Wednesday, 15 July 2015

Rounding conventions: what is the value of 1?

Smith & Nephew Plc v Convatec Technologies Inc [2015] EWCA Civ 607 (24 June 2015) is a patent case which turns on how to read a numerical range. The patent, which related to wound dressings in which silver - a known antimicrobial agent - was applied to the fibres. To prevent discolouration, the patent specified the use of a binding agent, at a concentration between 1 per cent and 25 per cent.

The trial judge, Birss J, interpreted this (using the conventions that I thought I had remembered from school maths lessons) to mean between 0.95 per cent and 25.5 per cent. But the Court of Appeal knew better (or at least they picked up on the argument).

Of course, like all interpretations of patent claims, the right approach is that set out in the Protocol on Interpretation of Article 69 - which requires us to consider what the person skilled in the art would think the claim was intended to mean.

It came down to whether the right approach was a "whole number" approach or a significant figure approach. The whole number approach would say that 1 per cent means all figures greater than or equal to 0.5 per cent and less than 1.5 per cent - or to put it another way, all values that round to 1 per cent when expressed in a whole number. The significant figure approach was explained by Kitchin LJ, who started by noting that the rules formed part of the common general knowledge: 
    i) non-zero digits are always significant;
    ii) zeros between non-zero digits are always significant;
    iii) leading zeros are never significant; if a decimal point appears in a number then trailing zeros are significant (before or after the decimal point);
    iv) in the absence of a decimal point, trailing zeros are not generally significant unless stated otherwise either expressly or with a bar over the zero.
Applying these rules, 1 per cent indicates a range from 0.95 per cent to 1.5 per cent. It is asymmetric because the values that round to 1 must comprise two significant figures: if there's only one significant figure, you can't round it up or down. This is how Birss J got to the conclusion that the defendants' 0.77 per cent concentration did not infringe. He reckoned that the skilled person would use the significant figures approach. The Court of Appeal preferred to think that the skilled person would assume that the values were expressed to the nearest whole number, so there was an infringement.

Why, oh why, didn't the person who drafted the claims (and the description, of course, as it has to support the claims) either write "up to 25 per cent" or "1.0 to 25 per cent" (especially when the teaching of the patent was that as little as 0.01 per cent would do the trick)? I'm sure they had their reasons, just as the patent agent who wrote the Catnic patents had his reasons for writing "vertical" (or was it "vertically"?). However, in neither case are the reasons known, while in both of them the effect was significant, indeed determinative, in the patentee's favour in both cases, though only just in the earlier one. Cases like these cause me to reflect on the received wisdom about patent claims, that they are technical documents written for technical people and not suited to the sort of legal scrutiny that a conveyance should get (I forget the precise quote). But in fact they are complicated legal documents (often incredibly so) written by highly-trained professional patent agents (soi disant attorneys, indeed) and to my mind that means that meticulous verbal analysis (there, I remembered the words!) is entirely appropriate. And the Protocol is wrong!



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