Scottish Government

5 Notes and Definitions

Sampling

92. The aim of surveys such as the SHCS is to select a sample which is representative of the population as a whole, thus ensuring that any estimates obtained from the survey data are as close as possible to the true population value. Although the SHCS uses a randomly selected sample, it is not necessarily representative of all households. Some households or dwelling types may be over-represented and others may be under-sampled, and response bias will have a further effect (for example non-response households consisting of a young, single male). In general, the smaller the sample size, the greater the likelihood the estimate could be misleading, so care must be taken when using subsets of the survey sample for analysis.

93. The 'paired' (social and physical) survey response rate is about 60%. Although the SHCS is re-weighted to take non-responses into consideration, we cannot be certain that the weighting process correctly represents the profile of the missing households and their residents.

Confidence intervals

94. Whilst we cannot quantify the extent of bias due to non-response, we can quantify the likely extent of sampling variability by calculating the 'standard error' associated with an estimate. By convention, a '95% confidence interval' is used to demonstrate the variability. On average there is a 1 in 20 chance that the true value will not fall within the given confidence interval or, conversely, there is a 95% chance that the true value will fall within the given confidence interval.

95. Table 35 shows the 95% confidence limits for estimates for a range of percentages calculated from sub-samples of a range of sizes. Note that the confidence limits for estimates of x% and (100-x)% are the same. The interpretation and use of this table are best demonstrated by an example.

96. In Table 13, 30% of detached houses were rated 'good' in terms of energy efficiency. To the right of the table it says that the sample size of detached houses was 727. Looking at Table 35 and reading across the 700 row to the 30% column (the nearest to our figures), we get a confidence interval of ±3.4% around the estimate. Thus we can say that we are 95% confident that the true proportion of detached houses with a 'good' energy rating lies between 26.6% and 33.4% (30% ± 3.2%).

Design effects

97. However, it is important to adjust these intervals by multiplying by the design effects for the survey. The design effect is the ratio between the variance (average deviation of a set of data points from their mean value) of a variable under the sampling method used (actual) and the variance computed under the assumption of simple random sampling (standard). In short, a design effect of 2 would mean doubling the size of a simple random sample to obtain the same volume of information; a design effect of 0.5 implies the reverse. Design effects adjustments are necessary when adjusting standard errors which are affected by the design and complexity of the survey.

98. Generally speaking, disproportionate stratification and sampling with non-equal probabilities tends to increase standard errors, giving a design factor greater than 1. However, this can be controlled by deliberately oversampling in stratum where the item of interest is either very rare or variable. The impact of non-response weighting on standard errors tends to be, although with exceptions, comparatively limited. The sampling design of the SHCS meets the criteria above in that disproportionate stratification is applied across the 32 local authority areas and oversampling of remote rural areas - for example in Shetlands and Orkney - is used to account for the aforementioned issues. As a result, one would expect the design factor to be above 1 although only modestly so.

Table 35: 95% Confidence Limits for estimates based on SHCS sub-samples of various sizes (without design effects)

99. Table 36 shows two distinct design factors for the SHCS 2007. They are 1.13 and 1.11 for the Physical and Social surveys respectively. When using a mixture of the Physical and Social survey, the Physical Survey design factor must be used. The design factors for the 2007 SHCS have followed a decreasing trend since the 2004/05 SHCS - this translates as improved effectiveness in the sampling method, though this may be chance. When producing estimates at local authority level, no design factor is necessary for adjusting standard errors as simple random sampling is carried out within each local authority.

100. Table 36 shows the design factors for all the SHCS surveys since 1991.

101. Confidence intervals for the mean NHER and SAP scores cannot be calculated from Table 35 (since they are not proportions) and so are provided explicitly in Tables 8, 11, 12, and 15-18.

102. The median tends to be a better measure of central tendency than the mean for most SHCS results as it is less affected by skewed distributions and the small number of outlying values which naturally occur in the data.

103. Numbers of cases are rounded to the nearest thousand and percentages to the nearest integer. This rounding may mean that in some cases the percentages do not add up to 100. A blank cell represents no survey cases. Zeroes correspond to either a count of less than 500 or a percentage of less than 0.5%.

Table 36: Design Factors for SHCS

Dwelling types

104. The SHCS uses the following definitions of dwelling types:

• Detached house - a house that is free standing with no party walls;
• Semi-detached house - a house that is only attached to one other dwelling, commercial premise etc. The two properties taken together should be detached from any other properties;
• Terraced house - a house forming part of a row of three or more dwellings, commercial premises etc;
• Tenement flat - a dwelling within a common block of two or more floors (commonly up to five storeys but may be higher in certain circumstances) where some or all of the flats have a shared or common vertical access. The selected dwelling need not share the access, but may be situated within the block with shared/common access (own door flat);
• 4-in-a-block - each flat in a block has its own independent access. Flats on the upper level have an internal or external stair;
• Tower/slab - flats in a high rise (ten or more storeys) or flats where the common circulation is predominantly horizontal (maisonette, balcony or gallery access);
• Flat from a conversion - flats resulting from the conversion of a house only. A flat converted from a non-residential building ( e.g. a warehouse) is classified according to the above flat types.

Household types

105. Households are allocated to one of eight types as shown below:

• Single adult: 1 adult of non-pensionable age and no children;
• Small adult: 2 adults of non-pensionable age and no children;
• Single parent: 1 adult of any age and 1 or more children;
• Small family: 2 adults and 1 or 2 children;
• Large family: 2 adults and 3 or more children or 3 or more adults and 1 or more children;
• Large adult: 3 or more adults and no children;
• Older smaller: 2 adults at least one of whom is of pensionable age and no children;
• Single pensioner: 1 adult of pensionable age and no children.

Energy Ratings and Labels

106. The way a building is constructed, insulated, heated and ventilated and the type of fuel used, all contribute to its energy consumption and carbon emissions. The use of 'energy labels' provides a method of demonstrating the results of complex calculations in an easy-to-understand way.

National Homes Energy Rating ( NHER)

107. The NHER assessment procedure is not based on what a household actually spends on fuel. It is based on a model (produced by the National Energy Services) of the theoretical costs ('Total Energy costs') of maintaining a standard heating regime for a standard level of occupancy derived from knowledge of the appliances, fuel sources, insulation, size and dwelling type of the premises. Total energy costs include space and water heating, lighting, standard domestic appliances ( e.g. washing machine) and standing charges. The model contains a factor for local climate variations which take into account differences across the UK. In reality household fuel use may be different to that assumed in the model.

108. Level 0 is the simplest of the four NHER assessment levels. It involves measuring up to 19 items and takes about 5 minutes per dwelling. The SHCS uses an enhanced level 0 assessment which includes many of the items recorded in the level 1 assessment and 1 item recorded in the level 2 assessment. At this level, the scale, used properly, does not allow scores for individual dwellings to be quoted, but does give the distribution of NHER across subsets of greater than 100 dwellings and therefore the stock as a whole. For further information see the SHCS 2002 National Report Technical Annex 9.

Standard Assessment Procedures ( SAP)

109. SAP is the Government's Standard Assessment Procedure for Energy Rating of Dwellings.

SAP 2005

110. SAP 2005 is adopted by government as part of the UK national methodology for calculation of the energy performance of buildings.

111. It is scaled from 1 (poor) to 100 (excellent) and reflects the energy cost per square metre for the lighting, space and water heating of a dwelling based on a single point in England. SAP takes no account of geography or climatic conditions.

112. It is used to demonstrate compliance with building regulations for dwellings - Part L (England and Wales), Section 6 (Scotland) and Part F (Northern Ireland) - and to provide energy ratings for dwellings.

SAP 2001

113. The SHCS also reports on the previous version of SAP ( SAP 2001) for comparability with previous surveys. It is scaled from 1 (poor) to 120 (excellent). SAP 2001 only takes space and water heating into account (whereas SAP 2005 also includes lighting). As with all versions of SAP it takes no account of regional or climatic conditions in its calculation.

Energy Performance Certificate ( EPC)

114. The EPC is a measure being introduced to promote the improvement of energy efficiency of our buildings. It is part of a programme of changes being rolled out across Europe to reflect legislation.

115. The Energy Efficiency Rating ( EER) letter of an EPC reflects the SAP 2005 score in a simplified form and, similar to those used for electrical white goods; the higher the rating the more energy efficient the home and the lower the fuel bills should be.

116. The SHCS emulates SAP 2005 and EPC's from the SAP 2001 scores together with the fuel use based on Table 16 of the SAP Methodology. Again, the EPC does not reflect climatic differences across Scotland. So a house in Galloway with the same EPC rating as a house in Shetland (or in Cornwall) indicates that the quality of the house is similar in respect of EPC factors, but does not fully reflect the actual level of energy required to heat a dwelling in those different climates.

Fuel Poverty

117. The concept of fuel poverty used in this report is based on a theoretical calculation of how much it would cost to heat a dwelling according to a specified regime (See foot notes 15, 16 and 17). It does not utilise information on how much a household actually spends on fuel. In reality households may choose to heat their dwellings in a different manner to that assumed in the model.

Energy Prices

118. Tables 37 and 38 show how energy prices have increased between May 1996 and July 2007 - the latest period for which SHCS data is available. Using changes in the index for fuel and light as a broad indicator of changes in fuel prices faced by households it is notable that between 1996 and 2002, when the proportion of households in fuel poverty fell to less than 38% of its 1996 value (Table 19), the price of fuel and light decreased by 17% in real terms. By 2005 the real price of fuel and light was 15% above its 1996 level and the proportion of fuel poor households was around two thirds of its 1996 level. The real price of fuel and light increased by 24% from May 2005 (the midpoint of the 2004/5 survey year) to May 2006 (the midpoint of the 2005/6 survey year) whereas the overall RPI (Retail Price Index) increase was 3% in the same period. In the period May 2006 and July 2007, fuel prices rose about 6% (depending on the mix of fuels used) - a relatively stable period of fuel price increases compared to 2008,

Table 37: Retail Price Index fuel components, May 1996 to May 2007

Source: Quarterly Energy Prices Tables ³⁰

Table 38: Retail Price Index fuel components relative to the GDP deflator, May 1996 to May 2007

Source: Quarterly Energy Prices Tables ³¹

The Tolerable Standard

119. The Tolerable Standard32 is the minimum condition required by Scottish Law for a dwelling to be habitable. It was introduced in the 1969 Housing Act and was updated in the 1987 and 2001 Acts. The requirements of the Act are that the dwelling:

• Is structurally stable;
• Is substantially free from rising and penetrating damp;
• Has satisfactory provision for natural and artificial light, for ventilation and for heating;
• Has an adequate piped supply of wholesome water within the house;
• Has a sink provided with a satisfactory supply of hot and cold water within the house;
• Has a WC available for the exclusive use of the occupants of the house suitably located within the house;
• Has a fixed bath/shower and wash-hand basin all with a satisfactory supply of hot and cold water suitably located within the house;
• Has an effective system for the drainage and disposal of foul and surface water;
• Has satisfactory facilities for the cooking of food within the house;
• Has satisfactory access to all external doors and outbuildings.
• A failure to meet one or more of these criteria will result in a dwelling being declared Below Tolerable Standard ( BTS). In such cases local authorities are required to act either through closure, demolition or improvement of the dwelling.Disrepair120. This report uses three different types of disrepair to describe the state of disrepair of a dwelling:Any disrepair - any disrepair, no matter how small, to any element of the dwelling;
• Urgent disrepair - any disrepair which if not rectified would cause the fabric of the building to deteriorate further and/or place the health and safety of the occupier at risk. Urgency of disrepair is only assessed for external and common elements;
• Disrepair to critical elements - any disrepair to the critical elements of the dwelling. The critical elements are those whose condition is central to a dwelling being wind and weather proof, structurally stable and safeguarded against further rapid deterioration. They are as follows:
• Roof covering;
• Roof structure;
• Chimney stacks;
• Flashings;
• Roof gutters and downpipes;
• External walls - finish;
• External walls - structure;
• Access decks and balustrades (common areas - flats only);
• Foundations;
• Damp proof course;
• External doors and windows (dwelling only);
• Doors, screens, windows and roof lights (common areas - flats only);
• Party walls - structure;
• Floor structure;
• Floor finish;
• Dry rot/wet rot.
• Extensive disrepair - a score of 2 or more on the 10-point repair scale and/or a score of 'medium' or 'renew' on the 5-point repair scale or dry/wet rot in two or more rooms. Extensive disrepair is calculated in order to identify those dwellings where any disrepair present is of a relatively greater severity.

Urban Rural Classifications

121. The SHCS uses the 8-category Scottish Executive Urban/Rural Classification 2005-0633. The definitions of the 8 categories are in Table 39 below. When looking for different characteristics in urban and rural areas, it can be more useful to group the 8 categories into 2 gross urban and rural categories. This requires a definition of what constitutes urban and rural. The Scottish Government's core definition of rurality classifies settlements of less than 3,000 people as rural. The 8-category urban/rural classification can be collapsed to the core definition:

• Urban = Large Urban Areas, Other Urban Areas, Accessible Small Towns, Remote Small Towns, Very Remote Small Towns i.e. categories 1 to 5;
• Rural = Accessible Rural, Remote Rural, Very Remote Rural i.e. categories 6 to 8.

122. The full classification can be grouped in differing ways to meet user needs, for example a six category version can be used. In that version, 'Remote Small Towns' and 'Very Remote Small Towns' are grouped into 'Remote Small Towns'; and 'Remote Rural' and 'Very Remote Rural' are grouped into 'Remote Rural'. In this report any urban/rural breakdown uses the core definition of rurality outlined in the previous paragraph.

Table 39: Scottish Executive 8 category Urban/Rural Classification 2005-2006