To answer this, we'll be using a panel data set called the National Longitudinal Survey of Youth (NLSY), which is a data set that is comprised of individuals who were in their teens or early twenties in 1979. Respondents have been surveyed almost every year from 1979 until the present. So, most individuals are represented by more - indeed, many more - than a single observation. One observation might be "Dan in 1979," a second might be "Dan in 1980," and so forth. So the level of observation is the person-year, and the number of observations equals the total number of unique individuals in the data times the number of years in which those individuals were surveyed