nips nips2012 nips2012-215 knowledge-graph by maker-knowledge-mining
Source: pdf
Author: Nilesh Dalvi, Aditya Parameswaran, Vibhor Rastogi
Abstract: In this paper, we consider the problem of debugging large pipelines by human labeling. We represent the execution of a pipeline using a directed acyclic graph of AND and OR nodes, where each node represents a data item produced by some operator in the pipeline. We assume that each operator assigns a confidence to each of its output data. We want to reduce the uncertainty in the output by issuing queries to a human, where a query consists of checking if a given data item is correct. In this paper, we consider the problem of asking the optimal set of queries to minimize the resulting output uncertainty. We perform a detailed evaluation of the complexity of the problem for various classes of graphs. We give efficient algorithms for the problem for trees, and show that, for a general dag, the problem is intractable. 1
Reference: text
sentIndex sentText sentNum sentScore
1 We represent the execution of a pipeline using a directed acyclic graph of AND and OR nodes, where each node represents a data item produced by some operator in the pipeline. [sent-9, score-0.346]
2 We want to reduce the uncertainty in the output by issuing queries to a human, where a query consists of checking if a given data item is correct. [sent-11, score-0.364]
3 In this paper, we consider the problem of asking the optimal set of queries to minimize the resulting output uncertainty. [sent-12, score-0.13]
4 However, the output uncertainties are highly correlated since different tuples share their lineage. [sent-23, score-0.128]
5 In this paper, we consider the following interesting and non-trivial problem : given a budget of k tuples, choose the k tuples to inspect that minimize the total uncertainty in the output. [sent-25, score-0.217]
6 We will formalize the notion of a data pipeline and uncertainty in Section 2. [sent-26, score-0.171]
7 We want to choose k tuples to inspect that minimize the total output uncertainty. [sent-54, score-0.17]
8 Resolving e3 completely resolves the uncertainty in w2 , which, in turn, completely resolves the uncertainty in e4 and reduces the uncertainty in e5 . [sent-75, score-0.333]
9 In general, one can also query intermediate nodes in addition to the output tuples, and choosing the best node is non-trivial. [sent-77, score-0.456]
10 In this paper, we consider the general setting of a data pipeline given by a directed acyclic graph that can capture both the motivating scenarios. [sent-78, score-0.141]
11 We define a measure of total uncertainty of the final output based on how close the probabilities are to either 0 or 1. [sent-79, score-0.173]
12 We give efficient algorithms to find the set of data items to query that minimizes the total uncertainty of the output, both under interactive and batch settings. [sent-80, score-0.317]
13 1 Related Work Our problem is an instance of active learning [27, 13, 12, 17, 2, 15, 5, 4, 3] since our goal is to infer probability values of the nodes being true in the DAG, by asking for tags of example nodes. [sent-82, score-0.254]
14 Unlike traditional active learning where we want to learn the underlying probabilistic model from iid samples, in our problem, we already know the underlying model and want to gain information about non-iid items with known correlations. [sent-85, score-0.16]
15 Our work is closely related to the field of active diagnosis [28, 19, 20], where the goal is to infer the state of unknown nodes in a network by selecting suitable “test probes”. [sent-93, score-0.214]
16 Our work is also related to the work on graph search [23], where the goal is to identify hidden nodes while asking questions to humans. [sent-99, score-0.244]
17 2 Problem Statement Execution Graph: Let G be a directed acyclic graph (dag), where each node n in G has a label from the set {∧, ∨} and a probability p(n). [sent-101, score-0.23]
18 the set of nodes having an outgoing edge to n, denote the set of data items which were input to the instance of the operator that produced n. [sent-109, score-0.224]
19 We further assume that, conditioned on the parents being correct, nodes are correct independently. [sent-114, score-0.264]
20 To state the semantics formally, we associate a set of independent Boolean random variables X(n) for each node n in G with probability p(n). [sent-115, score-0.149]
21 We also associate another set of random variables Y (n), which denotes whether the result at node n is correct (unconditionally). [sent-116, score-0.175]
22 , all nodes have a single parent, the labels of nodes do not have any effect, since Y (n) is the same for both ∧ and ∨ nodes. [sent-121, score-0.344]
23 We want all the final probabilities of L to be close to either 0 or 1, as the closer the probability to 1/2, the more uncertain the correctness of the given node is. [sent-128, score-0.278]
24 Then, we define the total output uncertainty of the DAG as I= ∑ f (Pr(Y (n))) (1) n∈L Our results continue to hold when different n ∈ L are weighted differently, i. [sent-130, score-0.142]
25 Now, our goal is to query a set of nodes Q that minimize the expected total output uncertainty conditioned on observing Q. [sent-135, score-0.452]
26 Given a G ∈ DAG, find the node q that minimizes the expected uncertainty I({q}). [sent-141, score-0.31]
27 Given a G ∈ DAG, find the set of nodes Q of size k that minimizes I(Q). [sent-143, score-0.222]
28 Suppose we have already issued a set of queries Q0 and obtained a vector v0 of their correctness values. [sent-145, score-0.128]
29 Given a new set of queries, we define the conditioned uncertainty as I(Q | Q0 = v0 ) = ∑v Pr(Q = v | Q0 = v0 ) ∑n∈L f (Pr(Y (n) | Q = v ∧ Q0 = v0 )). [sent-146, score-0.145]
30 Given a G ∈ DAG, and a set of already issued queries Q0 with answer v0 , find the best node q to query next that minimizes I({q} | Q0 = v0 ). [sent-148, score-0.391]
31 For any sets of queries Q1 , Q2 , I(Q1 ) ≥ I(Q1 ∪ Q2 ) Thus, expected uncertainty cannot increase with more queries. [sent-160, score-0.17]
32 Lastly, for the rest of the paper, we will assume that the query nodes Q are selected from only among the leaves of G. [sent-164, score-0.353]
33 There is a simple reduction of the general problem to this problem, where we attach a new leaf node to every internal node, and set their probabilities to 1. [sent-166, score-0.296]
34 Thus, for any internal node, we can equivalently query the corresponding leaf node (we will need to use the weighted form of the Eq. [sent-167, score-0.338]
35 (1), described in the extended technical report [11], to ensure that new leaf nodes have weight 0 in the objective function. [sent-168, score-0.281]
36 Let DAG(∧) and DAG(∨) denote the subclasses of DAG where all the node labels are ∧ and ∨ respectively. [sent-170, score-0.149]
37 (We define the depth to be the number of nodes in the longest root to leaf directed path in the dag. [sent-172, score-0.311]
38 ) Similarly, we define the class T REE where the dag is restricted to a tree, and T REE(d), consisting of depth-d trees. [sent-173, score-0.672]
39 For trees, since each node has a single parent, the labels of the nodes do not matter. [sent-174, score-0.321]
40 The parameter n is the number of nodes in the graph. [sent-183, score-0.172]
41 4 Best-1 Problem We start with the most basic problem: given a probabilistic DAG G, find the node to query that minimizes the resulting uncertainty. [sent-188, score-0.303]
42 ) Subsequently, we show that finding the best node to query is intractable for DAG(∨) of depth greater than 2, and is thus intractable for DAG as well. [sent-190, score-0.297]
43 , those nodes that have a directed path to n, including n itself. [sent-193, score-0.197]
44 1 T REE(2) Consider a simple tree graph G with root r, having p(r) = pr , and having children l1 , · · · , ln with p(li ) = pi . [sent-196, score-0.39]
45 Given a node x, let ex denote the event Y (x) = 1, and ex denote the event that Y (x) = 0. [sent-197, score-0.303]
46 We want to find the leaf q that minimizes I({q}), where: I({q}) = ∑ Pr(eq ) f (Pr(el | eq )) + Pr(eq ) f (Pr(el | eq )) (4) l∈L By a slight abuse of notation, we will use I(q) to denote the quantity I({q}). [sent-198, score-0.372]
47 It is easy to see the following (let l = q): Pr(el | eq ) = pl , Pr(eq ) = pr pq , Pr(el | eq ) = pr pl (1 − pq )/(1 − pr pq ) Substituting these expressions back in Eq. [sent-199, score-2.292]
48 We first compute ∑l pl and ∑l p2 , and then compute the objective l function for all q in linear time. [sent-202, score-0.171]
49 Now we consider the case when G is any general tree with the set of leaves L. [sent-203, score-0.127]
50 Thus, Px is the product of p(y) over all nodes y that are the ancestors of x (including x itself). [sent-206, score-0.234]
51 Given nodes x and y, let lca(x, y) denote the least common ancestor of x and y. [sent-207, score-0.22]
52 The following is immediate: Pr(eq ) = Pq Pr(el | eq ) = Pl Plca(l,q) Pr(el | eq ) = Pl (1 − Pq /Plca(l,q) ) 1 − Pq However, if we directly plug this in Eq. [sent-210, score-0.204]
53 Instead, we group all the leaves into equivalence classes based on their lowest common ancestor with q as shown in Fig. [sent-213, score-0.125]
54 Consider all leaves in the set Li such that their lowest common ancestor with q is ai . [sent-216, score-0.178]
55 Given a node x, let S(x) denote the sum of Pl2 over all leaves l reachable from x. [sent-217, score-0.226]
56 (4) over all leaves in Li , we get the following expression: ad −(S(ai ) − S(ai−1 )) 2 (Pq + Pai − 2Pq Pai ) + ∑ Pl 2 (1 − P ) Pai q l∈Li Define ∆1 (ai ) = S(ai ) − S(ai−1 ) and ∆2 (ai ) = (S(ai ) − 1−2P S(ai−1 )) P2 ai . [sent-219, score-0.13]
57 First, using a single top-down dynamic programming over the tree, we can compute Px for all nodes x. [sent-222, score-0.172]
58 Next, using a single bottom-up dynamic programming over G, we can compute S(x) for all nodes x. [sent-223, score-0.172]
59 In the third step, we compute ∆1 (x) and ∆2 (x) for all nodes in the tree. [sent-224, score-0.172]
60 In the fourth step, we compute ∑a∈anc(x) ∆i (x) for all nodes in the graph using another top-down dynamic programming. [sent-225, score-0.204]
61 Given a tree G with n nodes, we can compute the node q that minimizes I(q) is time O(n). [sent-230, score-0.249]
62 As before, we want to find the best node q that minimizes I(q) as given by Eq. [sent-233, score-0.232]
63 However, the expressions for probabilities Pr(eq ) and Pr(el | eq ) are more complex for DAG(2, ∨). [sent-235, score-0.133]
64 Subsequently, finding the best candidate node would require O(n2 ) time, giving us an overall O(n3 ) algorithm to find the best node. [sent-241, score-0.149]
65 5 Incremental Node Selection In this section, we consider the problem of picking the next best node to query after a set of nodes Q0 have already been queried. [sent-270, score-0.501]
66 We next pick a leaf node q that minimizes I({q} | Q0 = v0 ). [sent-272, score-0.339]
67 Here, we prove that incremental picking is intractable for DAG(∧) itself. [sent-276, score-0.144]
68 Thus, after each query, we can incorporate the new evidence by updating the probabilities of all the nodes along the path from the query node to the root. [sent-291, score-0.456]
69 Thus, finding the next best node to query can still be computed in linear time. [sent-292, score-0.253]
70 If the result is 0, p(q) is set to 0 and p(r) pr (1−pq ) is set to 1−pr pq . [sent-296, score-0.582]
71 Then, we can subsequently compute the leaf q that minimizes Eq. [sent-300, score-0.135]
72 For a fixed pr , ∑l pl , and ∑l p2 , this expression can be treated as l a rational polynomial in a single variable pq . [sent-302, score-0.809]
73 If we take the derivative, the numerator is a quartic in pq . [sent-303, score-0.349]
74 Using 4 binary searches, we can find the two pq closest to each of these roots (giving us 8 candidates for pq , plus two more which are the smallest and the largest pq ), and evaluate I(q) for each of those 10 candidates. [sent-306, score-1.044]
75 , the answer to each subsequent query), we can update the pr probability and the sum ∑l p2 in constant time. [sent-310, score-0.256]
76 If the p values of the leaf nodes are provided in sorted order, then, for a Depth-2 tree, the next best node to query can be computed in O(log n). [sent-314, score-0.51]
77 3 DAG(∧) For DAG(∧), while we can pick the best-1 node in O(n3 ) time, we have the surprising result that the problem of picking subsequent nodes become intractable. [sent-316, score-0.452]
78 In particular, we show that given a set S of queried nodes, the problem of finding the next best node is intractable in the size of S. [sent-318, score-0.201]
79 Our reduction, shown in in the extended technical report [11], is a weakly parsimonious reduction involving monotone-2-CNF, which is known to be hard to approximate, thus we have the following result: Theorem 5. [sent-324, score-0.154]
80 6 Best-K In this section, we consider the problem of picking the best k nodes to minimize uncertainty. [sent-329, score-0.248]
81 [19] give a log n approximation algorithm for a similar problem under the conditions of super-modularity: super-modularity states that the marginal decrease in uncertainty when adding a single query node to an existing set of query nodes decreases as the set becomes larger. [sent-331, score-0.64]
82 If we pick any leaf node with p = 1, the expected uncertainty is n/8. [sent-339, score-0.4]
83 If we pick a node with p = 1/2, the expected uncertainty is 25n/16 − 4/16. [sent-340, score-0.315]
84 Thus, if we sort nodes by their expected uncertainty, all the p = 1 nodes appear before all the p = 1/2 nodes. [sent-341, score-0.344]
85 If we pick greedily based on their expected uncertainty, we pick all the p = 1 nodes. [sent-343, score-0.142]
86 Thus, expected uncertainty after querying all p = 1 nodes is still n/8. [sent-345, score-0.283]
87 On the other hand, if we pick a single p = 1 node, and n − 1 nodes with p = 1/2, the resulting uncertainty is a constant. [sent-346, score-0.338]
88 Thus, picking nodes greedily can be O(n) worse than the optimal. [sent-347, score-0.28]
89 Consider a G ∈ DAG(2, ∧) having two nodes u and v on 7 the top layer and three nodes a, b, and c in the bottom layer. [sent-349, score-0.344]
90 Now consider the expected uncertainty Ic at node c. [sent-353, score-0.26]
91 Super-modularity condition implies that Ic ({b, a}) − Ic ({b}) ≥ Ic ({a}) − Ic ({}) (since marginal decrease in expected uncertainty of c on picking an additional node a should be less for set {} compared to {b}). [sent-354, score-0.336]
92 This can be used to show that conditioned on Y (b), expected uncertainty in c drops when conditioning on Y (a). [sent-359, score-0.145]
93 Then there exists a tree T ∈ T REE(d) such that for leaf nodes a , b, and c in T the following holds: Ic ({b, a}) − Ic ({b}) < Ic ({a}) − Ic ({}). [sent-367, score-0.307]
94 As in Section 4, assume the root r with p(r) to be pr , while the leaves L = {l1 , . [sent-370, score-0.362]
95 ) It is easy to check that that the first term is minimized with Q consists of nodes with p(l) closest to 1/2, and the second term is minimized with nodes with p(l) closest to 1. [sent-378, score-0.41]
96 Intuitively, the first term prefers nodes that are as uncertain as possible, while the second term prefers nodes that reveal as much about the root as possible. [sent-379, score-0.444]
97 This immediately gives us a 2-approximation in the number of queries : by picking at most 2k nodes, k closest to 1/2 and k closest to 1, we can do at least as well as the optimal solution for best-k. [sent-380, score-0.201]
98 This also makes intuitive sense, since the second term prefers nodes that reveal more about the root, and once we use sufficiently many nodes to infer the correctness of the root, we do not get any gain from asking additional questions. [sent-382, score-0.447]
99 Our reduction is a weakly parsimonious reduction involving monotone-partitioned-2-CNF, which is known to be hard to approximate, thus we have the following result: Theorem 6. [sent-391, score-0.161]
100 7 Conclusion In this work, we performed a detailed complexity analysis for the problem of finding optimal set of query nodes for various classes of graphs. [sent-394, score-0.297]
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