  Studies of the time-domain structure of fast radio bursts (FRBs) require an
accurate estimate of the FRB dispersion measure in order to recover the
intrinsic burst shape. Furthermore, the exact DM is itself of interest when
studying the time-evolution of the medium through which multiple bursts from
repeating FRBs propagate. A commonly used approach to obtain the dispersion
measure is to take the value that maximizes the FRB structure in the time
domain. However, various authors use differing methods to obtain this structure
parameter, and do not document the smoothing method used. Furthermore, there
are no quantitative estimates of the error in this procedure in the FRB
literature. In this letter, we present a smoothing filter based on the discrete
cosine transform, and show that computing the structure parameter by summing
the squares of the derivatives and taking the square root immediately lends
itself to calculation of uncertainty of the structure parameter. We illustrate
this with FRB181112 and FRB210117 data, which were detected by the Australian
Square Kilometre Array Pathfinder, and for which high-time-resolution data is
available.
