Question

2B- From Theory and Practice in Policy Analysis, Chapter 8 : What are the differences between risk, variability, and uncertainty? What do you think of the distinctions the author makes between different types of uncertainty - are they useful categories? What problems have you encountered where a proper treatment of uncertainty was important (and maybe was lacking)? What do you think about the methods of integrating uncertainty described here? Are they practical and useful? Do you the author's suggestions at the end to be useful?

Answer 1

Difference Between Risk and Uncertainty

The following are a few differences between risk and uncertainty:

• In risk you can predict the possibility of a future outcome while in uncertainty you cannot predict the possibility of a future outcome.
• Risk can be managed while uncertainty is uncontrollable.
• Risks can be measured and quantified while uncertainty cannot.
• You can assign a probability to risks events, while with uncertainty you can’t.

Example:

Let us say there are two well-known football teams consisting of renowned players, and they are going to play a football match the next day.

Can you tell me exactly which team is going to win?

Obviously not. However, you can make an educated guess by reviewing and analyzing the past performance of each individual player, the team, and the results of matches they played against each other.

Here you can come up with some number like there is a 40% chance of Team A or Team B winning, or there is a possibility of Team A or Team B losing the match by 70%.

Now, let us put the same football match in a different scenario.

Let us say again that two football teams are going to play a game, and no players have been selected for either team.

In this situation, if somebody asked you which team is going to win, what would you say?

You’re completely clueless. You don’t know which team consists of which players, and you have no idea how the teams will perform, etc.

In this situation, you don’t have any past information, are totally clueless, and hence cannot predict the outcome of the event, even though the match is the same, the rules are the same, and even the stadium is the same.

This situation is called uncertainty.

Difference Between Variability and Uncertainty

Variability

• Refers to the inherent heterogeneity or diversity of data in an assessment. It is "a quantitative description of the range or spread of a set of values" (U.S. EPA, 2011), and is often expressed through statistical metrics such as variance, standard deviation, and interquartile ranges that reflect the variability of the data.
• Variability cannot be reduced, but it can be better characterized.

Uncertainty

• Refers to a lack of data or an incomplete understanding of the context of the risk assessment decision. It can be either qualitative or quantitative (U.S. EPA, 2011).
• Uncertainty can be reduced or eliminated with more or better data.

For example, body weight varies between members of a study population. The average body weight of the population can be characterized by collecting data; collecting an exact measured body weight from each study participant will allow for better understanding of the average body weight of the population than if body weights are estimated using an indirect approach (e.g., approximating based on visual inspection). But, the assessor cannot change the individual body weights of the study population, and therefore cannot decrease the variability in the population.

Uncertainty can be qualitative or quantitative. Qualitative uncertainty may be due to a lack of knowledge about the factors that affect exposure, whereas quantitative uncertainty may come from the use of non-precise measurement methods. Chemical concentrations in environmental media can be approximated using assumptions (more uncertainty) or described using measured data (less uncertainty). Uncertainty can be introduced when defining exposure assumptions, identifying individual parameters (i.e., data), making model predictions, or formulating judgments of the risk assessment.

variability and uncertainty affect risk assessment

When variability is not characterized and uncertainty is high there is less confidence in the exposure and risk estimates; characterizing variability and reducing uncertainty increases the confidence in the estimates. A risk assessment report should also address variability and uncertainty to increase transparency and understanding of the assessment. Addressing variability and uncertainty can inform decision makers about the reliability of results and guide the process of refining the exposure assessment.

However, not all exposure evaluations are of the same complexity, and thus the level of complexity in evaluating uncertainty and variability can vary from one assessment to another. A tiered approach, starting with a simple assessment, is sometimes used to determine whether additional evaluation is required to further address uncertainty and variability. See the Tiers and Types Tool Set of EPA ExpoBox for more information on using a tiered approach for exposure assessment.