Unclear essential parameters
An inescapable trap at the EPO, or is there another way of assessing this issue?


EPO practice is strict on the assessment of the clarity of parameters, seemingly increasingly so following changes to the Guidelines for Examination made in 2019. 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 in question 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 a so-called “unclear essential parameter” trap, in which European patent applications may be refused purely on formal grounds, regardless of their substantive merit. This article 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 which may be more innovator-friendly.

This is the full-length version of a series published by IAM in three parts (see the first, second, and third parts).



The “inescapable trap” is a well-known issue at the EPO which can prove fatal to granted patents. This occurs when a granted patent is found to contain an amendment which adds new matter relative to the application as filed, but which cannot be un-done 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) EPC. It is impossible to fix this problem without contravening Article 123(3) EPC. A patent caught in this trap must be revoked, regardless of the substantive validity of the patent.

A further trap at the EPO is when an independent claim includes a parameter which does not meet the rigorous standards set by the EPO for the clarity of parameters, and can hence be referred to as the “unclear essential parameter trap”. Unless there is sufficient detail provided in the description on how to measure that parameter, such a claim is likely to be refused during examination for contravening Article 84 EPC. It is typically difficult to fix this problem by deleting the problematic parameter from the claim without contravening Article 123(2) EPC. A claim caught in this trap is likely to be refused, regardless of the substantive merit of that claim. Although the legal framework around this trap is long established, it is an issue which is becoming increasingly problematic in EPO prosecution. There appears to have been a change in EPO practice following amendments in 2019 to the Guidelines for Examination, discussed further below.

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

It should be noted that parameters are often attacked during opposition for insufficiency (Article 83 EPC), and may provoke objections under this ground during examination. This article 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. Instead, this article discusses how the strict assessment of clarity and requirement for unambiguous measurement is applied in such a way as to affect many different parameters across different technical fields.

As an example, suppose an application is drafted where each independent claim refers to a polymer having molecular weight in a certain range. The skilled person is aware of how to measure the molecular weight of that polymer. It may well be that a polymer having an appropriate molecular weight can be purchased from a chemicals supplier. However, unless that application contains a measurement method for the molecular weight, and unless that measurement method contains enough detail to meet the strict EPO standards, that application may well be caught in the trap and refused solely for that reason. In other jurisdictions, this might never be an issue.



The unclear essential parameter trap requires three elements. Unsurprisingly, these are (i) a parameter that is (ii) unclear and (iii) essential to the invention. These elements are discussed in turn.

Requirement (i): a parameter

A “parameter” refers to a property that needs to be measured. Typically, this is anything associated with a number. Examples of common parameters include particle size, molecular weight, viscosity, etc.

There is a class of parameter, so-called “unusual parameters”, discussed in the EPO Guidelines, F‑IV, 4.11.1. These are parameters which are not commonly used in the field of the invention and which are difficult to compare with the prior art, for instance because they have been “invented” by an applicant in an attempt to differentiate its alleged invention from the prior art. Such parameters are typically viewed with suspicion by the EPO, often provoke clarity objections, and often effectively ignored when the Examiner assesses patentability. This article is not concerned with unusual parameters. Although unusual parameters can trigger the trap, the main concern with the trap is that it can also be triggered by the use of parameters that are entirely normal in the field of the invention, i.e. “usual parameters”. This article is concerned with how the use of these “usual parameters” can be fatal to an application, if the application does not provide enough detail on how to measure them.

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, an invention of a new type of battery chemistry. The early applications in this field may be able to pursue valid claims on the basis of the chemistry alone. However, as the field is progressed, further inventions may build on that chemistry, which is no longer new, perhaps optimising the performance by controlling the microstructure. Later applications in the field may thus require the use of parameters to establish patentability over the earlier applications, specifying certain particle sizes, porosities, surface areas etc. 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 not drafted with an appropriate level of detail in the measurement methods.

Requirement (ii): an unclear parameter

The second requirement for the trap is for the parameter to be unclear. The EPO is particularly strict on the assessment of the clarity of parameters. This is nothing new – the legal framework around the unclear essential parameter trap is long established. However, the changes made in 2019 to how the Guidelines deal with the clarity of parameters appear to be encouraging Examining Divisions to take an even stricter view. The changes essentially merged two sub-sections of the previous Guidelines, incorporating the comments on measurement methods from previous F-IV, 4.18 into current F-IV, 4.11. Although in principle nothing much has changed, anecdotally, there has been a change in practice at the EPO. We are seeing increasing numbers of clarity objections to “usual” parameters which might not have been raised before. Reasons for this perceived stricter practice can be found in subtle changes to the wording of previous F-IV, 4.18 when it was incorporated into current F-IV, 4.11. Previous F-IV, 4.18 stated In principle the method of measurement is necessary for the unambiguous definition of the parameter” (emphasis added). The term “in principle” perhaps suggesting some flexibility in how strictly the rule should be applied. In contrast, current F-IV, 4.11 states unequivocally “the method for measuring a parameter (or at least a reference thereto) must appear completely in the claim itself”, the term “must” perhaps implying that the rule should now be enforced rigidly. Moreover, the EPO categorised[1] these changes to the Guidelines as part of “major amendments” which had “substantially rewritten sub-sections on the practice regarding the use of parameters in claims”, also implying that a shift in practice was intended.

The Guidelines provide two exceptions to the requirement for a measurement method to be included in a claim. These are:

(a) the measurement method to be employed belongs to the skilled person’s common general knowledge, e.g. because there is only one method, or because a particular method is commonly used; or

(b) all the measurement methodologies known in the relevant technical field for determining this parameter yield the same result within the appropriate limit of measurement accuracy.

However, proving that one of these exceptions applies to a given parameter may be difficult. In some circumstances, applicants may be able to submit experimental data and/or expert declarations to show that one of these exceptions applies, but that is often not possible. Accordingly, an Examining Division may insist that a claim including a parameter is amended to include a method for measuring that parameter. Although this might seem a straightforward fix, it is here where the difficulty lies. There are many scenarios in which an application which might appear, on the face of it, to have been drafted in a reasonable way, is in practice unsuitable for supporting such an amendment under the strict EPO rules.

Missing a measurement method

In one scenario, the application fails to include any reference to a measurement method, in which case evidently a claim cannot be amended to include one. This may occur if the parameter in question is considered “usual” enough by the attorney and/or inventor such that including a measurement method was omitted at the drafting stage. Although that circumstance would suggest that one of the exceptions (a) and (b) would apply, convincing an EPO examiner that this is the case is another matter. In particular, the omission of a measurement method at the drafting stage may be based on a gut feeling of “everyone knows how to measure that”; however, substantiating that gut feeling with documentary evidence is what is required to demonstrate to the EPO that one of the exceptions applies. This can be a particular problem for applications drafted outside Europe in a jurisdiction which has a more relaxed attitude towards the clarity of parameters and in which it is simply not necessary to include a measurement method for a “usual” parameter. An application drafted with a parameter in claim 1, but without a measurement method in the description, may be caught in the trap.

A measurement method missing detail

In a second scenario, the application refers to a measurement method but fails to include sufficient detail to satisfy the EPO that the parameter is clear. A common example in this regard is molecular weight. There are a number of general methods for measuring molecular weight (e.g. size-exclusion chromatography (SEC), gel permeation chromatography (GPC), light scattering etc.). An applicant might have considered that referring to one of these methods would be enough to clarify a claim with a molecular weight parameter, thereby meeting the requirement in the EPO Guidelines for the claim to include a measurement method. However, some decisions from the EPO Boards of Appeal conclude that the value of the molecular weight depends not only on the general measurement approach, but also on the specific conditions used for the measurement method (e.g. T 541/09, followed in T 2063/12). In some cases it has been concluded that the molecular weight obtained by GPC is highly dependent on conditions such as the sample concentration and calibration standard. Therefore, if an application merely refers to a generic way of measuring molecular weight, for example “molecular weight may be measured by GPC”, then this may still not provide sufficient detail about the measurement method to overcome an objection that the parameter is unclear. An application drafted with a parameter in claim 1 and a measurement method in the description, but without enough detail on that method, may be caught in the trap. Indeed, in practice we are seeing EPO examiners citing this case law as support for insisting that a claim including a molecular weight parameter must be provided with a detailed measurement method.

A measurement method incorporating trademarks

In a third scenario, the application includes a detailed measurement method but the level of detail leads to further issues. For instance, that method may refer to equipment or reagents by their trademarks. Amending a claim to include that measurement method may provoke further clarity objections because the EPO Guidelines only allow trademarks to be used in claims under very narrow circumstances (F-IV, 4.8). The use of trademarks in claims can also lead to insufficiency objections (F-III, 7).  Therefore care must be taken when drafting all aspects of a measurement method, keeping in mind that the measurement method may need to be incorporated into a claim in Europe.

A standard measurement method

In a fourth scenario, the application has been drafted with reference to a standard measurement method for a parameter, for instance an ISO or ASTM method. In many cases this approach is adequate, and the measurement method can readily be incorporated into a claim, if needed, to satisfy the EPO. The examiner is very likely to accept, without further investigation, that a standard method contains enough information for the unambiguous measurement of the parameter. However, for some standard measurement methods, a simple reference to the standard in the patent application may still fail to provide the level of detailed required. For instance, the temperature at which a parameter is measured may affect the result. A standard measurement method may not mandate a single temperature at which to measure; instead, it may allow a range of possible temperatures but stipulate that the measured value should be reported along with the temperature at which it was measured. In such instances a basic reference solely to the standard method may not be enough to clarify a parameter; the temperature of the measurement must be specified as well (and possibly further variable conditions). Another example of this pitfall is the measurement of the glass transition temperature of polymers by differential scanning calorimetry, in which the heating rate can affect the result. This can be a fatal flaw at the EPO. Patents have been revoked, even though they refer to standard testing methods, for not providing enough information on the measurement conditions (e.g. T 412/02 and T 1252/08). Accordingly, an application drafted with a parameter in claim 1 and a reference to a standard measurement method for that parameter in the description, may still be caught in the trap.

In summary, EPO practice typically requires a measurement method for a parameter to be included in a claim. However, it is not always straightforward to do this, e.g. if an application was drafted without a measurement method or if one was provided but without enough detail.

Requirement (iii): an essential parameter

The third requirement for the trap is that the parameter is deemed 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) EPC). Whether or not a feature can be removed from an independent claim is a long-standing issue at the EPO and this article is not intended to be an in-depth discussion of that case law. It suffices 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.

If the parameter cannot be retained in the claims without a measurement method, if the description is missing a measurement method or does not provide enough detail for the reasons given above, and if the parameter cannot be removed from the claims without adding matter, then an application is caught in the trap, and is likely to be refused. In such situations, there is no remedy for an application which 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 where exceptions (a) and (b) of F-IV, 4.11 do not apply. This is any parameter which might plausibly be measured in different ways, and when measured in different ways might provide significantly different results (i.e. not “the same result within the appropriate limit of measurement accuracy” as referred to in exception (b)). Many parameters can fail to meet these EPO-specific requirements, although this may depend on the specific facts of any particular invention. In practice, the trap is triggered when an examiner raises a clarity objection during examination. Accordingly, a key factor to 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 this issue. 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 EPC) is not a ground for opposition, the assessment of parameters in opposition is typically handled under the ground of insufficiency (Article 83 EPC), although parameters introduced into the claims from the description during the opposition 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 their 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 to a parameter.

What are the policy reasons for the trap?

So, we’ve seen how the strictness of EPO practice on the clarity of parameters coupled with strict practice on added matter can be fatal to an application, which might give the impression that the EPO is an overly harsh environment for applicants. However, it should be stressed that there are sound policy reasons for the EPO’s approach. Clarity is important. Article 84 EPC is there for a reason. A parameter with an unclear or missing measurement method can be open to widely different interpretations. A third party reading a claim should readily be able to assess whether it infringes that claim, but if a key feature in the claim is a parameter which is ambiguous in the absence of a properly defined measurement method, then it can be difficult or impossible for the third party to perform that assessment. An article might infringe a claim if measured by one method but not if measured by another. Accordingly, there is clear justification for the EPO’s insistence on parameters in claims being accompanied by an unambiguous measurement method.

Nevertheless, there is a balance to be struck between, on the one hand, providing certainty to third parties on the scope of protection by taking a strict view on parameters and, on the other hand, taking a more liberal view and minimising the likelihood of denying an applicant just reward for its invention purely due to a missing or incomplete measurement method. There is no easy answer to the question of where to set this balance. However, there is a possible alternative approach to the assessment of parameters which would tend towards a more applicant-friendly liberal view. This alternative approach is already taken by the EPO in other areas of claim interpretation, and arguably finds some support in the case law.

Is there another way of looking at parameters?

This alternative approach would be not to insist on a measurement method appearing in a claim but, instead, to interpret any parameter in a claim without a measurement method as encompassing that parameter when measured by any reasonable method. Such an interpretation – a “broadest reasonable interpretation” – would be deemed clear and meeting the requirements of Article 84 EPC, allowing examination to move on to the more substantive aspects of novelty and inventive step.

Under this approach, the applicant would have a degree of control as to how it intends the parameter to be interpreted. A detailed measurement method could be included in a claim, unambiguously specifying how that parameter is to be interpreted. Alternatively, an applicant may choose not to include a measurement method, knowing that its ambiguous parameter would be interpreted in the broadest reasonable manner. The latter option would arguably lead to a weaker claim – it would be broader, such that additional prior art may come into play compared to a claim with an unambiguous parameter, making it harder to establish novelty or an inventive step. In addition, the ambiguity may cast doubt over whether a technical effect is achieved across the full scope of the claim, adding to the difficulty in establishing an inventive step. Equally, the ambiguous parameter could be so broad that, if it is central to making the invention work, there would be a potential insufficiency issue.

Accordingly, adopting the broadest reasonable interpretation of a parameter would not amount to allowing an applicant to “get away with it”, if the applicant has not provided enough detail on how to measure its parameter. Instead, the applicant would have to accept the potentially severe negative consequences arising from such an approach, which would continue to encourage applicants to provide the requisite detail when drafting their applications. Given that the broadest reasonable interpretation would appear to solve the problem of applications being refused solely for not giving a detailed measurement method, it would seem an attractive option for both the EPO and its users.

On the other hand, however, an applicant may well be able to overcome the negative consequences arising if an ambiguous parameter is interpreted as clear, but broad. For example, any novelty, inventive step and insufficiency problems arising due to excessive claim breadth could be addressed by adding further limitations to the claim. It may be that the excessive breadth of a parameter is never an issue during examination, perhaps if the prior art evidently discloses the same parameter such that the argumentation is focussed on other features. In such instances, the negative consequences might not be so severe that they would eliminate the problem of applicants drafting applications with ambiguous parameters. Accordingly, allowing the broadest reasonable interpretation as a route for deeming parameters to meet the requirements of Article 84 EPC may not avoid ambiguous patents being granted, to the detriment of the certainly of third parties. This brings us back to precisely the reason why the EPO takes a strict view on the assessment of the clarity of parameters.

The broadest reasonable interpretation approach nevertheless finds support in other areas of EPO practice concerned with claim interpretation. For instance, a new section added to the Guidelines in 2021 (F‑IV, 4.24) adopts precisely this interpretation when assessing terms such as “identity” and “similarity” in relation to amino or nucleic acid sequences. It states the following (emphasis added):

Amino acid or nucleic acid sequences can be defined by a percentage of identity. The percentage of identity determines the number of identical residues over a defined length in a given alignment. If no algorithm or calculation method for determining the percentage of identity is defined, the broadest interpretation will be applied using any reasonable algorithm or calculation method known at the relevant filing date.

 Amino acid sequences can be defined by a degree of similarity (expressed as a percentage of similarity). The term similarity is broader than the term identity because it allows conservative substitutions of amino acid residues having similar physicochemical properties over a defined length of a given alignment. The percentage of similarity is determinable only if a similarity-scoring matrix is defined. If no similarity-scoring matrix is defined, a claim referring to a sequence displaying a percentage of similarity to a recited sequence is considered to cover any sequence fulfilling the similarity requirement as determined with any reasonable similarity-scoring matrix known at the relevant filing date.

A parallel can be drawn between traditional parameters and identity/similarity. Each is expressed by a numerical value which may differ depending on how the parameter is measured, or how the identity/similarity is calculated. For a parameter with no measurement method, the Guidelines suggest that the parameter is likely to be unclear. However, for identity/similarity, the Guidelines state that any reasonable calculation method can apply, implying that the identity/similarity is clear, but broad. Accordingly, in their current form, the Guidelines seem to be applicant-friendly for identity/similarity but applicant-hostile for other parameters. Were the EPO to consider softening its stance towards parameters, it could look to identity/similarity as a guide for how to do this.

The broadest reasonable interpretation approach also finds support in the case law on parameters, albeit in decisions concerning insufficiency (Article 83 EPC) rather than clarity (Article 84 EPC). For instance, T 2001/10 adopted the following interpretation of a particle size parameter with a missing measurement method, which had been attacked for insufficiency (Reasons 22, emphasis added):

In the absence of any definition in the patent in suit of the exact meaning of “average particle diameter” or of the specific method to be used for determining that parameter, the “average particle diameter” defining compound (D) in present claim 1 must be interpreted as encompassing any definition of that parameter and any method for its measurement which can be said to be standard in the art concerned.

Notably, the Board highlighted the possible negative consequences to the applicant of the broadest reasonable interpretation, stating the following (Reasons 26, emphasis added):

The absence of any additional information concerning the meaning of “average particle diameter” or of any specific method to be used for determining that parameter may result in a larger number of elastomeric particles meeting the requirements of “average particle diameter” of 10 to 700 nm, than would be the case where a specific definition or measurement method were given. This will have to be taken into consideration, should the “average particle diameter” be held to distinguish the claimed subject-matter from the prior art and/or considered essential for solving a technical effect vis-à-vis the closest prior art.

This decision shows that the broadest reasonable interpretation is already being used by the Boards of Appeal as a way of conferring a technically sensible meaning on a claim which recites a parameter, in the absence of a measurement method. Since this approach, and its possible negative consequences to the applicant, are already a part of EPO practice, applying this approach to the assessment of the clarity of parameters should be straightforward.


Summary and outlook

The current practice of the EPO’s Examining Divisions is to insist that a measurement method is provided in a claim. The clear justification for this approach is to avoid the grant of patents with ambiguous parameters. However, this does mean that some applications, drafted without a full understanding of EPO practice, will be refused on formal grounds. An alternative approach is available, namely, the broadest reasonable interpretation approach, but this also has its drawbacks.

The decision of an Examining Division to refuse an application for falling into the unclear essential parameter trap is subject to appeal. If appeals are filed in such cases, it remains to be seen whether the Boards of Appeal, when assessing parameters under the heading of clarity and Article 84 EPC (as opposed to the more common assessment under insufficiency and Article 83 EPC in an inter partes appeal) will tend more towards the “broadest reasonable interpretation” approach as a means for dealing with an ambiguous parameter, or whether they will follow the strict practice encouraged by the Guidelines in order to retain certainty for third parties. In other words, the Boards will need to decide which is worse: applications being refused for the sole reason that a parameter in claim 1 was not provided with enough detail on a measurement method, regardless of the substantive merit of that application; or patents being granted with ambiguous parameters.


Practice points

Although the broadest reasonable interpretation does offer a way out of the unclear essential parameter trap, the fact remains that current EPO practice is strict on the assessment of the clarity of parameters, such that the trap is very much a live issue. With the EPO Guidelines in their current form, i.e. requiring a claim to include a measurement method unless one of two narrow exceptions apply, examiners have little choice but to follow the Guidelines, potentially resulting 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 risking further objections in Europe.

This is particularly important for parameters in independent claims, but also applies to any parameter which might conceivably be used in the claims. For applications that have not been drafted in this way, applicants should be prepared for a rough ride.



[1] Guidelines for Examination in the European Patent Office (status November 2019), showing modifications, List of sections amended in 2019 revision – 1


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