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Frequently Asked Questions
E. Scientific
Denominator Data
DD - FAQ0001

Q: What denominators does the Mandatory Surveillance publication use to calculate rates of infections?

A: Population based analysis i.e. CCG attributed cases, Community-onset cases etc, use the available mid-year estimates provided by ONS, these can be found here. The mid-year estimates are annual estimates by calendar year, and therefore must be adjusted accordingly to reflect the period of time the analysis is being carried out for, see examples below:
  • monthly analysis of Jan – mid-year population data should be adjusted to reflect 31 days
  • quarterly analysis of Apr – Jun data – mid-year population data should be adjusted to 30 days in Apr, 31 days in May and 30 days in June = 91 days
  • the entire mid-year estimate is only used to calculate an annual rate using counts from Jan-Dec of the same year.

    • Trust based analysis i.e. Trust-apportioned cases, Hospital-onset cases etc are calculated using bed-days as the denominator for rate calculations. The bed-days denominator is calculated using KH03 data which is published here. The data is reported by Trust, and the collection is not managed by PHE. Guidance documents on the KH03 data is available at the same web address and a contact email address. Please note it is the Trusts responsibility to ensure that any changes within the organisation that affect bed days have been considered, as the KH03 data is submitted by the Trust. Only in extremely rare circumstances will we adjust published KH03 data.