Methods
The Million Women Study is a population-based cohort study. Women are recruited when they are invited for routine breast cancer screening, and the main outcomes to be examined at follow-up are the incidence of screen-detected breast cancer and cause-specific mortality.

Attendance at screening
About three-quarters of the women who are invited for screening by the NHSBSP subsequently attend for mammography [1]. Before the study could be launched, it was necessary to demonstrate that inviting women to join the Million Women Study would not reduce uptake of screening offered by the NHSBSP. During 1994 and 1995 a total of 6000 women who were due to be invited for breast cancer screening in Oxford and West London were randomly divided into two groups. One group was sent the usual invitation for screening and the other group was sent the study questionnaire, accompanying the usual invitation to screening. Attendance rates for screening were similar, at 71%, among those who were and were not sent an accompanying questionnaire [2].

Recruitment procedures
Women are asked to join the Million Women Study by participating NHSBSP screening centres at the time that or just before they are sent their usual invitation for routine breast cancer screening. A questionnaire is included with each woman's invitation and, if the woman wishes to join the study, she is asked to complete the questionnaire, to give signed permission for follow-up, and to return the questionnaire at the time she is screened. A freephone number is provided for women who have any questions or problems filling out the questionnaire. The questionnaire is four pages long (A4 size) and includes questions about lifestyle and sociodemographic factors, reproductive history, past use of oral contraceptives, use of HRT, past medical history and family history of breast cancer. Completed questionnaires are transferred periodically from the participating screening centres to the study co-ordinating centre at the Imperial Cancer Research Fund Cancer Epidemiology Unit (CEU), Oxford, UK.

Data storage, entry and checking
The confidential completed questionnaires are stored securely at all times. Once they reach the CEU they are checked and coded by trained staff and then scanned electronically. The scanned data are 'captured' using computerized intelligent character recognition and optical mark reading software (Eyes and Hands®; Readsoft Inc, Slough, UK). Range and logical checks are performed at the time of data entry. Any inconsistency or information that is not recognised by the data capture software is verified manually by trained data entry staff, who also validate computer-interpreted data and check each questionnaire to confirm whether signed consent for follow up has been granted. Each week the verified data for about 50 individuals are checked against the original questionnaires and the error rate is consistently below 1%. This partially automated process thus permits data to be entered rapidly and with high accuracy.

Follow up for breast cancer
Each screening centre of the NHSBSP is required to compile annual statistics on its activities, which include details of all breast cancers detected at mammography [1]. A list of women enrolled into the Million Women Study at each centre is cross-checked at regular intervals against the list of the women diagnosed with screen-detected breast cancer at that centre. If a breast cancer has been diagnosed at screening in a study participant, routinely recorded details of the cancer are abstracted, including tumour location, histology, size, grade, invasive status and involvement of axillary lymph nodes. Information on hormone receptor status and treatment is abstracted when it is available. Several approaches are being used to identify breast cancers diagnosed subsequent to screening. One will involve record linkage with cancer registry data. Also, women will be contacted directly 2-3 years after they were screened, and asked about new illnesses, including any new breast cancers, that may have been diagnosed (see Additional follow up, below). This will permit the identification of both screen-detected and interval cancers.

Follow-up for deaths
Deaths are identified annually by computerized matching of name, date of birth and NHS number of the women who gave signed consent for follow up in the Million Women Study, with the national death files held by the Office of National Statistics. For each death thus identified the date of death and underlying and associated causes of death are provided by the Office of National Statistics.

Additional follow up
Participants will be sent a follow-up questionnaire about 2-3 years after recruitment, to ascertain changes in use of HRT and incident morbidity, for example breast cancers, diagnosed outside the screening programme.

Validation
The most important variables for this study are the subjects' identification details, their use of HRT, any diagnosis of breast cancer and the recording of deaths. To assess the accuracy of the subjects' identification details and of the recording of deaths, a random sample of 5000 women recruited in 1996 has been selected for flagging on the NHS Central Register (NHSCR). Identification details recorded for the study (name, address, date of birth and NHS number) enabled all but 10 (0.2%) of the 5000 women to be identified on the NHSCR. The completeness and accuracy of the reported deaths will be validated in the future against those recorded in the NHSCR for these 5000 women.
The reliability of diagnosis of screen-detected breast cancers is monitored by various quality control procedures within the NHSBSP. Screen-detected breast cancers are verified according to defined procedures, and the invasive status, size and type of cancer are recorded for virtually 100% of the cancers.
The validity of reported information on use of HRT, including the type and dose, is being examined and a full report will be published in due course. Preliminary comparisons with the prescription records from one general practice in Oxfordshire indicate at least 95% agreement for reported current use of HRT, including the hormonal constituents of the preparation used most recently (Banks et al, unpublished data).