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Combined Certificates
(IWTO 31)

Multiple Certificates can be combined, provided they have been tested for Yield and contain all the tests you want to include on the Combination Certificate. The combined results are obtained using the calculations prescribed by the IWTO.


Combining Yield Results

Wool base, VM, Yield and Clean Weights are all calculated separately using the results from the component lots. Due to this process and the rounding up or down of results, the IWTO formulated results from the Combination Certificate do not always correlate with a manual calculation.

An example of this is the Combined Clean Weight which is simply the sum of the Clean Weights of the component lots. The Combined Yield is calculated separately so the Combined Clean Weight will not always match the Combined Yield on the Combination Certificate e.g.

  Component 1 Component 2 Combined
Nett Weight (Kg) 12000 14000 26000
Wool Base (%) 60.00 50.00 -
VMB (%) 1.0 1.5 -
IWTO 16%
Clean Weight (Kg) 8688 8554 17242

IWTO Combined Yield = Combined Clean Weight x 100 / Combined Nett Weight

IWTO Combined Yield = 17242 x 100 / 26000

IWTO Combined Yield = 66.3%

A manual calculation of 66.3% of 26000kg would give 17,238 kg whereas the actual IWTO formulated Combined Clean Weight is 17,242 kg.


Combining Mean Fibre Diameter, colour or length results

All component lots for any Combination must have a Yield test. This is because when any results are combined, we use clean weight averages of the lots to calculate the combined results e.g. if 2,000 kg of 24.0 micron wool was added to a 24,000 kg lot of 30.0 micron wool, the average would not simply be 27.0 microns as there is a great deal more of the 30.0 micron wool going into the Combined lot. Therefore, if you wish to combine only Mean Fibre Diameter, Colour, Conditioning or Length test results, you must have a complementing Yield test for all component lots to determine the clean weight of each lot. This applies to all wool lots including scoured and carbonised. N.B. Colour results are only valid for 6 months after testing, if you wish to add a colour result which is older than this onto a Combined Certificate, the wording "Colour reading expired" must be included on the Certificate.


Subtraction Combinations

If bales have been removed from a test lot after it has been cored, the sample will still contain cores from the omitted bales. To deduct bales from a lot, they must be re-cored and tested. Once tested, the same calculations are applied as when combining lots, the only difference being that the calculated weighted average results of the deducted portion are subtracted instead of added. Up to 5% of the total Nett weight of the original lot can be removed using a Subtraction Combination. If more than 5% of the lot has been removed, the sample is no longer representative of the lot and the remaining bales will therefore require re-coring and testing. Only two lots can be used in a Subtraction Combination but it is possible to remove bales from an existing Combination so lots can be combined and then bales can be removed. The Certificate for the subtracted portion is just like any other test Certificate and can be used to add the bales onto another lot if required.