Synthesis has both an age-structured and a size/age-structured version. Both are capable of simultaneously examining data from several fisheries and several surveys, each with its own pattern of selectivity. Synthesis calculates selectivity with modified logistic functions. Parameters of these functions can take on time-specific values, thus allowing flexibility to track changes in fishery selectivity. The goodness of fit is quantified in terms of a log-likelihood function composed of independent terms for each kind of observation from each type of fishery or survey. Synthesis estimates the best parameter values through numerical calculation of parameter derivatives and application of a modified Newton method
