  We examine the constraints that future lepton colliders would impose on the
effective field theory describing modifications of top-quark interactions
beyond the standard model, through measurements of the $e^+e^-\to bW^+\:\bar
bW^-$ process. Statistically optimal observables are exploited to constrain
simultaneously and efficiently all relevant operators. Their constraining power
is sufficient for quadratic effective-field-theory contributions to have
negligible impact on limits which are therefore basis independent. This is
contrasted with the measurements of cross sections and forward-backward
asymmetries. An overall measure of constraints strength, the global determinant
parameter, is used to determine which run parameters impose the strongest
restriction on the multidimensional effective-field-theory parameter space.
