  Human activity recognition (HAR) is a key challenge in pervasive computing
and its solutions have been presented based on various disciplines.
Specifically, for HAR in a smart space without privacy and accessibility
issues, data streams generated by deployed pervasive sensors are leveraged. In
this paper, we focus on a group activity by which a group of users perform a
collaborative task without user identification and propose an efficient group
activity recognition scheme which extracts causality patterns from pervasive
sensor event sequences generated by a group of users to support as good
recognition accuracy as the state-of-the-art graphical model. To filter out
irrelevant noise events from a given data stream, a set of rules is leveraged
to highlight causally related events. Then, a pattern-tree algorithm extracts
frequent causal patterns by means of a growing tree structure. Based on the
extracted patterns, a weighted sum-based pattern matching algorithm computes
the likelihoods of stored group activities to the given test event sequence by
means of matched event pattern counts for group activity recognition. We
evaluate the proposed scheme using the data collected from our testbed and
CASAS datasets where users perform their tasks on a daily basis and validate
its effectiveness in a real environment. Experiment results show that the
proposed scheme performs higher recognition accuracy and with a small amount of
runtime overhead than the existing schemes.
