At a glance

Performing ISSA 5000 engagements requires applying professional judgement over inherent uncertainty.

Practitioners, preparers and users require understanding of what assurance can and cannot provide.

The report illustrates how practitioners can apply certain requirements of ISSA 5000.


By bringing together key ISSA 5000 requirements with respect to estimates and forward-looking information and a practical case study, the report aims to promote a consistent and proportionate approach to assurance in this challenging area. It is intended to assist assurance practitioners, preparers and users in understanding what assurance over estimates and forward-looking information can and cannot provide.

The assurance challenge 

ISSA 5000 recognises that while estimates and forward-looking information are conceptually different the approach to testing them often involves similar assurance procedures.

Unlike historical information, which reflects actual events or outcomes, estimates and forward-looking information are based on assumptions about future conditions and events that may or may not occur. They are therefore subject to greater measurement or evaluation uncertainty.

Estimates and forward-looking information cannot be evaluated with the same level of precision as historical information. As uncertainty increases, the focus of assurance moves away from simple verification of outcomes and towards assessing the reasonableness of assumptions, methods and disclosures.

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Under ISSA 5000, when providing assurance over sustainability information, the practitioner needs to consider what constitutes sufficient and appropriate evidence, given the inherent uncertainty and judgement involved. This is particularly relevant in the case of estimates and forward-looking disclosures, given their exposure to greater measurement or evaluation uncertainty.

The expectation gap reflects the difference between what users often believe assurance can deliver and what it is actually designed to provide under professional standards. This gap has the potential to be particularly significant for sustainability-related information, and even more so for estimates and forward-looking information, because such information is inherently less grounded in objective, observable data than historical financial information. For that reason, the practitioner has a responsibility to be transparent about what assurance over this type of information means, including its scope and limitations.

Key requirements

Key requirements under ISSA 5000 include:

  • Determining whether estimates and forward-looking information are capable of assurance.
  • Identifying and assessing risks of material misstatement.
  • Responding to identified risks under limited and reasonable assurance.
  • Evaluating disclosure including uncertainty and neutrality.
  • Forming the assurance conclusion and communication limitations.

Summary of key requirements

  • Confirm preconditions, appropriateness of subject matter, and suitability/availability of criteria.
  • Assess risks of material misstatement (limited: disclosure level; reasonable: assertion level).
  • Respond to assessed risks with procedures appropriate to the level of assurance.
  • Apply the dedicated requirements for estimates/forward-looking information (limited vs reasonable).
  • Evaluate evidence reliability and the description of criteria, which are central to transparency about uncertainty.
  • Form the assurance conclusion on the basis of sufficient appropriate evidence.

Key learnings from our case study 

The report is in the form of a case study following same fictional entity, ABC plc, building on the scenario introduced in the first guide (See Demystifying materiality in accordance with ISSA 5000).

The case study illustrates how practitioners apply ISSA 5000 when assuring estimates and forwardlooking information, demonstrating both limited and reasonable assurance procedures and highlighting key areas where professional judgement is required.

The case study focuses on how the practitioner responds to identified risks of material misstatement and applies professional judgement when performing procedures and evaluating evidence relating to underlying estimates and forwardlooking information.

Limitations of our approach 

This report has been developed to support understanding of how the requirements of ISSA 5000 may be applied when providing assurance over estimates and forward-looking information. It is intended as practical guidance, rather than as a comprehensive or prescriptive interpretation of the Standard and does not replace the need for professional judgement in individual engagements.

The effective date for ISSA 5000 applies to assurance engagements on sustainability information reported for periods beginning on or after 15 December 2026, or as at a specific date on or after that date. Earlier application is permitted. Consequently, practical experience with the application of ISSA 5000 in this area remains limited, and approaches to applying the Standard are expected to continue to evolve as assurance practitioners, firms and regulators gain experience.

Practical consideration and recommendations 

Practitioners need to apply robust planning, maintain strong professional scepticism throughout the engagement, and clearly communicate the basis, limitations and nature of the information being assured, to support users’ understanding.

Practitioners should be mindful of how assurance conclusions are framed, ensuring that users understand what assurance can and cannot provide in relation to targets, projections and transition plans.

Sustainability disclosures increasingly extend beyond historical data and incorporate estimation uncertainty, methodological choices, scenario assumptions and strategic commitments such as emissions reduction targets and transition plans.

Further reading