Add, edit or delete nearness criteria

A nearness case is made of a collection of criteria which define the cuts and properties that will be used to determine the similarity between an assay (the Comparison Assay) and the Reference Assay. The criteria in the nearness case are managed using the icons below the Criteria section:

• - add a new criterion to the nearness case

• - edit the selected criterion

• - remove the selected criterion from the nearness case

  

  Tip
  Right-click the column headers in the Criteria section dialog to group parameters.

  

  By grouping together cuts or properties, it is easier to see related settings together.

  

Each nearness criterion is defined by the following information:

• Cut - the cut of whole crude that will be used for comparison

• Property - the property value that will be used for comparison

• Scale - a number that defines the range the property value of the comparison assay may differ from the reference assay value and still be considered similar; click the to automatically calculate a value based on the spread of cut/property values for Database assays

• Method - whether the property value of the comparison assay and reference assay are compared directly (Linear), or using a logarithmic scale (Logarithmic)

• Weighting - the importance of the this particular criterion to the overall Nearness calculation.

Each criterion evaluates to a number between 0 and 1, where a value of 1 indicates that the cut/property value of the reference assay is equal to the cut/property value of the comparison assay. Values close to 1 indicate for this criterion the comparison assay is similar to the reference assay.

To calculate the overall Nearness, the nearness for each individual criterion is combined using a weighted average. See Nearness calculation.

Scale

The scale of the nearness criterion is used to define how much the property value of the assay being compared to the reference assay may differ from property value of the reference assay. That is, it defines the range of property value that may be considered 'near'.

After setting the Cut and Property click the Calculate Scale button to automatically determine an appropriate value for the Scale. This is based on the spread of the cut/property value for the assays present in the Database.

The Nearness Calculation shows how the Scale is used in the calculation of each Nearness criterion. The Nearness value for each criterion is inversely proportional to the Scale for that criterion. Increasing the value for the Scale increases the range over which the comparison assay property value could differ from the reference assay property value and still be considered near. For a single criterion, if the property value of the comparison assay is already close to the property value of the reference assay then a change in Scale will not have a significant effect on the criterion Nearness, as the assays are already 'near'. However, for a comparison assay with a significantly different property value, increasing the Scale would result in a significant increase in Nearness. See Nearness calculation.

Note: Increasing the Scale increases the number of assays that return a high Nearness value for the particular criterion.

The following table shows the result of changing the value of Scale when comparing the freeze point of the kerosene fraction using Escravos as the reference assay, and comparing Brent Blend and Arab Light.

Method

Two options are available for the calculation method:

• Linear - the property value of the reference assay is compared directly to the property value of the comparison assay

• Logarithmic - the logarithm (base 10) of the property value of the reference assay is compared to the logarithm of the property value of the comparison assay.

  The logarithm method should be used when the property profile increases exponentially. For example, when comparing viscosities where the cut property values for an assay may vary substantially it may be more appropriate to compare the logarithm value rather than the original value.

Weighting

The Weighting of the criterion indicates its importance to the overall Nearness calculation. Each criterion evaluates a Nearness value, and these values are combined to give a single overall Nearness value. Each separate Nearness value is combined using its weighting, so Criteria with high weightings have more influence on the final Nearness value. See Nearness calculation.

For example, when comparing crudes similarities between their yield profiles would often be considered important, and so yield criteria would often have high weightings associated. Similarities between metal content may also be considered, but would probably not be as important, so these therefore would have a lower value for their weighting.

Tip: After creating a new Nearness Case, close the Nearness Manager and immediately add the new Nearness type to the current Database View with a standard reference assay. Re-open the Nearness Manager and add the different criteria to the Nearness Case as normal. After adding each criterion click the Apply button to see the effect the criterion has on the Nearness column in the Database view for many assays at once.