A longitudinal study and a cross-sectional study are methods of collecting scientific data.A longitudinal study is the method that gathers data on a subject for a particular period of time and the subject's response to particular variables.A cross-sectional study is where more than one subject is used for the collection of data at different points in time in response to particular variables.These types of studies are sometimes used to determine correlation. longitudinal study and a cross-sectional study are methods of collecting scientific data.