Cross-sectional, longitudinal, and panel data (example):

Consider, for example, a set of 1000 households randomly chosen from all households of New York City. The observed variable is the gross annual income.

A set of 1000 annual income values for year 1995 for each household is an example of cross-sectional data . From such data one could derive information on how income was distributed among households in New York City in 1995.

A series of 10 values of the average annual income of ALL 1000 households for each year since 1991 to 2000 is an example of longitudinal data . From such data one could derive information on how average income changed during the decade from 1991-2000