As alluded to yesterday, my dissertation compares growth in an extinct animal with growth in living humans; this study is necessarily cross-sectional, meaning that it examines individuals at a single point in time. Alternatively, longitudinal data sample individuals from several points in time. So for instance if I constructed a growth curve by measuring the stature of a bunch of people of different ages in just a day, that would be cross-sectional. But if I had the time and wherewithal to measure some people’s heights once a year from birth to adulthood, well that’d be longitudinal. Cross-sectional data lack the resolution of longitudinal data, whereas longitudinal data can be prohibitively difficult to collect (such as in long-lived, slow-maturing animals like humans, or in extinct animals like Australopithecus robustus).
Some researchers abhor cross-sectional data, pointing out that the intricacies of individuals’ longitudinal growth will not be adequately captured in with cross-sectionally. American anthropology founder Franz Boas himself discussed this in a paper nearly 82 years ago. Anyway, I was reminded of this dichotomy today when perusing a paper that examined longitudinal brain activity in a cohort of adolescent kids (right, from Campbell et al. in press). The mess of jagged lines are individuals’ measurements from age 9-18, and the smoothed blue and red curves are the cross-sectionalized curves calculated from these kids. Oy, look at all that variation and caprice that gets left out in the cross-sectionalized curves!
Of course, this doesn’t mean that we should never use cross-sectional data to study growth – like I’d mentioned above, the fossil record necessitates a cross-sectional approach to the study of growth. As always, you have to understand and acknowledge the limits of your data.
Boas, F. (1930). OBSERVATIONS ON THE GROWTH OF CHILDREN Science, 72 (1854), 44-48 DOI: 10.1126/science.72.1854.44
Campbell, I., Grimm, K., de Bie, E., & Feinberg, I. (2012). Sex, puberty, and the timing of sleep EEG measured adolescent brain maturation Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1120860109