Selection of appropriate study design and analytical methods is critical for producing robust research, as the design and analytical approach used can ultimately shape results and their interpretation. The objective of this research was to examine how findings from a large repeat cross-sectional data system compare to those from a sample of longitudinal data nested within the same data system, using milk and milk alternative consumption as a case study. Overall, the repeat cross-sectional and longitudinal findings were consistent, and this article highlights the strengths and limitations of both methodologies under examination. Application to social research and public health and policy are discussed.