Here encodes the true, unobserved user preferences. We write to indicate a maximal utility configuration. The goal of the system is to suggest high utility configurations without direct access to . A common strategy is to iteratively improve an estimate of the true preferences through interaction with the user, while keeping the user’s cognitive cost at a minimum.
We follow the Coactive Learning [Shivaswamy and Joachims2015] paradigm, which we describe briefly^{2}^{2}2We only consider a “contextless” version of Coactive Learning, which is sufficient for our purposes; our method can be trivially extended to support contexts. See shivaswamy2015coactive shivaswamy2015coactive for further details.. In Coactive Learning, the learner maintains an estimate of the user preferences. At each iteration , the algorithm computes a most preferable configuration , by maximizing the current estimate of the utility . The configuration is then presented to the user, who is tasked with providing an improved configuration , e.g. by direct manipulation of . The two options and provide an implicit ranking constraint . The latter is employed to update the preference estimate, in the simplest case with a perceptron update^{3}^{3}3Other update strategies can be applied, see for instance [Shivaswamy and Joachims2015]; we will stick with the classical perceptron with unit step size for simplicity.:
In the remainder we assume the user to be informative [Shivaswamy and Joachims2015]: if the configuration is not optimal, the user can always produce an improvement with higher true utility (modulo mistakes). Formally, informativity implies that there exists a constant such that, for all , it holds that:
Improvement errors are absorbed by the (possibly negative) slack term . Under this assumption, the average regret incurred by Coactive Learning after iterations, defined as:
is bounded from above as follows.
Theorem 1 (shivaswamy2015coactive shivaswamy2015coactive).
For an informative user with true preference vector and bounded length feature vectors , the average regret incurred by Coactive Learning after iterations is upper bounded by
As a consequence, so long as the user is not too noisy, the slacks will be small enough, and the bound guarantees that the average regret will shrink accordingly. While similar bounds have been proposed for more general users [Shivaswamy and Joachims2015], here we restrict ourselves to informative users for simplicity. In our presentation we do not impose any restriction on the type of features used. We note in passing, however, that the choice of feature type can heavily impact the complexity of the inference step. There are however ways to make Coactive Learning work with approximate inference procedures [Goetschalckx, Fern, and Tadepalli2014].
Coactive Critiquing.
Coactive Learning presupposes the user and the learner to have unlimited access to the complete feature function at all times. This assumption is often unrealistic. It is well known that users may discover their own quality criteria while exploring the option catalogue [Payne, Bettman, and Johnson1993]; further, critique queries can be employed to stimulate the users to discover their own criteria [Faltings et al.2004]. We amend to this deficiency by augmenting Coactive Learning with support for example critiquing interaction.
At a high level, Coactive Critiquing works as shown in Algorithm 1. The algorithm maintains estimates of both the user preferences and feature function . The initial set of features is supposedly taken from a reasonable default set, provided by a domain expert, by the user herself (e.g. through a questionnaire), or other sources [Chen and Pu2012]. At each iteration , the algorithm performs an improvement query, as in Coactive Learning (lines 4, 5), but can additionally submit a critique query to the user. Critiques are only queried when specific conditions are met (line 7), as described in the next subsection.
Given the proposed and improved configurations, and respectively, a query critique (line 8) amounts to asking the user why she thinks the improved configuration is preferable to the suggested one. Ideally, the user would respond with a critique that maximally explains the utility difference between the two configurations. This interaction protocol is based on a modest “local rationality” assumption: when presented with two distinct configurations , the user can state at least one critique that contributes a significant utility difference between the configurations. The user is free to reply with suboptimal critiques, according to her current awareness and the required cognitive effort. We will discuss the impact of suboptimal critiques in our theoretical analysis.
The feedback of the critique query consists of a single, arbitrary critique constraint . We interpret the latter as a feature function that captures whether (or how much) the constraint is satisfied. In principle, all kinds of features are acceptable, including indicators and numerical degrees of satisfaction. For instance, the critique “I prefer indoor activities during winter” would equate to a feature that indicates the conjunction of the season being winter and whether the trip includes one or more indoor activities. The feature is appended to the current feature vector ; the weight vector is padded accordingly by appending a zero element (lines 9 and 10). The learner traverses increasingly more expressive feature spaces , , as critiques are collected. The perceptron update remains the same as in coactive learning (line 12). The algorithm terminates after a fixed number of iterations , or when the user is satisfied (e.g. when the regret of the current suggestion is small enough).
When to ask for critiques.
Critique queries are key in improving the expressiveness of the feature space. Critiques are only elicited at the iterations selected by the NeedCritique procedure (line 7). The design of this procedure is crucial. On one hand, if the procedure is too lazy, not enough critiques are elicited, impairing the representation ability of the traversed feature spaces. This may in turn make it impossible to learn the true utility . On the other hand, if the procedure is too eager, the algorithm may end up eliciting more critiques than necessary, thus wasting cognitive effort. We will show in the next section that, unsurprisingly, the design of the procedure affects the regret incurred by the learner.
In order to balance between the two, we design a simple selection criterion, as follows. The idea is to submit a critique query as soon as algorithm realizes the true utility can no longer be represented in the current feature space. Since we do not have access to the true utility, we use the collected ranking feedback (i.e. the dataset, indicated as in Algorithm 1) as a proxy. To decide whether to ask for a critique or not, we check for the existence of a weight vector that correctly ranks the pairwise preference examples in , i.e. more formally: .
This criterion is guaranteed to work in noiseless scenarios. When the user is noisy though, a vector satisfying the ranking constraints may not exist in any subspace of . In this case, Coactive Critiquing may end up querying for a critique at every iteration. We did not experience this problem in practice. We also designed a more sophisticated criterion, based on estimating the likelihood of inconsistencies in the dataset being due to noise or lack of features. However, we did not see any improvements using this strategy in our empirical tests.
Theoretical analysis.
Theorem 1 assumes that the feature space is fixed. In coactive critiquing, however, this is not the case. Our goal is to extend the theorem to this more general case.
In Coactive Learning, at each iteration , the utility gain provided by two configurations over is , and is lower bounded by the informativity assumption (Eq Coactive Learning.). Our algorithm however works in a lower dimensional feature space than the user’s one, and has access to the partial utility only. In the lower dimensional space, the utility gain amounts to , so it “misses out” on the contribution of the unobserved features. We write to denote the missing part, quantified as:
(1)  
where is the number of features acquired up to iteration . Note that can be either positive or negative, depending on whether ignoring the missing features worsens or improves the utility gain, respectively. The latter case can occur when the update is negatively correlated with with respect to the missing features.
We formalize this intuition in the following proposition, which is an adaptation of Theorem 1.
Proposition 2.
For an informative user with true preference vector and bounded length feature vectors , the average regret incurred by Coactive Critiquing after iterations is upper bounded by
See the Appendix for the proof. The sum on the right hand side depends on the effectiveness of the user’s critiques and how often they are asked, as well as the problem structure. The latter factor is beyond our control, but the former can be (partially) controlled by properly designing the interaction with the user. By explicitly asking for the critique contributing the most to the utility gain, we are effectively removing the largest summand from (in practice, the user errors may make it decrease by a smaller amount). Furthermore, the amount of critiques may reduce the sum of the , at the price of additional cognitive effort for the user. In the next section we will show that our proposed NeedCritique heuristic offers a good tradeoff.
Empirical Evaluation
We evaluate Coactive Critiquing on two preference elicitation tasks. All experiments were run on a 2.8 GHz Intel Xeon CPU with 8 cores and 32 GiB of RAM. Our implementation makes use of MiniZinc [Nethercote et al.2007] with the Gecode backend. The cc source code and the full experimental setup are available at: goo.gl/cTFOFq.
User simulation.
We simulated the user feedback as follows. In improvement queries, the user is asked to produce an improvement of the suggested configuration . A real user would choose the improved configuration by balancing between cognitive effort and perceived quality of the improvement. To account for this fact, our simulated user (line 5 of Algorithm 1) computes the improvement by finding a minimal change to with improved true utility. This is done by solving the combinatorial problem:
s.t. 
Here measures the difference between and , and
is a normally distributed (
) perturbation that simulates user noise. The user is informative as per Eq Coactive Learning.. In order not to artificially advantage our method, our simulated user returns a minimal improvement, consequently providing a minimal utility gain.In critiquing queries (line 8), the user is asked to return the critique contributing the most to the utility gain of over . Formally, the contribution of feature is . Ideally, the user would respond with the feature with the highest contribution (with ties broken at random). In practice, she may choose a suboptimal critique. We simulate user noise by sampling
from a multinomial distribution where the probability of choosing
is set to . This model favors features with higher contribution, while still leaving room for suboptimal choices.Synthetic Experiment.
First, we evaluate our method on a synthetic task. The configurations are 2D points with integer coordinates, taking values in a discrete bounding box of size , for a total of feasible configurations. There are rectangles inside the bounding box. The position and size of the rectangles are sampled uniformly at random once and kept fixed for all runs. Each feature , , acts as an indicator for the corresponding rectangle : it evaluates to if is within the rectangle, and to otherwise. The true weights establish a preference over the rectangles: if the user prefers configurations contained in , and outside of it otherwise. It can be readily seen that most features are uncorrelated, and thus a sufficiently expressive subset of features is needed to find an optimal solution. The inference and improvement simulators were implemented as mixed integerlinear problems and solved accordingly.
First, we compare our NeedCritique heuristic against an uninformed baseline randomly choosing when to ask for critiques. Specifically, we replace our heuristic at line 7
with a binomial distribution, varying the parameter
.We run all methods over 20 users independently sampled from a dimensional standard normal distribution. We compute the median utility loss over all users (the lower, the better) as well as the average number of acquired features. Execution times are omitted, as the difference between algorithms is negligible. We report the results in the left column of Figure 1. As shown by the plots, our heuristic strikes a good balance between user satisfaction and cognitive effort. In terms of utility loss, it fares inbetween the (most informed baseline) and the (second most informed) variants, while eliciting fewer critiques than both. The other baselines are not up to par.
Next, we compare cc with our NeedCritique heuristic against cl. cc always starts from features and acquires new ones dynamically through query critiques. In contrast, cl has fixed access to % of the features, for . In order not to bias the results, for each we take the average of five different cl runs, each over a randomly drawn subspace of of the appropriate size. We refer to this setting as . Given that there is no standard, accepted way to estimate the real cognitive cost of replying to improvement or critique queries, we avoid computing a single unified measure of user effort and rather count the number of queries separately. We report the results in the middle column of Figure 1.
In median, reaches zero loss after 11 iterations, which is hardly surprising, considering its unrestricted (and unrealistic) access to the full feature space; cc instead takes 41 iterations. All other methods fail to converge. Notably, cc acquires about 30 features to reach zero median loss, and beats in the same metric after 18 iterations, with 14 acquired features. These results highlight the effectiveness of cc in acquiring relevant features, with consequent savings of cognitive effort.
Realistic Experiment.
We applied Coactive Critiquing to an interactive touristic trip planning task. We collected a dataset including 10 cities and 15 possible activities from the Trentino Open data website: http://dati.trentino.it/. The goal is to suggest a trip route between (some of) the cities. Each city has a particular offering of activities (e.g. luxury resorts, points of interest, healthcare services) and an overnight cost. Cities may be visited more than once. Traveling between cities takes a time proportional to their distance. In our experiments we set the trip length to 10.
We distinguish between base features and full features . The former include the amount of time spent at each location and the time spent performing each activity, for a total of base features. The latter include the number of distinct visited locations, the total time spent travelling, the total cost, the number of visited geographic regions, among others, for a total of acquirable features. We omit the full list for space restrictions.
As in the synthetic experiment, we compare cc against variants of cl obtained by varying the percentage of available features over 20 users sampled from a dimensional standard normal distribution. We report the results in the right column of Figure 1.
The problem is significantly more difficult than the synthetic one, due to the combinatorial size of the space of configurations. The plots show that cc is very critiqueeffective: by the last iteration it acquires about as many features as (approx. ), which is the least informed method, but it performs considerably better. The baselines converge faster, having access to most of the features from the beginning. Our approach performs comparably to from iteration 50 onwards, notwithstanding the much fewer acquired features ( versus , respectively). Although uses (from iteration ) about twice the number of features eventually acquired by cc, it is surpassed by the latter roughly at the iteration.
Conclusion
In this paper we described an approach to preference elicitation that combines Coactive Learning with example critiquing interaction. Contrary to coactive learning, the feature space is acquired dynamically through interaction with the user. We discussed the theoretical guarantees of the method, and a heuristic query selection strategy that balances between user effort and expressivity of the acquired feature space. We presented experimental evidence in support of our findings. Coactive Critiquing is competitive with more informed baselines, often requiring many less features to obtain comparable (or better) recommendations. Like conversational recommenders, Coactive Critiquing could in principle handle freeform textual or speech critiques, see for instance grasch2013recomment grasch2013recomment.
Coactive Critiquing is especially suited for constructive preference elicitation tasks [Teso, Passerini, and Viappiani2016]. Given that the computational cost of inference can become prohibitive in these settings, it may be fruitful to integrate support for approximate inference, as discussed by goetschalckx2014coactive goetschalckx2014coactive. Another promising research direction involves allowing the user to reply with nonfeasible improved configurations. In this case, the projection of the improvement on the feasible space may break informativity. We are currently investigating how to tackle this issue.
Acknowledgments
ST is supported by the CARITRO Foundation through grant 2014.0372. PD is a fellow of TIMSKIL Trento and is supported by a TIM scholarship.
Appendix A Appendix A: Proof of Proposition 2
We split the proof in three steps.
(i) The update equation of Algorithm 1 (line 12) is
We expand the dot product using the above, obtaining
The optimality of in the current feature space (line 4) implies that the second term is no greater than zero. Given that by assumption, it follows that
(ii) Let be a 01 vector, of the same shape as such that the only nonzero elements of are those corresponding to the features elicited up to iteration . We expand the dot product using the above update rule to obtain
where is the elementwise product. We unroll the recursion to get
By applying the definition of utility to the first term and the definition of to the second one, we get
(iii) The CauchySchwarz inequality states that . We plug Equations A and A to obtain:
Now we use the informativity assumption (Eq Coactive Learning.) and the definition of average regret (Eq Coactive Learning.) to obtain the claim.
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