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5 Notes and Definitions
48. 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 given that it is 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.
49. The 'paired' (social and physical) survey response rate is about 60%. Although the SHCS is reweighted to take non-responses into consideration we cannot be certain that the weighting process correctly represents the profile of the missing population.
50. 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 one in twenty 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.
51. Table 20 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.
52. In Table 9, 24% 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 772. Looking at Table 20, and reading across the 800 row to the 25% 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 20.6% and 27.4% (24% ± 3.4%).
53. Where no sample size has been given, for example in Table 1, it can be assumed that the base of the figures is the full sample, which was 3,783 households in 2004/05 for Tables 1 and 2, which only use information from the social questionnaire, and 3,093 paired social and physical cases for the other tables.
Table 20: "95% Confidence Limits" for estimates based on SHCS sub-samples of various sizes.
Sub-sample size. (i.e the "n=" value corresponding to 100%) | Estimate |
|---|
1%
or
99% | 2%
or
98% | 5%
or
95% | 10%
or
90% | 15%
or
85% | 20%
or
80% | 25%
or
75% | 30%
or
70% | 35%
or
65% | 40%
or
60% | 45%
or
55% | 50% |
|---|
percentage points ( + / - ) |
|---|
100 | 2.2 | 3.1 | 4.9 | 6.7 | 8.0 | 8.9 | 9.7 | 10.2 | 10.7 | 10.9 | 11.1 | 11.2 |
|---|
150 | 1.8 | 2.6 | 4.0 | 5.5 | 6.5 | 7.3 | 7.9 | 8.4 | 8.7 | 8.9 | 9.1 | 9.1 |
|---|
200 | 1.6 | 2.2 | 3.4 | 4.7 | 5.6 | 6.3 | 6.8 | 7.2 | 7.5 | 7.7 | 7.9 | 7.9 |
|---|
250 | 1.4 | 2.0 | 3.1 | 4.2 | 5.0 | 5.7 | 6.1 | 6.5 | 6.7 | 6.9 | 7.0 | 7.1 |
|---|
300 | 1.3 | 1.8 | 2.8 | 3.9 | 4.6 | 5.2 | 5.6 | 5.9 | 6.2 | 6.3 | 6.4 | 6.5 |
|---|
350 | 1.2 | 1.7 | 2.6 | 3.6 | 4.3 | 4.8 | 5.2 | 5.5 | 5.7 | 5.9 | 5.9 | 6.0 |
|---|
400 | 1.1 | 1.6 | 2.4 | 3.4 | 4.0 | 4.5 | 4.8 | 5.1 | 5.3 | 5.5 | 5.6 | 5.6 |
|---|
450 | 1.0 | 1.5 | 2.3 | 3.2 | 3.8 | 4.2 | 4.6 | 4.8 | 5.0 | 5.2 | 5.2 | 5.3 |
|---|
500 | 1.0 | 1.4 | 2.2 | 3.0 | 3.6 | 4.0 | 4.3 | 4.6 | 4.8 | 4.9 | 5.0 | 5.0 |
|---|
600 | 0.9 | 1.3 | 2.0 | 2.7 | 3.3 | 3.6 | 3.9 | 4.2 | 4.4 | 4.5 | 4.5 | 4.6 |
|---|
700 | 0.8 | 1.2 | 1.8 | 2.5 | 3.0 | 3.4 | 3.7 | 3.9 | 4.0 | 4.1 | 4.2 | 4.2 |
|---|
800 | 0.8 | 1.1 | 1.7 | 2.4 | 2.8 | 3.2 | 3.4 | 3.6 | 3.8 | 3.9 | 3.9 | 3.9 |
|---|
900 | 0.7 | 1.0 | 1.6 | 2.2 | 2.7 | 3.0 | 3.2 | 3.4 | 3.6 | 3.6 | 3.7 | 3.7 |
|---|
1,000 | 0.7 | 1.0 | 1.5 | 2.1 | 2.5 | 2.8 | 3.1 | 3.2 | 3.4 | 3.5 | 3.5 | 3.5 |
|---|
1,100 | 0.7 | 0.9 | 1.5 | 2.0 | 2.4 | 2.7 | 2.9 | 3.1 | 3.2 | 3.3 | 3.4 | 3.4 |
|---|
1,200 | 0.6 | 0.9 | 1.4 | 1.9 | 2.3 | 2.6 | 2.8 | 3.0 | 3.1 | 3.2 | 3.2 | 3.2 |
|---|
1,300 | 0.6 | 0.9 | 1.4 | 1.9 | 2.2 | 2.5 | 2.7 | 2.8 | 3.0 | 3.0 | 3.1 | 3.1 |
|---|
1,400 | 0.6 | 0.8 | 1.3 | 1.8 | 2.1 | 2.4 | 2.6 | 2.7 | 2.8 | 2.9 | 3.0 | 3.0 |
|---|
1,500 | 0.6 | 0.8 | 1.3 | 1.7 | 2.1 | 2.3 | 2.5 | 2.6 | 2.8 | 2.8 | 2.9 | 2.9 |
|---|
1,600 | 0.6 | 0.8 | 1.2 | 1.7 | 2.0 | 2.2 | 2.4 | 2.6 | 2.7 | 2.7 | 2.8 | 2.8 |
|---|
1,700 | 0.5 | 0.8 | 1.2 | 1.6 | 1.9 | 2.2 | 2.3 | 2.5 | 2.6 | 2.7 | 2.7 | 2.7 |
|---|
1,800 | 0.5 | 0.7 | 1.1 | 1.6 | 1.9 | 2.1 | 2.3 | 2.4 | 2.5 | 2.6 | 2.6 | 2.6 |
|---|
1,900 | 0.5 | 0.7 | 1.1 | 1.5 | 1.8 | 2.1 | 2.2 | 2.3 | 2.4 | 2.5 | 2.6 | 2.6 |
|---|
2,000 | 0.5 | 0.7 | 1.1 | 1.5 | 1.8 | 2.0 | 2.2 | 2.3 | 2.4 | 2.4 | 2.5 | 2.5 |
|---|
2,100 | 0.5 | 0.7 | 1.1 | 1.5 | 1.7 | 2.0 | 2.1 | 2.2 | 2.3 | 2.4 | 2.4 | 2.4 |
|---|
2,200 | 0.5 | 0.7 | 1.0 | 1.4 | 1.7 | 1.9 | 2.1 | 2.2 | 2.3 | 2.3 | 2.4 | 2.4 |
|---|
2,300 | 0.5 | 0.7 | 1.0 | 1.4 | 1.7 | 1.9 | 2.0 | 2.1 | 2.2 | 2.3 | 2.3 | 2.3 |
|---|
2,400 | 0.5 | 0.6 | 1.0 | 1.4 | 1.6 | 1.8 | 2.0 | 2.1 | 2.2 | 2.2 | 2.3 | 2.3 |
|---|
2,500 | 0.4 | 0.6 | 1.0 | 1.3 | 1.6 | 1.8 | 1.9 | 2.0 | 2.1 | 2.2 | 2.2 | 2.2 |
|---|
2,600 | 0.4 | 0.6 | 1.0 | 1.3 | 1.6 | 1.8 | 1.9 | 2.0 | 2.1 | 2.1 | 2.2 | 2.2 |
|---|
2,700 | 0.4 | 0.6 | 0.9 | 1.3 | 1.5 | 1.7 | 1.9 | 2.0 | 2.1 | 2.1 | 2.1 | 2.2 |
|---|
2,800 | 0.4 | 0.6 | 0.9 | 1.3 | 1.5 | 1.7 | 1.8 | 1.9 | 2.0 | 2.1 | 2.1 | 2.1 |
|---|
2,900 | 0.4 | 0.6 | 0.9 | 1.2 | 1.5 | 1.7 | 1.8 | 1.9 | 2.0 | 2.0 | 2.1 | 2.1 |
|---|
3,000 | 0.4 | 0.6 | 0.9 | 1.2 | 1.5 | 1.6 | 1.8 | 1.9 | 1.9 | 2.0 | 2.0 | 2.0 |
|---|
3,093 | 0.4 | 0.6 | 0.9 | 1.2 | 1.4 | 1.6 | 1.7 | 1.8 | 1.9 | 2.0 | 2.0 | 2.0 |
|---|
3,783 | 0.4 | 0.5 | 0.8 | 1.1 | 1.3 | 1.5 | 1.6 | 1.7 | 1.7 | 1.8 | 1.8 | 1.8 |
|---|
15,000 | 0.2 | 0.3 | 0.4 | 0.5 | 0.7 | 0.7 | 0.8 | 0.8 | 0.9 | 0.9 | 0.9 | 0.9 |
|---|
54. Confidence intervals for the mean NHER and SAP scores cannot be calculated from Table 20, and so are provided explicitly in Tables 5, 8, 10 and 11.
55. 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.
56. 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.
57. 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
58. 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 20) of the theoretical 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 to take into account differences across the UK. In reality household fuel use may be different to that assumed in the model.
59. 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 recorded in the level 2 assessment. At this level the scale 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 21 Technical Annex 9 and 10.
60. The Standard Assessment Procedure is the UK government's preferred system for rating the energy efficiency of dwellings. It is scaled from 1 (poor) to 100 (excellent). It is based on the total fuel costs for space and water heating per square metre of floor space. However, unlike the NHER, there is no factor to take into account regional variations in climate in the UK. Furthermore it does not consider lighting, domestic appliances and standing charges in its modelling of fuel costs, nor does it include any information on occupancy.
61. 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 10,9,10. 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.
62. Tables 21 and 22 show how energy prices have increased between May 1996 and May 2006, the latest year 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 12), the price of fuel and light had decreased by 17% in real terms. By 2005 the real price of fuel and light was 7% below its 1996 level and the proportion of households fuel poor was around half of its 1996 level.
Table 21: Retail Price Index fuel components, May 1996 to May 2006
| Coal and smokeless fuels | Gas | Electricity | Heating oils | Fuel and light | Petrol and oil |
|---|
May-96 | 118.6 | 112.7 | 120.9 | 94.2 | 116.4 | 134.7 |
|---|
May-02 | 139.3 | 114.0 | 105.2 | 117.2 | 111.1 | 182.8 |
|---|
May-03 | 142.2 | 115.5 | 105.9 | 118.6 | 112.2 | 184.3 |
|---|
May-04 | 146.6 | 123.5 | 111.9 | 146.2 | 120.3 | 199.3 |
|---|
May-05 | 163.6 | 139.8 | 123.1 | 172.2 | 134.8 | 209.0 |
|---|
May-06 | 178.5 | 185.2 | 150.1 | 222.0 | 170.2 | 236.1 |
|---|
% change May-96 to May-05 | 38.0% | 24.1% | 1.9% | 82.8% | 15.8% | 55.1% |
|---|
% change May-02 to May-05 | 17.5% | 22.7% | 17.1% | 46.9% | 21.3% | 14.3% |
|---|
Source: DTI Quarterly Energy Prices Tables 22
Table 22: Retail Price Index fuel components relative to the GDP deflator, May 1996 to May 2006
| Coal and smokeless fuels | Gas | Electricity | Heating oils | Fuel and light | Petrol and oil | GDP Deflator |
|---|
May-96 | 96.5 | 91.7 | 98.4 | 76.6 | 94.7 | 109.6 | 122.9 |
|---|
May-02 | 98.4 | 80.6 | 74.3 | 82.8 | 78.5 | 129.2 | 141.5 |
|---|
May-03 | 97.6 | 79.3 | 72.7 | 81.4 | 77.0 | 126.5 | 145.7 |
|---|
May-04 | 98.1 | 82.6 | 74.8 | 97.8 | 80.5 | 133.3 | 149.5 |
|---|
May-05 | 107.2 | 91.6 | 80.6 | 112.8 | 88.3 | 136.8 | 152.7 |
|---|
May-06 | 114.3 | 118.7 | 96.2 | 142.2 | 109.0 | 151.2 | 156.1 |
|---|
% change May-96 to May-05 | 11.1% | -0.1% | -18.1% | 47.3% | -6.8% | 24.8% | 24.2% |
|---|
% change May-02 to May-05 | 8.9% | 13.6% | 8.5% | 36.2% | 12.5% | 5.9% | 7.9% |
|---|
Source: DTI Quarterly Energy Prices Tables 23
63. The Tolerable Standard 24 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 or demolition or improvement of the dwelling.
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