|
<< Click to Display Table of Contents >> Navigation: 2. ShipWeight Basics > 2.9 The Item window: Entering Weight Information > 2.9.30 Risk Analysis |
ShipWeight Basics > The Item Window: Entering Weight Information
Use Risk Analysis in the Items window to inspect the combined weight uncertainty of selected items. The analysis reads a custom code field containing a standard deviation for each selected item, then opens the Risk Analysis window with Gaussian and cumulative S-curves.
Configure the standard-deviation field
Create or choose a numeric custom code field for item standard deviation in Code Definition. Give it a clear title, for example Weight standard deviation, with kg or % in the title to match the chosen interpretation. The field must contain numerical uncertainty values, rather than category IDs or text descriptions.
Open View > Options in the main window, select Codes, enable Item std.dev. field and choose this code type in the adjacent list. This identifies the custom code field as the source for item uncertainty; naming the field alone does not enable the analysis.
Choose how this code field is interpreted with the Percentage checkbox. Leave it cleared for absolute standard deviations in kg, or select it for standard deviations expressed as percentages of each item's weight. This choice applies to the configured code field for all analysed items; do not mix kg and percentage values in the same field. Changing the checkbox does not convert values already stored in the field.
Click OK to save the Options settings. Add the selected code field to the Items view setting if necessary so that you can enter and inspect its values.
See also: 4.13 The Code Definition Window
Enter uncertainty values
Absolute value (Percentage cleared): Enter the standard deviation as a non-negative value in kilograms. For example, an item weighing 1,000 kg with a standard deviation of 50 kg should contain 50 in the configured code field. Use kg for the source value even if item weights or the graph are displayed in another unit.
Percentage selected: Enter a non-negative percentage as a number: enter 5 for 5%, not 0.05. The program calculates each item's absolute standard deviation as item weight multiplied by the percentage divided by 100. For an item weighing 1,000 kg, a value of 5 therefore gives a standard deviation of 50 kg.
For example, an item weighing 12,877.050 kg with a code-field value of 10 has a standard deviation of 1,287.705 kg, displayed as 1,287.71 kg when two decimals are used. For a single item, Minimum and Maximum are equal.
Both alternatives specify one standard deviation, not a maximum possible error or an added weight margin. In percentage mode the program converts each selected item's percentage separately before combining the standard deviations; it does not apply one percentage to the total selection.
Use values that reflect the uncertainty of the item weights. Review missing or blank values before analysing the selection; a missing value should not be treated as evidence that an item has no uncertainty. At least one positive standard deviation is needed for a useful curve.
Select items and open the graph
Select the required rows in the Items window. Check the selection after applying a filter: the analysis uses the selected rows, rather than automatically including every item in the project.
Choose View > Risk Curve, or right-click the selection and choose Risk Curve. The Risk Analysis window shows the combined weight and identifies the selected weight groups/items.
Minimum: Show the curve calculated by combining independent item uncertainties: square the individual standard deviations, add them and take the square root.
Maximum: Show the curve calculated by adding the individual standard deviations directly, representing fully positively dependent variations.
Gaussian: Show the probability-density curve. A broader curve represents greater uncertainty.
S: Show the cumulative curve. At a chosen weight limit, its height estimates the probability of being at or below that limit under the normal-distribution model.
For example, two selected items weighing 1,000 kg each, with standard deviations of 50 kg each, give a combined weight of 2,000 kg. Enter either 50 with Percentage cleared or 5 with Percentage selected for each item. Both methods give Minimum approximately 70.71 kg and Maximum 100 kg. If the graph displays tonnes, the corresponding values are 2 t, 0.07071 t and 0.1 t.
The two curves describe different dependency assumptions; they are not minimum and maximum possible item weights. Changing the curve options does not edit the selected item weights or code values.
For graph interpretation, printing and copying, see 3.6.1 The Risk Analysis Window.
If the window does not open or no useful curve appears
Check that Item std.dev. field is enabled in Options > Codes, that a code type is selected, that the intended item rows are selected, and that Percentage matches the meaning of the stored code-field values. Absolute values must be entered in kg; percentages must be entered as numbers such as 5 for 5%. Close and reopen Risk Analysis after changing the source values or the interpretation.