What is uncertainty?

Definitions

Uncertainty and associated terms such as error, risk and ignorance are defined and interpreted differently by different authors, see Walker et al. (2003) for a review. The different definitions reflect the underlying scientific philosophical way of thinking and therefore typically vary among different scientific disciplines. In addition they vary depending on their purpose. Some are rather generic, such as Funtowicz and Ravetz (1990), while others apply more specifically to model based water management, such as Beck (1987).

In this document we will use the terminology of Klauer and Brown (2003) that has emerged after discussions between social scientists and natural scientists specifically aiming at applications in model based water management. More details and discussion on the definitions given below can be found in Klauer and Brown (2003). By doing so we adopt a subjective interpretation of uncertainty in which the degree of confidence that a decision maker has about possible outcomes and/or probabilities of these outcomes is the central focus. For reasons of completeness and comparison we will also briefly sketch the objective interpretation of each form of uncertainty we discuss.

Uncertainty

The notion of uncertainty includes both subjective and objective aspects. Becoming confident or establishing lack of confidence is an act of subjective judgement about the validity of some information. However, the judgement might be supported and informed by the evaluation of ‘objective’ facts and other forms of evidence.

 Definition (Uncertainty): A person is uncertain if s/he lacks confidence about the specific outcomes of an event or action. Reasons for this lack of confidence might include a judgement of the information as incomplete, blurred, inaccurate or potentially false or might reflect intrinsic limits to the deterministic predictability of complex systems or of stochastic processes.

Similarly, a person is certain if s/he is confident about the outcome of an event. It is possible that a person feels certain but has misjudged the situation (i.e. s/he is wrong).

Example: A person may be uncertain about the exact value of a river discharge value due to uncertainty related to the instruments used for measurements, representativeness of measurements and the method of transforming measurements (of often secondary variables) to discharge. Two different people may have different perceptions of the magnitude of this uncertainty.

Note that other authors define the term uncertainty not as a property (state of confidence) of the decision maker but as a property (state of perfection) of the total body of knowledge or information that is available at the moment of judgement. Uncertainty is then seen as an expression of the various forms of imperfection of the available information and depends on the state-of-the-art of scientific knowledge on the problem at the moment that the decision needs to be made (assuming that the decision maker has access to the state-of-the-art knowledge).

Ignorance

Awareness of the information on the potential outcomes of an event is a precondition for any grade of certainty or uncertainty. There are different, gradual stages of awareness starting from awareness of the existence of information to a deep understanding of the information.

Definition (ignorance): A person is ignorant with respect to an event if s/he is unaware of the (potential) outcomes of that event or of the event itself.

Examples: Oestrogens were not suspected of being harmful to fishes until a few decades ago. Clayey till was supposed to be virtually impermeable for pesticides and other contaminants and, therefore, provide good protection of groundwater until it was discovered 10-20 years ago that some clayey layers contain fractures through which pollutants can be transported very rapidly.

Note that ignorance refers to unawareness of the entire scientific community about potential outcomes of an event or side effects of an activity in the interpretation that sees ignorance as a property of the state of scientific knowledge rather than as the state of the individual decision maker.

Risk situations

If uncertainty is recognised as being an important issue, then the most common strategy to cope with this is to use probabilities. However, the use of probabilities presupposes a number of things about the available representation. First, it assumes that all potential outcomes of the event are known. In other words, that the event is properly characterised by the set of potential outcomes. Secondly, it assumes that the probabilities of each outcome are also known. We will call such a situation a risk situation.

Definition (risk situation): A risk situation is a person’s representation of an event, where s/he assumes to know all potential outcomes as well as the probabilities of each outcome.

In some disciplines risk is defined as being equivalent to probability, while in others it is defined as damage multiplied by probability. Our definition is compatible with the latter (but not the first) of these.

Example: A water manager has to decide whether to implement a certain measure; for example, cleaning a polluted aquifer or to make additional field measurements to improve the data basis and, thereby, reduce the uncertainty involved (i.e. two possible outcomes). The water manager believes s/he can calculate the uncertainty (in terms of a probability distribution function) for how polluted the aquifer is and how much this uncertainty will change in case of new data. At the same time s/he knows the costs of making a wrong decision and the costs of the additional field program.

Precaution

The Precautionary Principle (PP) has become an underlying rationale over the past decades for the satisfactory and ethically justified management of uncertain risks to public health, society or environment. The PP aims to protect humans and the environment against uncertain risks of human action by means of pre-damage control (anticipatory measures). The PP is to supplement, but not necessarily replace, other management strategies that fall short of being able to handle large scale scientific uncertainty and ignorance. It is incorporated in a large and increasing number of international treaties and declarations in the fields of, inter alia, sustainable development, environmental protection, health, trade, and food safety. The PP is on its way to becoming a widely accepted part of international law. In its basic form, the Precautionary Principle states that actions to protect human health and the environment against possible danger of severe and irreversible damage, need not wait for rigorous scientific proof (Weiss, 2003).

Definition (precautionary principle): In order to protect the environment, the precautionary approach shall be widely applied by States according to their capabilities. The lack of full scientific certainty shall not be used as a reason for postponing cost-effective measures to prevent environmental degradation where there are threats of serious or irreversible damage.

The triple negative notion in this definition stemming from the Rio Declaration(1992), that the absence of rigorous proof of harm does not justify inaction, is perceived as being weak. It forces the consideration of proactive intervention but does not require such intervention. Other definitions are stronger and put the burden of proof on the proponent of an action to show that it does not pose a danger of environmental harm. For examples, the Wingspread Statement on the Precautionary Principle defines it as follows: “When an activity raises threats of harm to human health or the environment, precautionary measures should be taken even if cause and effect relationships are not fully established scientifically . . .[The] proponent of the activity, rather than the public, should bear the burden of proof.”