We propose novel model transfer-learning methods that refine a decision forest model M learned within a "source" domain using a training set sampled from a "target" domain, assumed to be a variation of the source. We present two random forest transfer algorithms. The first algorithm searches greedily for locally optimal modifications of each tree structure by trying to locally expand or reduce the tree around individual nodes. The second algorithm does not modify structure, but only the parameter (thresholds) associated with decision nodes. We also propose to combine both methods by considering an ensemble that contains the union of the two forests. The proposed methods exhibit impressive experimental results over a range of problems.

We present an algorithm for identifying users who share a common condition from anonymized search engine logs. Input to the algorithm is a set of seed phrases that identify users with the condition of interest with high precision albeit at a very low recall. We expand the set of seed phrases by clustering queries according to the pages users clicked following these queries and the temporal ordering of queries within sessions, emphasizing the subgraph containing seed phrases. To this end, we extend modularity-based clustering such that it uses the information in the initial seed phrases as well as other queries of users in the population of interest. We evaluate the performance of the proposed method on two datasets, one of mood disorders and the other of anorexia, by classifying users according to the clusters in which they appeared and the phrases contained thereof, and show that the area under the receiver operating characteristic curve (AUC) obtained by these methods exceeds 0.87. These results demonstrate the value of our algorithm for both identifying users for future research and to gain better understanding of the language associated with the condition.

We introduce a new discrepancy measure between two distributions that gives an indication on their similarity. The new measure, termed the Perturbed Variation (PV), gives an intuitive interpretation of similarity; it optimally perturbs the distributions so that they best fit each other. The PV is defined between continuous and discrete distributions, and can be efficiently estimated from samples. We provide bounds on the convergence of the estimated score to its distributional value, as well as robustness analysis of the PV to outliers. A number of possible applications of the score are presented, and its ability to detect similarity is compared with that of other known measures on real data. We also present a new visual tracking algorithm based on the PV, and compare its performance with known tracking algorithms.

Detecting changes in data-streams is an important part of enhancing learning quality in dynamic environments. We devise a procedure for detecting concept drifts in data-streams that relies on analyzing the empirical loss of learning algorithms. Our method is based on obtaining statistics from the loss distribution by reusing the data multiple times via resampling. We present theoretical guarantees for the proposed procedure based on the stability of the underlying learning algorithms. Experimental results show that the detection method has high recall and precision, and performs well in the presence of noise.

We introduce a new discrepancy score between two distributions that gives an indication on their \emph{similarity}. While much research has been done to determine if two samples come from exactly the same distribution, much less research considered the problem of determining if two finite samples come from similar distributions. The new score gives an intuitive interpretation of similarity; it optimally perturbs the distributions so that they best fit each other. The score is defined between distributions, and can be efficiently estimated from samples. We provide convergence bounds of the estimated score, and develop hypothesis testing procedures that test if two data sets come from similar distributions. The statistical power of this procedures is presented in simulations. We also compare the score's capacity to detect similarity with that of other known measures on real data.