3. Sample design
From 1999 to 2015, the survey was conducted with adults aged 18 or over. For the first time in 2016, the age range for the survey was extended to include 16 and 17 year olds to reflect the lowering of the age limit for voting in Scottish elections.
The survey is designed to yield a representative sample of adults aged 16 or over, living in private households in Scotland. The sample frame is the Postcode Address File (PAF), a list of postal delivery points compiled by the Post Office. The detailed procedure for selecting the 2019 sample was as follows:
1. 93 postcode sectors were selected from a list of all postal sectors in Scotland, with probability proportional to the number of addresses in each sector for addresses in urban areas and a probability of twice the address count for sectors in rural areas (i.e. the last 3 categories in the Scottish Government's 6 fold urban-rural classification). Prior to selection the sectors were stratified by Scottish Government urban-rural classification, region and percentage of household heads recorded as being in non-manual occupations (SEG 1-6 and 13, taken from the 2011 Census).
2. 30 addresses were selected at random from each of these 93 postcode sectors.
3. Interviewers called at each selected address and identified its eligibility for the survey. Where more than one dwelling unit was present at an address, all dwelling units were listed systematically and one was selected at random using a computer generated random selection table. In all eligible dwelling units with more than one adult aged 16 or over, interviewers had to carry out a random selection of one adult using a similar procedure.
Addresses that are ineligible or out of scope include: empty / derelict addresses, buildings under construction, holiday homes, businesses, other non-residential (such as schools, offices and institutions), and addresses that had been demolished. Students are included at either their main term-time or their main out-of-term address.