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216 nips-2010-Probabilistic Inference and Differential Privacy


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Author: Oliver Williams, Frank Mcsherry

Abstract: We identify and investigate a strong connection between probabilistic inference and differential privacy, the latter being a recent privacy definition that permits only indirect observation of data through noisy measurement. Previous research on differential privacy has focused on designing measurement processes whose output is likely to be useful on its own. We consider the potential of applying probabilistic inference to the measurements and measurement process to derive posterior distributions over the data sets and model parameters thereof. We find that probabilistic inference can improve accuracy, integrate multiple observations, measure uncertainty, and even provide posterior distributions over quantities that were not directly measured. 1

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Summary: the most important sentenses genereted by tfidf model

sentIndex sentText sentNum sentScore

1 com Abstract We identify and investigate a strong connection between probabilistic inference and differential privacy, the latter being a recent privacy definition that permits only indirect observation of data through noisy measurement. [sent-3, score-1.221]

2 Previous research on differential privacy has focused on designing measurement processes whose output is likely to be useful on its own. [sent-4, score-1.027]

3 We consider the potential of applying probabilistic inference to the measurements and measurement process to derive posterior distributions over the data sets and model parameters thereof. [sent-5, score-0.564]

4 We find that probabilistic inference can improve accuracy, integrate multiple observations, measure uncertainty, and even provide posterior distributions over quantities that were not directly measured. [sent-6, score-0.316]

5 1 Introduction There has recently been significant interest in the analysis of data sets whose individual records are too sensitive to expose directly, examples of which include medical information, financial data, and personal data from social networking sites. [sent-7, score-0.274]

6 Although agencies with the resources to collate such data are unable to grant outside parties direct access to them, they may be able to safely release aggregate statistics of the data set. [sent-9, score-0.145]

7 Progress in this area has so far been driven by researchers inventing sophisticated learning algorithms which are applied directly to the data and output model parameters which can be proven to respect the privacy of the data set. [sent-10, score-0.814]

8 Proving these privacy properties requires an intricate analysis of each algorithm on a case-by-case basis. [sent-11, score-0.688]

9 While this does result in many valuable algorithms and results, it is not a scalable solution for two reasons: first, to solve a new learning problem, one must invent and analyze a new privacy-preserving algorithm; second, one must then convince the owner of the data to run this algorithm. [sent-12, score-0.225]

10 In this paper, we show a natural connection between differential privacy, one of the leading privacy definitions, and probabilistic inference. [sent-14, score-1.007]

11 Specifically, differential privacy exposes the conditional distribution of its observable outputs given any input data set. [sent-15, score-1.059]

12 Combining the conditional distributions of differentially-private observations with generative models for the data permits new inferences about the data without the need to invent and analyze new differentially-private computations. [sent-16, score-0.243]

13 In some cases, one can rely on previously reported differentially private measurements. [sent-17, score-0.301]

14 As well as this flexibility, probabilistic inference can improve the accuracy of existing approaches, provide a measure of uncertainty in any predictions made, combine multiple observations in a principled way, and integrate prior knowledge about the data or parameters. [sent-19, score-0.334]

15 In Section 3 we explore the marginal likelihood of the differentially-private observations given generative model parameters for the data. [sent-21, score-0.151]

16 Section 4 shows several experimental results validating our hypothesis that probabilistic inference can be fruitfully applied to differentially-private computation. [sent-23, score-0.206]

17 In particular, we show how the application of principled, probabilistic inference to measurements made by an existing, heuristic algorithm for logistic regression improves performance, as well as providing confidence on the predictions made. [sent-24, score-0.49]

18 1 Related work There is a substantial amount of research on privacy, and differential privacy in particular, connected with machine learning and statistics. [sent-26, score-0.902]

19 Nonetheless, we are unaware of any research that uses exact knowledge of the conditional distribution over outputs given inputs to perform inference over model parameters, or other features of the data. [sent-27, score-0.127]

20 Chaudhuri and Monteleoni [5, 6] introduced the NIPS community to the problem of differentiallyprivate logistic regression. [sent-31, score-0.151]

21 Although we will also consider the problem of logistic regression (and compare our findings with theirs) we should stress that the aim of the paper is not specifically to attack the problem of logistic regression. [sent-32, score-0.195]

22 Rather, the problem serves as a good example where prior work on differentially-private logistic regression can be improved through probabilistic inference. [sent-33, score-0.198]

23 2 Differential Privacy Differential privacy [7] applies to randomized computations executed against a dataset and returning an aggregate result for the entire set. [sent-34, score-0.763]

24 It prevents inference about specific records by requiring that the result of the computation yield nearly identical distributions for similar data sets. [sent-35, score-0.295]

25 Formally, a randomized computation M satisfies -differential privacy if for any two possible input data sets A and B, and any subset of possible outputs S, P (M (A) ∈ S) ≤ P (M (B) ∈ S) × exp( × |A B|) , (1) where A B is the set of records in A or B, but not both. [sent-36, score-0.948]

26 When A B is small, the relative bound on probabilities limits the inference an attacker can make about whether the true underlying data were actually A or B. [sent-37, score-0.253]

27 Inferences about the presence, absence, or specific values of individual records are strongly constrained. [sent-38, score-0.154]

28 One example of a differentially private computation is the exponential mechanism[8], characterized by a function φ : Dn × R → R scoring each pair of data set and possible result with a real value. [sent-39, score-0.413]

29 While any differentially-private mechanism can be expressed as a φ function, verifying that a function φ satisfies the constraint | ln φ(z, A) − ln φ(z, B)| ≤ |A B| is generally not easy, and requires some form of proof on a case by case basis. [sent-41, score-0.228]

30 This subclass is useful practically, as data providers can ensure differential privacy by clamping each φ(z, x) value to the range [e−1 , e+1 ], without having to understand the φ function. [sent-43, score-1.04]

31 We will refer to this subclass as the factored exponential mechanism. [sent-44, score-0.232]

32 As we can see from the definition of the exponential mechanism, a differentially-private mechanism draws its guarantees from its inherent randomness, rather than from secrecy about its specification. [sent-45, score-0.246]

33 Although differential privacy has many other redeeming features, it is this feature alone that we 2 i=1. [sent-46, score-0.902]

34 (a) If the data X = {xi } are directly observable (shaded nodes), the canonical learning task is to infer the posterior over θ given a model relating X and θ. [sent-51, score-0.242]

35 (b) In the private setting, the data are not observable; instead we observe the private measurement z, related to X by a known measurement process. [sent-52, score-0.64]

36 By the same token, although there are many other privacy definitions with varying guarantees, we can apply inference to any definition exhibiting one key feature: an explicit probabilistic relationship between the input data sets and output observations. [sent-54, score-1.024]

37 3 Inference and privacy Differential privacy limits what can be inferred about a single record in a data set, but does not directly limit inference about larger scale, aggregate properties of data sets. [sent-55, score-1.685]

38 For example, many tasks in machine learning and statistics infer global parameters describing a model of the data set without explicit dependence on any single record, and we may still expect to be see a meaningful relationship between differentially-private measurements and model parameters. [sent-56, score-0.154]

39 One way to model a data set is to propose a generative probabilistic model for the data p(X|θ). [sent-57, score-0.185]

40 (3) Armed with the marginal likelihood, it is possible to bring all the techniques of probabilistic inference to bear. [sent-62, score-0.271]

41 This will generally include adding a prior distribution over θ, and combining multiple measurements to form a posterior p(zj |θ) p(θ|z1 . [sent-63, score-0.175]

42 Therefore, the remainder of this section is devoted to the development of several bounds on the marginal likelihood for cases in which the measurement is generated via the factored exponential mechanism. [sent-69, score-0.479]

43 1 Factored exponential mechanism The factored exponential mechanism of Section 2 is a special case of differentially-private mechanism that admits efficient approximation of the marginal likelihood. [sent-72, score-0.775]

44 We will be able to use the independence in p(X|θ) = i p(xi |θ) and φ(z, X) = i φ(z, xi ) to factorize lower and upper 3 bounds on the integral (3), resulting in a small number of integrals over only the domain of records, rather than the domain of data sets. [sent-73, score-0.266]

45 φ(z , x) dx p(x|θ) φ(z, x) p(z|θ) ≥ z ∈Z p(z|θ) ≤ e−H[q] n −1 (5a) φ(z, x) z ∈Z φ(z , x) dx p(x|θ) n (5b) q(z ) where the upper bound is defined in terms of a variational distribution q(z) [9] such that z∈ q(z) = 1. [sent-75, score-0.41]

46 Notice that the integrations appearing in either bound are over the space of a single record in a data set and not over the entire dataset as they were in (3). [sent-77, score-0.169]

47 Proof of lower bound To prove the lower bound, we will apply Jensen’s inequality with the function f (x) = 1/x to the marginal likelihood of the exponential mechanism. [sent-78, score-0.342]

48 p(xi |θ) dxn i φ(z , xi ) φ(z, xi ) dxi p(xi |θ) = i = dx p(x|θ) φ(z , xi ) φ(z, xi ) φ(z , x) φ(z, x) n . [sent-82, score-0.347]

49 Proof of upper bound We can lower bound the normalizing term z ∈Z φ(z , X) in (2) by introducing a variational distribution q(z), and applying Jensen’s inequality with the function f (x) = log x. [sent-83, score-0.272]

50 z ∈Z Applying this bound to the marginal likelihood gives us the bound dX p(X|θ) φ(z, X) ≤ e−H[q] z ∈Z φ(z , X) dX p(X|θ) = e−H[q] p(xi |θ) dX i = e−H[q] φ(z, X) z ∈Z φ(z , X) dx p(x|θ) q(z ) φ(z, xi ) z ∈Z φ(z , xi ) φ(z, x) z ∈Z φ(z , x) q(z ) n q(z ) . [sent-85, score-0.487]

51 While the upper bound is true for any q distribution, the tightest bound is found for the q which minimizes the bound. [sent-86, score-0.186]

52 4 Upper bound Lower bound Actual p(z|theta) 0. [sent-87, score-0.132]

53 8 theta (b) Figure 2: Error in upper and lower bounds for coin-flipping problem. [sent-99, score-0.218]

54 (a) For each epsilon, we plot the maximum across all θ of the error between the true distribution and each of the upper and lower bounds is plotted. [sent-100, score-0.197]

55 5, we show the shape of the upper bound, lower bound, and true distribution when differentially-private measurement returned was z = 0. [sent-102, score-0.225]

56 1 Chosing a φ function The upper and lower bounds in (5) are true for any admissible φ function, but leave unanswered the question of what to chose in this rˆ le. [sent-106, score-0.204]

57 In the absence of privacy we might try to find a good fit for o the parameters θ by maximum likelihood. [sent-107, score-0.688]

58 In the private setting this is not possible because the data are not directly observable, but the output of the factored exponential mechanism has a very similar form: Max likelihood: Exp. [sent-108, score-0.617]

59 mechanism: θ∗ = arg max θ∈Θ p(xi |θ) z ∗ = noisy max z∈Z (6a) i φ(z, xi ) (6b) i f (z) where noisy maxz∈Z f (z) samples from . [sent-109, score-0.146]

60 By making the analogy between (6a) and (6b), z ∈Z f (z ) we might let z range over elements of Θ (or a finite subset), and take φ(z, xi ) to be the likelihood of xi under parameters z. [sent-110, score-0.167]

61 The exponential mechanism is then likely to choose parameters z that fit the data well, informing us that the posterior over θ is likely in the vicinity of z. [sent-111, score-0.35]

62 For φ to be admissible, we must clamp very small values of φ up to 1/e, limiting the ability of very poorly fit records to influence our decisions strongly. [sent-112, score-0.154]

63 2 Evaluation of the bounds To demonstrate the effectiveness of these bounds we consider a problem in which it is possible to analytically compute the marginal likelihood. [sent-114, score-0.213]

64 We see in figure 2a that the error in both the upper and lower bounds, across the entire density function, is essentially zero for small epsilon. [sent-122, score-0.123]

65 As epsilon increases the bounds deteriorate, but we are most interested in the case of small values of epsilon, where privacy guarantees are meaningfully strong. [sent-123, score-0.979]

66 Figure 2b shows the shape of the two bounds, and the true density between, for epsilon = 0. [sent-124, score-0.193]

67 This large value was chosen as it is in the region for which the bounds are less tight and the difference between the bounds and the truth can be seen. [sent-126, score-0.148]

68 5 The upper bound is defined in terms of a variational distribution q. [sent-127, score-0.164]

69 In general, however, these (and other) test show that both bounds are equally good for reasonable values of and we therefore use the lower bound for the experiments in this paper, since it is simpler to compute. [sent-129, score-0.182]

70 First, we consider applying probabilistic inference to an existing differentially-private computation, specifically a logistic regression heuristic taken from a suite of differentially-private algorithms. [sent-131, score-0.401]

71 The heuristic is not representable in the factored exponential mechanism, and as such we must attempt to approximate the full integral over the space of data sets directly. [sent-132, score-0.364]

72 In our second experiment, we choose a problem and measurement process appropriate for the factored exponential mechanism, principal components analysis, previously only ever addressed through noisy observation of the covariance matrix. [sent-133, score-0.356]

73 1 Logistic Regression To examine the potential of probabilistic inference to improve the quality of existing differentiallyprivate computations, we consider a heuristic algorithm for logistic regression included in the Privacy Integrated Queries distribution [10]. [sent-135, score-0.483]

74 This heuristic uses a noisy sum primitive to repeatedly compute and step in the direction of an approximate gradient. [sent-136, score-0.185]

75 When the number of records is large compared to the noise introduced, the approximate gradient is relatively accurate, and the algorithm performs well. [sent-137, score-0.178]

76 When the records are fewer or the privacy requirements demand more noise, its performance suffers. [sent-138, score-0.842]

77 Probabilistic inference has the potential to improve performance by properly integrating the information extracted from the data across the multiple gradient measurements and managing the uncertainty associated with the noisy measurements. [sent-139, score-0.329]

78 We test our proposals against three synthetic data sets (CM1 and CM2 from [5] and one of our own: SYNTH) and two data sets from the UCI repository (PIMA and ADULT) [11]. [sent-140, score-0.189]

79 SYNTH Records Dimensions Positive examples Test set records CM1 CM2 PIMA ADULT 1000 4 497 1000 17500 10 8770 17500 17500 10 8694 17500 691 8 237 767* 16000 6 7841 8000 Table 1: Data sets used and their statistics. [sent-144, score-0.194]

80 PIMA and ADULT are standard data sets [11] containing diabetes records, and census data respectively, both of which correspond to the types of data one might expect to be protected by differential privacy. [sent-147, score-0.374]

81 1 Error Rates and Log-Likelihood Tables 2 and 3 report the classification accuracy of several approaches when the privacy parameter is set to 0. [sent-151, score-0.715]

82 These results are computed from 50 executions of the heuristic gradient descent algorithm. [sent-154, score-0.17]

83 We can see a trend of general improvement from the heuristic approach to the probabilistic inference, both in terms of the average error rate and the standard deviation. [sent-155, score-0.234]

84 1 All measurements are in per cent; errors are reported as the mean ± one standard deviation computed from 50 independent executions with random starting points. [sent-188, score-0.133]

85 Benchmark is the best maximum likelihood solution found by gradient ascent when the data are directly observable and forms a baseline for expected performance. [sent-191, score-0.248]

86 Each colored path describes an execution with a fixed level of accuracy in each iteration, and all are plotted on the common scale of total privacy consumption. [sent-226, score-0.715]

87 Even with very few additional examples, probabilistic inference is capable of exploiting this information and performance improves dramatically. [sent-234, score-0.206]

88 2 Principal components To demonstrate inference on another model, and to highlight the applicability of the factored exponential mechanism, we consider the problem of probabilistically finding the first principal compo7 0. [sent-236, score-0.309]

89 (b) Incorporating non-private observations A compelling benefit of probabilistic inference is how easily alternate sources of information are added. [sent-246, score-0.237]

90 The horizontal line indicates the performance of the benchmark maximum likelihood solution computed from the data without privacy. [sent-247, score-0.133]

91 1 Figure 4: Posterior distribution as a function of The same synthetic data set under differentiallyprivate measurements with varying epsilon. [sent-251, score-0.239]

92 The posterior is noticeably more concentrated and accurate as epsilon increases. [sent-253, score-0.279]

93 nent of a data set where we model the data as iid draws from a Gaussian p(x|θ) = N (0, θθT + σ 2 I). [sent-254, score-0.128]

94 Figure 4 demonstrates three instances of inference applied to the same data set with three different values of . [sent-256, score-0.141]

95 We stress that the posterior and its concentration are returned to the analyst; each image is the result of a single differentially-private measurement, rather than a visualization of multiple runs. [sent-258, score-0.178]

96 5 Conclusions Most work in the area of learning from private data forms an intrinsic analysis. [sent-262, score-0.237]

97 That is, a complex algorithm is run by the owner of the data, directly on that data, and a single output is produced which appropriately indicates the desired parameters (modulo noise). [sent-263, score-0.126]

98 In contrast, this paper has shown that it is possible to do a great deal with an extrinsic analysis, where standard, primitive, measurements are made against the data, and a posterior over model parameters is inferred post hoc. [sent-264, score-0.199]

99 This paper brings together two complementary lines of research: the design and analysis of differentially-private algorithms, and probabilistic inference. [sent-265, score-0.129]

100 Our primary goal is not to weigh-in on new differentially-private algorithms, nor to find new methods for probabilistic inferences – it is to present the observation that the two approaches are complementary in way that can be mutually enriching. [sent-266, score-0.178]


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