"for non-experimental study designs, the approach that will likely optimize our ability to understand how the attitudes of particular individuals change over time is the"

This is called "Panel Study".

Panel studies are a specific outline of longitudinal investigation in which the unit of examination is taken after at indicated interims over a long time, usually for numerous years. The key element of Panel studies is that they gather rehashed measures from a same sample at various times. Most Panel studies are intended for quantitative examination and utilize structured survey information.