Weight average fields |
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Weight average fields |
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Weight average fields |
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Weight average fields |
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If a data field is specified as being weight average, then data is accumulated up the database structure and in totals using an average which has been weighted against some other relevant data field. This means that the data field values from each of the component records contribute to the result, but to varying degrees. The following example of a branch of database structure illustrates how a weight average data field is accumulated for an upper level record.
Some examples of weight average data fields are "Fe2O3 Grade" and "Insitu Coal Ash".
When a data field is assigned as weight average, you must also assign a Weighting Field. The weighting field is selected from the drop down list on the Field Type window.
Note that the data field must have the same activity number as the weighting field and the weighting field must be an additive type. Consequently, the drop down list of weighting fields is restricted to those additive data fields with the same activity number as the data field that you are assigning as weight average.
Zero values for the weight average field (not the weighting field) may or may not be included in the determination of the weight average value, by deselecting or deselecting the Include Zeros box on the Field Type screen.
Once you have decided that a data field is weight average, you must then select the field that it is to be weighted against. There is a rule which is useful in making this decision.
The units for any weight average parameter can be thought of as:
"Type 1 Units" per "Type 2 Units"
The weighting field should always be the parameter represented by the "Type 2 Units", ie. the numerator. This is best illustrated through examples.
| • | "Gold Grade" is a weight average field and the units for it are "grams of gold per tonne of ore". Hence, it should be weighted against tonnes of ore. |
| • | "Insitu Coal Ash" is a weight average field and the units for it are "percent by weight". If the units are thought of as "tonnes of ash per tonne of insitu coal", then it can be seen that it should be weighted against tonnes of insitu coal. |
| • | A parameter which is often weighted against the wrong data field is "Density" (eg. coal or ore). The units are "tonnes per bcm" and it should therefore be weighted against the volume of material, not the tonnes of the material. |
| • | "Area" and "Thickness" often cause confusion when it comes to the weighting field. Either one of these parameters can be weight average, depending on the database structure. When summing up a number of panels which are located side by side and together make up a pit, the area should be additive whereas the thickness should be weight average. Alternatively, for a layered deposit where you may need to sum up a number of seams into a strip, the area would be weight average and the thickness would be additive. These two examples can occur within one database and it is therefore sometimes necessary to assign a different field type for each level in your database. |
| • | Having decided whether to weight area or thickness, you then need to determine which field to weight them against. Using the rule, "Area" can be thought of as "bcm of volume per metre of thickness" and it should therefore be weighted against thickness. Similarly, "Thickness" can be thought of as "bcm of volume per square metre of area" and it should therefore be weighted against area. |