In October 2002, a computer-generated random sample of 10,000 individuals (sampling rate 2.3%, N = 434,690), aged 18 and over, was extracted from the Grampian CHI. Each individual was allocated a unique, anonymised study identifier. Demographic information, including deprivation levels based on Jarman scores, was stored on a secure, confidential database set up on behalf of NHS Grampian.
The lead author, who is also the Data Manager for the Faculty of Medicine and Medical Sciences, University of Aberdeen, has an honorary position with NHS Grampian and liaises directly with the Director of Public Health for the administration of all research studies involving CHI. Under this arrangement, no patient-identifiable information is passed to researchers without prior written consent from potential subjects. For this particular study, only anonymised data was provided to other members of the research team. The questionnaire mailings were administered by the lead author from a confidential database.
The confidential database was used to generate letters from the Grampian Director of Public Health, inviting people to agree to receive either a postal or electronic version of a questionnaire about communicating their views on NHS issues for a research study by a consortium of Grampian Local Health Council, Health Services Research Unit, University of Aberdeen and NHS Scotland. An information leaflet and a consent form, including a section for the provision of an e-mail address, were enclosed with the invitation letter, together with a pre-paid reply envelope to be returned to the Director of Public Health.
Non-respondents at the consent stage are referred to in this manuscript as "Group A". No reminder letters were sent, in accordance with local confidentiality guidelines. People who gave consent but no e-mail address, are referred to as "Group B" and were sent a postal questionnaire with a pre-paid reply envelope to be returned to Grampian Local Health Council. Postal questionnaires were despatched on receipt of subject consent. The questionnaires were fairly short, consisting of 4 pages (16 main questions, most of which had multiple choice response options and some which included boxes for free text comments). People who consented and provided an e-mail address were allocated alternately to "Group C" (sent a postal questionnaire) or "Group D" (sent an e-mail to enable them to access the electronic questionnaire). The e-mail message contained a hyperlink to the electronic questionnaire, an access code and a unique study identifier that served as a username to allow on-line completion of the questionnaire. For the postal questionnaire groups, if no response was received within 3 weeks, reminder letters were sent out with a duplicate questionnaire and pre-paid envelope. Similarly, reminder e-mails were sent to non-respondents in the electronic questionnaire group.
The confidential study database was used to record mailing and receipt dates for consent forms, to administer the mailing of questionnaires and reminders, and to record mailing and receipt dates for these. Electronic questionnaire data was recorded on the web server together with a date and time stamp. These were downloaded weekly from the web server to update the database.
Response rates to the consent form were calculated after a period of 10 weeks had elapsed following the initial mailing, and to the subsequent postal and electronic questionnaires after 3 weeks (first questionnaire) and a further 7 weeks (reminder questionnaire) had elapsed. In the light of recent experience with similar research studies, 10 weeks for each stage of the study was considered sufficient for the vast majority of people to respond to the consent form and subsequent questionnaire, respectively. However, 4 responses to the consent form (0.04%) were received several weeks later and were excluded from the survey. Individual response times were calculated as the time elapsed in days between the mailing of the consent form or questionnaire and receipt of a reply.
Median response times and interquartile ranges (IQR) were calculated for return of consent forms and questionnaires.
Response rates by age, gender and postal area deprivation status were analysed for each stage of the study. Odds ratios (OR) were calculated, together with 95% confidence intervals (CI) using male, 1970–79 and no deprivation as reference groups. A 5% significance level was used throughout for statistical comparisons and the Chi-squared (χ2) test used to compare categorical data.