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5. AgeX population

5.1 Eligibility for cluster randomisation

Women were eligible for cluster randomisation if they were aged 47-49 or 71-73 (see Section 3.2.1 for the definition of age) and registered with an NHS general practitioner (a necessity for routine NHS breast screening invitations).

5.2 Patient throughput from randomisation to inclusion in the main analyses population

The flow of invitees and controls from randomisation to inclusion in the main analyses population is shown in Table 1, which gives more details than a standard CONSORT flow-chart. Briefly,  women were included if, when randomised, they were the right age, alive, linkable to routinely collected NHS England electronic records, without a history of cancer or other breast disease, and (based on their previous screening history) likely to accept if invited. For, inclusion of women who would be unlikely to accept if invited would dilute any effects of screening, reducing the sensitivity of analyses that compare all invitees vs all controls. The requirement of being likely to accept if invited excluded older women who had missed their previous breast screening invitation (or whose last invitation was not 1.0-5.0 years ago). It also excluded younger women who had no NHS record of cervical screening in the past 5 years (despite having no NHS record of having had a hysterectomy).

All these exclusion criteria were based unbiasedly on information recorded before randomisation, all were defined blind to analyses of mortality differences between invitees and controls, and no exclusion criterion was significantly unbalanced between invitees and controls (Table 1).

5.3 Follow-up, and loss to follow-up

Information on ICD-coded mortality and cancer registration, and on which participants still being followed up, is sent to AgeX regularly every 3 months by NHS England. The average lag in this is assumed in AgeX analyses to be 3 months for death and 15 months for cancer diagnosis. Other linked datasets (Section 6.2) generally arrive about annually.

If a woman chooses to opt out of having her further NHS records used for research then she will cease to be mentioned in all subsequent datasets sent to AgeX (and even her opt-out will not be mentioned), so the AgeX mortality and cancer onset analyses stop 3 and 15 months before she was last included.

If a woman is recorded by NHS England as having moved permanently or semi-permanently from England to another part of the UK (or abroad) then this too is reported, and the AgeX analyses stop at her emigration date unless death, cancer or return is reported.

Analyses of mortality and cancer incidence will include all cases reported to AgeX, regardless of whether follow-up was thought to have stopped. Any slight uncertainty about effective numbers at risk does not bias the comparison of invitees versus controls. Estimated proportions lost to follow-up for reasons other than mortality will be plotted against year since randomisation.

5.4 Baseline characteristics

The only baseline characteristics planned to be used are those that defined the exclusion criteria used in Table 1. Further subdivision of the younger and the older women by age would not be usefully informative, as the age ranges 47-49 and 71-73 are extremely narrow.

In this section