Jaeger's recommendation to leave the definition of an indicator open and to determine the status of potential indicators on pragmatic rather than strict definitional grounds is a wise one. An education system can be conceived as having underlying properties that are not directly or perfectly measurable. For example, we can talk about the quality of the teaching force but also recognize that there is no direct way to measure it. At best, several statistics can be combined into an indicator that gets at our notion of teacher quality. Education indicators are statistics that reflect important aspects of the education system, but not all statistics about education are indicators.
Statistics qualify as indicators only if they serve as yardsticks.
That is, they must tell a great deal about the entire system by reporting the condition of a few particularly significant features of it. For example, the number of students enrolled in schools is an important fact, but it does little to tell us how well the education system is functioning. However, data on the proportion of secondary students who have successfully completed advanced study in mathematics can provide considerable insight into the health of the system, and can be appropriately considered an indicator. Another central concept in the discussion of indicators is that of the indicator system. Whether indicators are single or composite statistics, a single indicator can rarely provide useful information about such a complex phenomenon as schooling.