It is well known that accurate estimates of population information can be ascertained through the use of a sample of the population, rather than having to census the entire population [3]. However, to ensure that the information received from a sample is representative of the underlying population, the sampling strategy must be chosen carefully to ensure that the underlying population structure (age, gender, postal area) is accurately represented [4]. Of the various sampling frames available for population-based surveys, those containing both individual names and associated demographic data are particularly valuable because they make it possible to use personalised mailings and stratified sampling (sampling that accounts for the underlying structure of the population) to compare respondent and non-respondent profiles. The UK electoral roll is one possible sampling frame but this holds very little demographic information and has long been acknowledged to be a biased sampling frame because it excludes foreign nationals and those choosing not to register. A better sampling frame is Scotland's Community Health Index (CHI) (equivalent to Primary Care Trust (PCT) patient registration databases in England and Wales), which contains details of all people registered with a general practitioner, together with data such as age, gender and postal area. These sampling frames have been used extensively in the past with success, achieving response rates of 70–80% [5,6]. However, the introduction of a number of legislative reforms in recent years [7-9], has resulted in severe limitations being imposed on access to these registers [10]. For example, due to a legal challenge to the release of personal details from the electoral roll, access is now only allowed to an edited register, which excludes people withholding consent to be listed, thus introducing an additional bias in the demographic profile [11].

In particular, the strengthening of legislation and guidance to protect the confidentiality of individual data has restricted the ways in which previously recorded information about individuals can be used for secondary purposes [7-9]. In the health field, "Caldicott Guardians" (senior health professionals appointed to be responsible for ensuring that patients' rights to confidentiality are respected) are now required to obtain consent from individuals before releasing their demographic information to researchers [12]. In Scotland, Directors of Public Health act as Caldicott Guardians for the CHI. Within our region (NHS Grampian), confidentiality protocols now require that potential survey participants identified from the Community Health Index are sent a letter from the Director of Public Health asking them for their consent for a researcher to contact them directly. This is an "opt in" process. Non-respondents to this initial approach are assumed to have withheld consent and further contact with them (even to issue a reminder letter) is not permitted.

The implications of these new requirements for the conduct and findings of surveys are as yet unclear. In this paper, we examine response rates, non-response bias and response times for initial requests for consent and subsequent self-completion questionnaires in the context of a population-based survey carried out in Grampian according to the new protocols.

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.

Differences between the demographic profiles of respondents and non-respondents in this study were more marked at the consent stage than at the questionnaire stage. Respondents at the consent stage were more likely to be older, female and not living in postal areas of deprivation.

For both consent and questionnaire stages, the majority of replies were received within a 4-week time interval. The use of reminders, which was only possible at the questionnaire stage, did appear to increase the overall response rate, which is consistent with previous studies [14].

Low response rates, such as that seen in this study, are not a regular feature of surveys conducted among the population under study. Previous surveys that have used the Grampian CHI as a sampling frame and sent questionnaires about health service topics directly (without a consent stage) have achieved response rates of 70–80% [5,6]. The questionnaire stage of this study achieved a response rate at the upper end of this range – although it was based on a self-selected subset of the original population sample.