If the two pandas have a cub, the new contract states the Memphis Zoo does not have to pay China a yearly fee on that. However, the cub will have to be returned to China after four years.
Panel study. A panel study is a longitudinal study design where a cross-sectional sample of units is selected and surveyed at usually regular intervals. The observation units may be individuals, households, firms or other organisations, or even countries. Household panel surveys are an important sub-type of panel study.
1 Cohort study or Panel study: a particular form of longitudinal study where a group of patients is closely monitored over a span of time.  Ecological study: an observational study in which at least one variable is measured at the group level.