A formalism called Sequential Input Graphical Model Assessment (SIGMA) diagrams is introduced. It shares with conventional graphical models features for specifying probabilistic inferences. In addition, it provides facilities for describing temporal patterns of evidence. The formalism was motivated by a need for transparent representations of assessment processes in the INFACT online learning environment. An editor and interpreter for SIGMA diagrams have been implemented and embedded in INFACT. The interpreter works with either stored event data, real-time event processing, or a combination of both. Examples diagrams are given, and the expressive power of the formalism is discussed.