We undertook a cross-sectional study in March-April 2022 with a target sample size of 2,000 adults aged 50-79 years old residing in Spain. Sample selection was randomized using Spanish landline telephone contacts. The sample size was random and we aimed at a proportional sample to the population according to the following variables: municipality size (under 10,000, between 10,000 and 50,000, between 50,000 and 100,000 and over 100,000 inhabitants), age (50-64 and 65-74), sex (men, women), and highest educational attainment (four educational groups are distinguished).
The field work was undertaken by Opiniòmetre. Data were collected by trained interviewers using a standardized computer-assisted telephone interview (CATI). Interviews took place between March 2nd and April 12th and lasted, approximately, 15 minutes. Because of the different timing of filling the quotas by educational attainment and some difficulties in reaching the lowest educated groups, the quotas for the highest educational groups were opened in the last days of the fieldwork. When picking up the telephone, the interviewer introduced him/herself on behalf of Opiniòmetre, briefly explain the project aims and the data protection regulations (confidential, anonymous and uniquely used for statistical purposes) (Appendix 1). The interview was in Spanish or either in Spanish or Catalan in the Catalan speaking regions, upon the choice of the interviewee.
The sample was collected using a Computer Assisted Telephone Interviewing (CATI). In total 49,742 telephone numbers were used. Most telephone numbers could be reached within the first three attempts (45,425, 91.3%), whereas up to 15 attempts were done in order to get in contact others. Out of the used telephone numbers, 2,011 (4.0%) have completed the interview, 29,077 (58.5%) could not be reached, 15,537 (31.2%) rejected to participate, 2,570 (5.2%) could not continue the interview as they were either out of the age range or the quotas were fulfilled, and a remaining 1.1% did not participate for other reasons (company telephone, second residence). These are reasonable numbers for a study of such characteristics given the limited age range and the quotas to be fulfilled.
The DDHealth questionnaire has four main modules. The first module is the sociodemographic module, and next to age and sex, there are questions related to the socioeconomic position such as highest educational attainment or financial constraints. The second module focuses on digital divide aspects such as internet use, frequency of internet use or use of internet for medical purposes. The third module contains questions related with health and wellbeing, for instance self-reported health, chronic conditions or smoking. Finally, the last module contains the 16 questions belonging to the short health literacy questionnaire from the EU (HLS-EU-Q16).
