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A.1 Totals may appear to differ slightly from the apparent sums of their component parts, in cases where they have been calculated by adding up the "unrounded" values of the components and then rounding each figure independently. Similarly, percentages may appear not to sum to 100%.
A.2 In tables which analyse the results of questions for which multiple answers were allowed, the percentages may total more than 100%, because some interviewees gave more than one response.
A.3 The underlying sample numbers shown in different tables may not be the same. In some cases, this is because the tables relate to different populations (such as all households, all adults and all people). In addition, the SHS only collects certain kinds of information for particular sub-groups of the population (which are identified in the relevant tables' headings), and therefore some questions were not asked of all respondents because they only applied in certain circumstances (eg questions about children would not be asked in a household without any children). In some cases, the bases differ because some people were unable to, or did not want to, answer certain questions (e.g. some households did not wish to provide details of their income).
A.4 Highest Income Householder: the household reference person for the first part of the interview. This must be a person in whose name the accommodation is owned or rented, or who is otherwise responsible for the accommodation. In households with joint householders, the person with the highest income is taken as the household reference person. If householders have exactly the same income, the older is taken as the household reference person.
A.5 Adult: for the purposes of the SHS, an adult is someone who was aged 16 or over at the time of the interview; a child is someone who was aged 15 or under.
A.6 Household types
A single pensioner household consists of just one adult of pensionable age (60+ for women, and 65+ for men) and no children
A single parent household contains an adult of any age and one or more children.
A single adult household consists of an adult of non-pensionable age and no children.
An older smaller household contains either (a) an adult of non-pensionable age and an adult of pensionable age and no children or (b) two adults of pensionable age and no children.
A large adult household has three or more adults and no children.
A small adult household contains two adults of non-pensionable age and no children.
A large family household consists of either (a) two adults and three or more children or (b) three or more adults and one or more children.
Small family households consist of two adults and one or two children.
A.7 Social class: the social class categories shown in this bulletin were developed for the analysis of the results of the 1991 Census of Population, and have been used since then. The basis of the groupings is given in the OPCS and Employment Department Standard Occupational Classification, Volume 3 (HMSO, 1991). The method used is designed to group together, as far as possible, people with similar levels of occupational skills. In general, each occupation group is assigned as a whole to one social class, and no account is taken of differences between people within the same occupation group (such as differences in education). However, those who have the employment status of a manager or a foreman may be allocated to a higher class than other people who have the same occupation. The six occupational social classes are as follows, with examples of the occupations in each which are taken from Regional Trends (no. 35 / 2000 edition, pages 242-243):
Because the SHS only collects occupational information for people in employment, and for people who are not in work but who have been in paid work in the five years prior to the survey, the social class is not known in many cases (e.g. people who have been retired for many years). For the purposes of classifying households, the social class of the Highest Income Householder is used.
A.8 Annual net household income: this is the total annual net income (i.e. after taxation and other deductions) from employment, benefits and other sources, which is brought into the household by the highest income householder and/or his/her spouse or partner. This includes any contribution to household finances made by other household members (e.g. for "digs"). Some of the figures given in this bulletin differ from those published in "Scottish Transport Statistics no. 19 / 2000 edition", due to the subsequent imputation of missing or incomplete income data. Cases with missing information (because of refusals or ‘don’t knows’) in relation to any of the main components of household income were excluded from the "income" analyses published in "Scottish Transport Statistics" in August 2000, which therefore covered only about 66% of households. Subsequently, the SHS contractors have imputed the missing components of income for most of the remaining 34% of households, using information that was obtained from other households that appeared similar. Depending upon the component of income, the contractors used either "hot deck" imputation (where the sample is divided into sub-groups based on relevant household characteristics, and the imputed values are obtained from randomly-chosen "donor" cases) or "predictive mean" imputation (where the data are used to construct a statistical model of the relationship between income and other household characteristics, which is then used to "predict" the income in cases where a value is to be imputed). The analyses by income given in this bulletin therefore cover all but a couple of percent of households.
A.9 Scottish MOSAIC is a neighbourhood classification system developed by Experian. It draws on a large number of Census variables, augmented by some published non-Census information, to distinguish between postcodes in terms of types of housing, housing densities and household characteristics. It uses statistical analysis of variables such as home ownership, car ownership, age, health, employment status and occupation to identify types of neighbourhoods with similar characteristics. The ten broad areas of the MOSAIC system (shown in some of the tables in this bulletin) have been used in both the sampling and analysis of the SHS. All households within a given postcode are regarded as being in the same type of neighbourhood: that to which the postcode as a whole is classified. Further information about MOSAIC can be obtained from Experian (the company’s Web site is at www.experian.com).
A.10 The SHS urban / rural classification
A.10.1 The urban / rural classification shown in some of the tables was developed for use in analysing the results of the SHS. It is based on settlement sizes, and (for the less-populated areas) the estimated time that would be taken to drive to a settlement with a population of over 10,000. The classification is based on postcodes. First, each postcode in Scotland was classed as either "urban" or "non-urban" on the basis of its "density" (measured in terms of the numbers of [a] residential and [b] non-residential addresses per hectare). Then, clumps of adjacent "urban" postcodes, which together contained more than a certain total number of addresses, were grouped together to form "settlements". (Any apparently "non-urban" postcodes which were entirely surrounded by "urban" postcodes and/or coastline were reclassified as "urban", and included in the relevant settlements.)
A.10.2 Six categories were then defined:
The four cities - the settlements of Aberdeen, Dundee, Edinburgh and Glasgow. Because of the way in which settlements are defined, this category may (a) include some areas outwith the boundaries of the relevant local authorities, in cases where a city settlement extends into a neighbouring local authority, and (b) exclude some "non-urban" areas within the boundaries of the city local authorities.
Other urban areas - other settlements of over 10,000 population
Small "accessible" towns - settlements of between 3,000 and 10,000 people, which are within 30 minutes drive of a settlement of over 10,000 people
Small "remote" towns - settlements of between 3,000 and 10,000 people, which are not within 30 minutes drive of a settlement of over 10,000 people
"Accessible" rural areas - settlements of less than 3,000 people, which are within 30 minutes drive of a settlement of over 10,000 people
"Remote" rural areas - settlements of less than 3,000 people, which are not within 30 minutes drive of a settlement of over 10,000 people
A.11 Possible sampling variability
A.11.1 Although the SHS's sample is chosen at random, the people who take part in the survey will not necessarily be a representative cross-section of the people of Scotland. For example, purely by chance, the sample could include disproportionate numbers of certain types of people, in which case the survey's results would be affected. In general, the smaller the sample from which an estimate is produced, the greater the likelihood that the estimate could be misleading. As an example, suppose that the percentage of people in a particular population sub-group (those aged 16-19, say) who travel to work in a particular way (eg by bicycle) is calculated from SHS data for a total of only 100 or so commuters from that sub-group. Should the SHS sample contain, purely by chance, just two or three more 16-19 year olds who cycle to work, the resulting estimate would be two or three percentage points higher. Results produced from a small sample could therefore be greatly affected by sampling variability. The larger the sample, the less likely it is that the results will be affected greatly by sampling variability.
A.11.2 The likely extent of sampling variability can be quantified, by calculating the "standard error" associated with the estimate of a quantity produced from a random sample. Statistical sampling theory states that, on average:
By convention, the "95% confidence interval" for a quantity is defined as the estimate plus or minus about twice the standard error (from sampling theory, the interval is plus or minus 1.96 times the standard error), because there is only a 5% chance (on average) that a sample would produce an estimate that differs from the true value of that quantity by more than this amount.
A.11.3 There is no simple "rule of thumb" for the size of standard errors: the standard error of the estimate of a percentage depends upon several things:
A.11.4 The table below, which is a reduced version of a table produced by the SHS contractors for "Scotland's People" volume 1 (see section B.4), shows the "95% confidence limits" for estimates of a range of percentages calculated from sub-samples of a range of sizes.
"95% confidence limits" for estimates of various percentages which are calculated from SHS sub-samples of various sizes
| Estimate* ( % ) | Sub-sample size (i.e. the "n =" value which corresponds to 100%) | ||||||||
|
100 | 500 | 1,000 |
2,000 | 3,000 | 5,000 |
7,000 | 10,000 | 15,000 | |
|
percentage points (plus or minus) | |||||||||
|
5 or 95 | 4.7 |
2.1 | 1.5 |
1.1 | 0.9 |
0.7 | 0.6 |
0.5 | 0.4 |
| 10 or 90 |
6.5 | 2.9 |
2.0 | 1.4 |
1.2 | 0.9 |
0.8 | 0.6 |
0.5 |
| 15 or 85 | 7.7 |
3.4 | 2.4 |
1.7 | 1.4 |
1.1 | 0.9 |
0.8 | 0.6 |
| 20 or 80 |
8.6 | 3.9 |
2.7 | 1.9 |
1.6 | 1.2 |
1.0 | 0.9 |
0.7 |
| 25 or 75 | 9.3 |
4.2 | 3.0 |
2.1 | 1.7 |
1.3 | 1.1 |
0.9 | 0.8 |
| 30 or 70 |
9.9 | 4.4 |
3.1 | 2.2 |
1.8 | 1.4 |
1.2 | 1.0 |
0.8 |
| 35 or 65 | 10.3 |
4.6 | 3.3 |
2.3 | 1.9 |
1.5 | 1.2 |
1.0 | 0.8 |
| 40 or 60 |
10.6 | 4.7 |
3.3 | 2.4 |
1.9 | 1.5 |
1.3 | 1.1 |
0.9 |
| 45 or 55 | 10.7 |
4.8 | 3.4 |
2.4 | 2.0 |
1.5 | 1.3 |
1.1 | 0.9 |
| 50 | 10.8 | 4.8 |
3.4 | 2.4 |
2.0 | 1.5 |
1.3 | 1.1 |
0.9 |
* the confidence limits are the same for estimates of x% and for (100-x)%
A.11.5 The interpretation of an entry in the table is best explained by an example:
A.11.6 As an example of the use of this table, it will be seen from Table 1 that there were 514 households with an annual net income of "over £30,000, up to £40,000" in the survey in 1999. The first column of the relevant row of Table 1 shows that an estimated 50% of such households had an up to 3 minutes walk to the nearest bus stop. Because that estimate was produced from data for only 514 such households, sampling variability could (by chance) produce an error of several percentage points. The entry in the cell at the intersection of the "50%" row and the "500" column in the table which appears above shows that the confidence limits for the estimate will be about 4.8%-points. This means that there is a 1-in-20 chance that the estimate differs from the true value by more than 4.8%-points. It follows that there is roughly a 1-in-3 chance that the estimate differs from the true value by more than 2.4%-points. Clearly, estimates based on small samples have wide confidence limits, and so could be quite unreliable.
A.11.7 The above information relates only to sampling variability. The survey's results could also be affected by non-contact / non-response bias: the characteristics of the people who should have been in the survey but who could not be contacted, or who refused to take part, could differ markedly from those of the people who were interviewed. If that is the case, the SHS's results will not be representative of the whole population. Without knowing the true values (for the population as a whole) of some quantities, one cannot be sure about the extent of any such biases in the SHS. However, comparison of SHS results with information from other sources suggests that they are broadly representative of the overall Scottish population, and therefore that any non-contact or non-response biases are not large overall. However, such biases could, of course, be more significant for certain sub-groups of the population. In addition, because it is a survey of private households, the SHS does not cover some sections of the population - for example, it does not collect information about many students in halls of residence (see section B.2.3). "Scotland's people" volume 2 (see section B.4) provides more information on these matters.
A.12 Changes to the method of recording the answers to some questions
A.12.1 The SHS interview includes a number of questions which ask why a person does (or did) something - e.g. why he/she uses a particular means of travel to work. Some of these questions were originally "open-ended", with the interviewer typing a summary of the person's answer into the computer. If there were a number of reasons, it could take a long time for the interviewer to type them all in. Therefore, once a few months' answers had been obtained, the SHS contractors scrutinised them, and identified the reasons that were given often. They then changed some of the questions to use pre-coded lists of reasons, so that the interviewer could simply "tick" each one that was given by the respondent, which is much quicker than typing them in. The option of typing in something that the person said was retained for use on those occasions on which some of the answer could not be recorded using the entries in the pre-coded list. In such cases, the contractors subsequently examine the typed-in answers, and decide how to code them and whether there is a need to add new entries to the pre-coded lists.
A.12.2 Comparison of the results of the two methods of recording the answers indicated that, on average, more reasons were recorded per respondent after the pre-coded lists of answers were introduced. Clearly, the data for the first part of 1999, which were collected using the "open-ended" forms of questions, are not on the same basis as the data for the second part of the year, which were collected using the "pre-coded list" forms of questions. Therefore, in such cases, the results reported in this bulletin relate only to the period for which the later forms of the questions were used, in order that they will be comparable with the results for the year 2000, which were collected using the same method.
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