We introduce the questionable observer detection problem: Given a collection of videos of crowds, determine which individuals appear unusually often across the set of videos. The algorithm proposed here detects these individuals by clustering sequences of face images. To provide robustness to sensor noise, facial expression and resolution variations, blur, and intermittent occlusions, we merge similar face image sequences from the same video and discard outlying face patterns prior to clustering. We present experiments on a challenging video dataset. The results show that the proposed method can surpass the performance of a clustering algorithm based on the VeriLook face recognition software by Neurotechnology both in terms of the detection rate and the false detection frequency.
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