Being clear on unclear parameters – how to stay clear of patent traps at the EPO

EPO practice appears increasingly strict with regard to the assessment of parameter clarity following 2019 changes to the Guidelines for Examination. The EPO typically requires that a measurement method for a parameter is included in the claim itself. However, this may not be possible if the application was drafted without consideration of the EPO-specific practice in this area. This strict practice, coupled with existing strict practice on added matter, leads to the so-called ‘unclear essential parameter’ trap, in which European patent applications may be refused purely on formal grounds regardless of their substantive merit. 

This three-part series considers the various ways in which a parameter may fall foul of the trap, the justification for the strict EPO practice in this regard and a possible alternative approach that may be more applicant-friendly.  


The inescapable trap is a well-known issue at the EPO, which can prove fatal to patents. It occurs when a patent is found to contain an amendment that adds new matter relative to the application as filed but which cannot be undone without broadening the scope of protection relative to the granted claims. A patent caught in this trap has nowhere to go. It is invalid due to the amendment contravening Article 123(2) of the European Patent Convention (EPC) and is impossible to fix without contravening Article 123(3) of the EPC. A patent caught in this trap must be revoked regardless of the substantive validity of the patent. 

Another potential pitfall at the EPO as mentioned above – is when an independent claim includes a parameter that does not meet the rigorous standards for clarity. Unless there is sufficient detail in the description on how to measure that parameter, such a claim will likely be refused during examination for contravening Article 84 of the EPC. 

However, it is often difficult to fix this problem by deleting the parameter from the claim without contravening Article 123(2) of the EPC. A claim caught in this trap is thus likely to be refused regardless of its substantive merit. Although the legal framework around this trap is long established, it is an issue which is becoming increasingly problematic in EPO prosecution. In addition, there appears to have been a change in EPO practice following amendments in 2019 to the Guidelines for Examination (discussed further in the second article in the series).   

The unclear essential parameter trap is, in principle, less fatal than the added matter inescapable trap. The key issue is one of clarity. As such, the unclear essential parameter trap typically arises during examination rather than opposition and there is typically more freedom for the applicant to amend the claim and resolve the issue (eg, as Article 123(3) of the EPC is not in play). Nevertheless, if an application is drafted such that the unclear parameter is essential to the invention (eg, if it is in claim 1 and all independent claims), in practice this unclear parameter can be just as fatal to a pending application as the added matter inescapable trap is to a granted patent. 

Parameters are often attacked during opposition for insufficiency (Article 83 of the EPC) and may provoke objections on this ground during examination. This series is not an in-depth discussion of the sufficiency of parameters which, under current EPO practice, typically depends on the importance of the parameter to achieving the claimed invention. Rather it focuses on how the strict assessment of clarity and requirement for unambiguous measurement affects many different parameters and technical fields.  

As an example, suppose an application is drafted where each independent claim refers to a polymer having molecular weight within a certain range. A skilled person would know a method of measuring the molecular weight of that polymer. However, unless the application sets out a measurement method for the molecular weight, and unless that measurement method contains enough detail to meet the strict EPO standards, that application may fall into the trap and be refused. In other jurisdictions this might never be an issue. 

The unclear essential parameter trap thus requires three elements: a parameter that is unclear and essential to the invention. These elements are discussed in turn. 

Requirement one: a parameter

A ‘parameter’ refers to a property that requires measurement, typically anything associated with a number. Examples of common parameters include particle size, molecular weight and viscosity. 

The EPO Guidelines (F IV, 4.11.1) include a class of so-called ‘unusual parameters’. These are parameters that are not commonly used in the field of the invention and that are difficult to compare with the prior art, perhaps because they have been created by an applicant in an attempt to differentiate their invention from the prior art. Such parameters are typically viewed with suspicion by the EPO, they often provoke clarity objections and are frequently ignored when the examiner assesses patentability. 

Although unusual parameters can trigger the trap, the main concern is that it can also be set off by the use of parameters that are entirely normal in the field of the invention (ie, usual parameters).  

Parameters are commonly used in claims, perhaps increasingly so as a technical field matures. One can envisage an invention opening up a new technical field, say, the invention of a new type of battery chemistry. Early patent applications in this field may be able to pursue valid claims on the basis of the chemistry alone. However, as the field fills up, further inventions may build on that chemistry, perhaps optimising performance by controlling the microstructure. Later applications may therefore require the use of parameters to establish patentability over the earlier applications, specifying certain particle sizes, porosities or surface areas. Similarly, for inventions in the field of polymer chemistry, it may be essential to specify molecular weight or viscosity. Such applications may be vulnerable if drafted without an appropriate level of detail in the measurement methods.

Requirement two: an unclear parameter

The second requirement is for the parameter to be unclear. This will be discussed in detail in the second article in the series. 

Requirement three: an essential parameter

The third requirement for the trap is that the parameter is essential to the invention such that it cannot be removed from the claims without contravening the strict EPO rules on added matter (Article 123(2) of the EPC). Whether a feature can be removed from an independent claim is a longstanding issue at the EPO. This series is not intended to be an in-depth discussion of that case law. It is sufficient to say that if an independent claim is drafted including a parameter, it can be very difficult indeed to remove that parameter without provoking an added matter objection.  

The application will be caught in the trap and likely refused if:

  • the parameter cannot be retained in the claims without a measurement method;
  • the description is missing a measurement method or does not provide enough detail for the reasons which will be discussed in the second article; and
  • the parameter cannot be removed from the claims without adding matter.

In such situations, there is no remedy for an application that was drafted without consideration of the EPO-specific practice in this area.

Which parameters can be caught in the trap?

The answer is: any parameter that might plausibly be measured in different ways, which in turn might provide significantly different results. Many parameters fail to meet these EPO-specific requirements although this may depend on the specific facts of a particular invention. In practice the trap is triggered when an examiner raises a clarity objection during examination. Accordingly, a key factor in determining which parameters might be problematic is the opinion of that examiner or perhaps the internal practice taken by a particular technical sub-division of examiners at the EPO. 

Apart from the opinion of the examiner another key factor is the ever-growing body of case law on parameters. A parameter in a claim may be seen as an area of weakness, which an opponent can exploit when challenging a patent. Since clarity (Article 84 of the EPC) is not a ground for opposition, the assessment of parameters in opposition is typically handled under the ground of insufficiency (Article 83) although parameters introduced into the claims from the description during the opposition proceedings are open to attack for lacking clarity. 

One common strategy for opponents is to attempt to show that a claim is insufficient because it contains a parameter without enough detail on how it is measured. Accordingly, over the years there have been a number of decisions on parameters and measurement methods. These include molecular weight (T 541/09, T 2063/12), viscosity (T 808/09, T 2403/11) and particle size (T 45/10, T 83/01, T 2001/10). Although most of these decisions concern sufficiency and indeed some conclude that a parameter is sufficient despite a sub-optimal measurement method, these decisions can be used by an examiner as justification for raising clarity objections during examination.  

Practice points

Current EPO practice is strict on the assessment of the clarity of parameters such that the trap is very much a live issue. Examiners have little choice but to follow the EPO Guidelines in their current form (discussed in more detail next week), which can potentially result in refusals. Accordingly, the practice points for users of the European patent system are abundantly clear: 

  • any parameter used to define the invention should be accompanied by a measurement method; 
  • the measurement method should have sufficient detail to unambiguously define that parameter; and 
  • the measurement method should be drafted in such a way that it can be readily incorporated into a claim without leading to further issues. 

This is particularly important for parameters in independent claims but also applies to any parameter that might conceivably be used in those claims. 

For applications that have not been drafted in this way applicants should be prepared for a rough ride. However, there is an alternative, more applicant-friendly way of assessing parameters which could mitigate these issues. This will be discussed in the third article in the series. 

This is an Insight article, written by a selected partner as part of IAM's co-published content. Read more on Insight

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