PADS, Variables and Formulae

ISAE allows the analyst to define the various key probabilities as formulae. This uses a set of values called Parametric Variables. Once the model is complete ISAE allows the Parametric Variables to be manipulated to see how this affects the probabilities at the other end of the model. For example if the failure of a smoke alarm is related to the life expectancy of a dry cell battery, then add a variable to state so. Then after the model is complete you will be able to see the effect of upgrading the quality of the cells has on the overall failure.

PADs - Parametric Datasets

Each set of these Parametric Values is known as a PAD. ISAE 7 can have many different PADs. The name of the PAD currently being used is shown after the title Project Explorer in the Explorer Screen. A list of the PADs that have already been set up is shown under Hazard Processing in the Explorer Screen. To add a new PAD do a RH mouse click on the PADs icon and select Add new PAD. PADs can also be imported and exported.

The current PAD is highlighted. To change to a different set of Parametric variables, do a RH mouse click on the new PAD name and select Set as Current. The new values will automatically be used when editing the Parametric Formula of a Symbol or when generating an ALF. Other functions that can be carried out on the PAD itself are:

Change the name
Delete it
Generate a report showing all the current values

Setting Parametric Variables

The system administrator can define Parametric Variables which are the foundation building blocks defining the inputs to your system.

A Parametric Variable is a representation of a value that is replaced by a pneumonic code. This code can be used in formula, and has the advantage of being able to be changed with immediate effect on all relevant formulae.

To define the Parametric Variables within a PAD, do a RH mouse click on the PADs icon in the Explorer and select Edit PAD Details.

A list of the Variables already defined is shown.

Using the buttons on the right, new variables can be added and old ones edited or deleted.

When editing or adding a new Parametic Variable, the following screen is shown:-

The following fields can be entered:

Reference	Unique name of parameter
Global Parameter	Is this parameter to be allowed to be used global.
Description	A detailed description of the field to assist the analyst in selecting the correct parameter.
User Prompt	This is the question that will be asked when setting up the parameters for a scenario.
Default	This is the default value that will be used when the user offers no values for a scenario.
Units	These are the units for the variables eg. Km, :1, per month
Maximum	The maximum value the user is permitted to enter for a scenario
Minimum	The minimum value the user is permitted to enter for a scenario
Stochastic	Add additional information to allow stochastic modelling to the ALF.
PADs	This shows the different values of this parameter defined in each PAD. This allows the Model to be applied to completely different environments.

Entering a Parametric Formula in a Symbol

From the relevant Symbol Screen (Barrier, Critical Event or Base Event) select the Parametric data button. The above Screen is shown.

Enter the formulae in the Formula box as you would in Excel, save for the initial equals sign. The form allows you to immediately evaluate the formula, and the system will warn if the resultant value is greater than one. (This is a probability after all) You should therefore take care in creating the formulae. If the probability is linked directly to a Parametric Variable then enter the name as the formula.

There is a risk that if every user is allowed to enter Parametric Variables an unwieldy list soon grows. In previous large projects the list was reviewed, compiled and entered by the system administrator. This proved to be very successful, so ISAE now only allows a system administrator with the ability to add and delete parameters.

If the value is acceptable from the formulae, click on the …Write Back button to save the value as the barriers probability. The Parametric Variables can also be changed from this Screen as shown on the previous page.