Indicators may vary greatly in their structure. 

One important distinction in this context is between simple and compound (or composite) indicators. 

Simple indicators, as their name suggests, target a specific element of the system.  Examples (relating to an assessment of air pollution) include:

• in terms of health outcome, hospital admissions for acute respiratory infections;
• in terms of exposures (or more strictly environmental concentrations), mean annual PM_2.5 concentrations. 

Compound indicators, in contrast, combine different elements into a single measure. Examples include:

• in terms of health outcome, disease adjusted life years (DALYs) attributable to air pollution or the monetary costs of air pollution-related health effects;
• in terms of exposure, an overall air quality index (e.g. obtained by combining concentrations of particulates, nitrogen oxides, benzene and ozone).

Compound indicators clearly provide more information, and fewer indicators may therefore be needed to summarise the results of an assessment.  Care is needed, however, in combining the different elements since this almost invariably involves some form of weighting (e.g. on the basis of toxicity or severity).  Since these weights cannot usually be defined on the basis of scientific data, they rely on some form of judgement; the quality of these judgements inevitably determines the reliability of the indicators.  In addition, it may be difficult for users to unravel a compound indicator and understand which of its different elements is really driving any effect or change.