What is Fixed Effects

This chapter describes the System for Population Kinetics (SPK), a novel Web service for performing population kinetic analysis. Population kinetic analysis is a widely-used tool for extracting information about the probability distributions of unknown parameters in kinetic models. The statistical population model is usually hierarchical, with a nested structure encompassing both variation between subjects and residual unexplained variation associated with the model predictions. The complexity of the analysis is largely driven by the nonlinearity of the models employed. Here, we provide a concise introduction to the topic and a historical perspective for the benefit of the reader who is new to these concepts. Next, we briefly describe the SPK open source system and its multi-tiered architecture, indicating the user goals it set to achieve and elucidating its practical usage with examples.