# Risk Evaluation

Risk evaluation is the method to measure and compare risk .

Risk is originally a psychological index . The step of risk evaluation starts from the definition of risk from the view of mathematics and engineering.

## Factors

Risk is not measured directly. Basic factors of risk are "the generation provability" and "the size of effect".

There is a case that "the generation provability" or "the size of effect", itself is also called "risk". In this page, risk is defined the quantity calculated from "the generation provability" and "the size of effect".

In reliability engineering , there are more factors. For example, "unevenness of the generation provability and the size of effect" and "easiness to find the generation". But I explain only "the generation provability" and "the size of effect" below.

## Units of indexes

Units of "the generation provability" and "the size of effect" are some kinds.

For the unit of "the generation provability" is not always "percent".
When we measure with experience, the unit is "pointing". For example,"Rare is 1. Frequent is 5."

Pointing is used for "the size of effect" too.

Pointing is basic measuring in Questionnaire .

## Definition of Risk

There are three definition of risk, Multiplication, addition and scatter plot. They use the factors, "the generation provability" and "the size of effect".

Multiplication type is popular. But in the view of risk cognition , addition type and scatter plot type is close to human sense.

### Multiplication Type

In multiplication type,
"Risk" = "the generation provability" * "the size of effect"

This type is used in Risk of Chemical Substances and Risk of Machines . Some methods use more than two factors, bur concept is same.

"Risk" = "the generation provability" + "the size of effect"

In this type, units of factors are same. If different units are used, it is wrong in the sense of dimension analysis .

### Dot Plot Type

In scatter plot type, "Risk" is not a quantity.
"Risk" = The place of plot of "the generation provability" and "the size of effect" on the graph

This type is called "risk map" and "risk matrix".

We take quick decision making in daily life. Dot plot type may be the closest model of our sense.

## Scenario

To calculate risk, we make scenarios of generation of risk. If we make "all" scenarios, the sum of provability is "1".

For Risk of Machines , the range of scenario is in a factory or a machine. But for Environmental Risk , the range is very wide.

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