Technical details / Sample design

Sample design

The British Social Attitudes survey is designed to yield a representative sample of adults aged 18 or over. Since 1993, the sampling frame for the survey has been the Postcode Address File (PAF), a list of addresses (or postal delivery points) compiled by the Post Office.[1]

For practical reasons, the sample is confined to those living in private households. People living in institutions (though not in private households at such institutions) are excluded, as are households whose addresses were not on the PAF.

The sampling method involved a multi-stage design, with three separate stages of selection.

Selection of sectors

At the first stage, postcode sectors were selected systematically from a list of all postal sectors in Britain. Before selection, any sectors with fewer than 500 addresses were identified and grouped together with an adjacent sector; in Scotland all sectors north of the Caledonian Canal were excluded (because of the prohibitive costs of interviewing there). Sectors were then stratified on the basis of:

  • 37 sub-regions;
  • population density, (population in private households/area of the postal sector in hectares), with variable banding used in order to create three equal-sized strata per sub-region; and
  • ranking by percentage of homes that were owner-occupied.

This resulted in the selection of 261 postcode sectors, with probability proportional to the number of addresses in each sector.

Selection of addresses

Twenty-six addresses were selected in each of the 261 sectors or groups of sectors. The issued sample was therefore 261 x 26 = 6,786 addresses, selected by starting from a random point on the list of addresses for each sector, and choosing each address at a fixed interval. The fixed interval was calculated for each sector in order to generate the correct number of addresses.

The Multiple-Occupancy Indicator (MOI) available through the PAF was used when selecting addresses in Scotland. The MOI shows the number of accommodation spaces sharing one address. Thus, if the MOI indicated more than one accommodation space at a given address, the chances of the given address being selected from the list of addresses would increase so that it matched the total number of accommodation spaces. The MOI is largely irrelevant in England and Wales, as separate dwelling units (DUs) generally appear as separate entries on the PAF. In Scotland, tenements with many flats tend to appear as one entry on the PAF. However, even in Scotland, the vast majority (98.9 per cent) of MOIs in the sample had a value of one. The remainder had MOIs greater than one. The MOI affects the selection probability of the address, so it was necessary to incorporate an adjustment for this into the weighting procedures (described below).

Selection of individuals

Interviewers called at each address selected from the PAF and listed all those eligible for inclusion in the British Social Attitudes sample – that is, all persons currently aged 18 or over and resident at the selected address. The interviewer then selected one respondent using a computer-generated random selection procedure. Where there were two or more DUs at the selected address, interviewers first had to select one DU using the same random procedure. They then followed the same procedure to select a person for interview within the selected DU.

Notes
  1. Until 1991 all British Social Attitudes samples were drawn from the Electoral Register (ER). However, following concern that this sampling frame might be deficient in its coverage of certain population subgroups, a ‘splicing’ experiment was conducted in 1991. We are grateful to the Market Research Development Fund for contributing towards the costs of this experiment. Its purpose was to investigate whether a switch to PAF would disrupt the time-series – for instance, by lowering response rates or affecting the distribution of responses to particular questions. In the event, it was concluded that the change from ER to PAF was unlikely to affect time trends in any noticeable way, and that no adjustment factors were necessary. Since significant differences in efficiency exist between PAF and ER, and because we considered it untenable to continue to use a frame that is known to be biased, we decided to adopt PAF as the sampling frame for future British Social Attitudes surveys. For details of the PAF/ER ‘splicing’ experiment, see Lynn and Taylor (1995).
  2. This includes households not containing any adults aged 18 or over, vacant dwelling units, derelict dwelling units, non-resident addresses and other deadwood.
  3. In 1993 it was decided to mount a split-sample experiment designed to test the applicability of Computer-Assisted Personal Interviewing (CAPI) to the British Social Attitudes survey series. CAPI has been used increasingly over the past decade as an alternative to traditional interviewing techniques. As the name implies, CAPI involves the use of a laptop computer during the interview, with the interviewer entering responses directly into the computer. One of the advantages of CAPI is that it significantly reduces both the amount of time spent on data processing and the number of coding and editing errors. There was, however, concern that a different interviewing technique might alter the distribution of responses and so affect the year-on-year consistency of British Social Attitudes data.

    Following the experiment, it was decided to change over to CAPI completely in 1994 (the self-completion questionnaire still being administered in the conventional way). The results of the experiment are discussed in the British Social Attitudes 11th Report (Lynn and Purdon, 1994).
  4. Interview times recorded as less than 20 minutes were excluded, as these timings were likely to be errors.
  5. An experiment was conducted on the 1991 British Social Attitudes survey (Jowell et al., 1992) which showed that sending advance letters to sampled addresses before fieldwork begins has very little impact on response rates. However, interviewers do find that an advance letter helps them to introduce the survey on the doorstep, and a majority of respondents have said that they preferred some advance notice. For these reasons, advance letters have been used on British Social Attitudes surveys since 1991.
  6. Because of methodological experiments on scale development, the exact items detailed in this section have not been asked on all versions of the questionnaire each year. 
  7. In 1994 only, this item was replaced by: Ordinary people get their fair share of the nation’s wealth. [Wealth1]
  8. In constructing the scale, a decision had to be taken on how to treat missing values (“Don’t know” and “Not answered”). Respondents who had more than two missing values on the left–right scale and more than three missing values on the libertarian–authoritarian and welfarism scales were excluded from that scale. For respondents with fewer missing values, “Don’t know” was recoded to the mid-point of the scale and “Not answered” was recoded to the scale mean for that respondent on their valid items.
  • Notes
    1. Until 1991 all British Social Attitudes samples were drawn from the Electoral Register (ER). However, following concern that this sampling frame might be deficient in its coverage of certain population subgroups, a ‘splicing’ experiment was conducted in 1991. We are grateful to the Market Research Development Fund for contributing towards the costs of this experiment. Its purpose was to investigate whether a switch to PAF would disrupt the time-series – for instance, by lowering response rates or affecting the distribution of responses to particular questions. In the event, it was concluded that the change from ER to PAF was unlikely to affect time trends in any noticeable way, and that no adjustment factors were necessary. Since significant differences in efficiency exist between PAF and ER, and because we considered it untenable to continue to use a frame that is known to be biased, we decided to adopt PAF as the sampling frame for future British Social Attitudes surveys. For details of the PAF/ER ‘splicing’ experiment, see Lynn and Taylor (1995).
    2. This includes households not containing any adults aged 18 or over, vacant dwelling units, derelict dwelling units, non-resident addresses and other deadwood.
    3. In 1993 it was decided to mount a split-sample experiment designed to test the applicability of Computer-Assisted Personal Interviewing (CAPI) to the British Social Attitudes survey series. CAPI has been used increasingly over the past decade as an alternative to traditional interviewing techniques. As the name implies, CAPI involves the use of a laptop computer during the interview, with the interviewer entering responses directly into the computer. One of the advantages of CAPI is that it significantly reduces both the amount of time spent on data processing and the number of coding and editing errors. There was, however, concern that a different interviewing technique might alter the distribution of responses and so affect the year-on-year consistency of British Social Attitudes data.

      Following the experiment, it was decided to change over to CAPI completely in 1994 (the self-completion questionnaire still being administered in the conventional way). The results of the experiment are discussed in the British Social Attitudes 11th Report (Lynn and Purdon, 1994).
    4. Interview times recorded as less than 20 minutes were excluded, as these timings were likely to be errors.
    5. An experiment was conducted on the 1991 British Social Attitudes survey (Jowell et al., 1992) which showed that sending advance letters to sampled addresses before fieldwork begins has very little impact on response rates. However, interviewers do find that an advance letter helps them to introduce the survey on the doorstep, and a majority of respondents have said that they preferred some advance notice. For these reasons, advance letters have been used on British Social Attitudes surveys since 1991.
    6. Because of methodological experiments on scale development, the exact items detailed in this section have not been asked on all versions of the questionnaire each year. 
    7. In 1994 only, this item was replaced by: Ordinary people get their fair share of the nation’s wealth. [Wealth1]
    8. In constructing the scale, a decision had to be taken on how to treat missing values (“Don’t know” and “Not answered”). Respondents who had more than two missing values on the left–right scale and more than three missing values on the libertarian–authoritarian and welfarism scales were excluded from that scale. For respondents with fewer missing values, “Don’t know” was recoded to the mid-point of the scale and “Not answered” was recoded to the scale mean for that respondent on their valid items.