CMI Working Paper 200

We released the latest version of the CMI Mortality Projections Model, CMI_2024, on 30 June 2025, accompanied by Working Paper 201. 

We issued Working Paper 197 in February 2025 which consulted on our proposed method for CMI_2024:

• Extending the existing age-period-cohort improvement (APCI) model to introduce multiple period terms, with three period terms and a linear basis; 
• Adding a “fitted overlay” to the APCI model to model the impact of the COVID-19 pandemic on mortality;
  • We proposed the name “age-period-cohort-overlay improvement” (APCOI) model for the version of the APCI model which includes the overlay. 
• Applying cohort constraints after fitting the APCOI model, rather than as part of the fitting process; and
• Changing the algorithm used to fit the APCOI model to be quicker and more robust.

Working Paper 200 summarises the responses to the consultation and our decisions for CMI_2024.

While respondents to the consultation have a range of views, the responses as a whole are supportive of the proposals set out above. 

CMI_2024 takes a similar approach to our proposals in Working Paper 197, using multiple period terms and a fitted overlay with the updated fitting algorithm and cohort constraint method. However, the published version differs in the following ways:

•  The APCOI model has five period terms with a quadratic basis, rather than three period terms with a linear basis.
• We have shortened the length of the cohort convergence period for those aged 21-59 at the start of the projection. The cohort convergence period is 10 years for ages 20-30 and then increases linearly to 40 years at age 60.
• We have changed the implementation of cohort constraints so that the transition is smooth rather than piecewise-linear as initially proposed.
• We have made a minor technical change to the method used to fit the APCOI model. This improves numerical accuracy when solving the system of linear equations but has negligible impact on results.
• We have made the single equivalent period smoothing parameter an Advanced rather than an Extended parameter.

The Committee hosted a webinar on 17 July 2025 to discuss CMI_2024. A link to the recording of the webinar is available in this document. The presentation slides are available here and we have published a document that answers a number of questions asked using the webinar Q&A feature that the presenters ran out of time to answer.

Documents

• CMI Working Paper 200
• Spreadsheet of values used in charts in Working Paper 200
• CMI MPC July 2025 webinar recording document

This paper and accompanying documents are available to Authorised Users only.