TECHNICAL NOTE 2 DATA RELIABILITY


INTRODUCTION

1 For 2006-07, the electricity, gas, water and waste services industries collection was a sample survey designed primarily to deliver national estimates at the industry subdivision and group level.


SAMPLING ERROR

2 The majority of data in this publication have been obtained from a sample of electricity, gas, water and waste services businesses. As such, these data are subject to sampling variability; that is, they may differ from the figures that would have been produced if the data had been obtained from all electricity, gas, water and waste services businesses in the population. One measure of the likely difference is given by the standard error, which indicates the extent to which an estimate might have varied by chance because the data were obtained from only a sample of units.

3 There are about two chances in three that a sample estimate will differ by less than one standard error from the figure that would have been obtained if the data had been obtained from all units, and about 19 chances in 20 that the difference will be less than two standard errors.

4 The standard error can also be expressed as a percentage of the estimate, and this is known as the relative standard error (RSE). RSEs at the industry subdivision and group level for selected data items representing the full range of data contained in this publication are shown in the table below. Detailed RSEs can be made available on request.

Relative standard errors
		Employment at end of June	Sales and service income	Wages and salaries	Industry value added
		%	%	%	%
26	Electricity supply	0.4	0.3	0.3	0.2
261	Electricity generation	1.7	1.1	1.2	0.6
262	Electricity transmission	-	-	-	-
263	Electricity distribution	-	-	-	-
264	On selling electricity and electricity market operation	0.7	0.1	0.5	0.6
27	Gas supply	3.5	4.4	2.7	10.4
28	Water supply, sewerage and drainage services	3.4	4.2	3.1	3.2
29	Waste collection, treatment and disposal services	5.4	4.3	4.5	6.0
26-29	Electricity, gas, water and waste services	1.7	0.9	1.1	1.2
- nil or rounded to zero (including null cells)

5 To illustrate the above, the estimate of sales and service income for the Australian electricity supply industry in 2006-07 was $39,516m. The RSE of the estimate is shown as 0.3%, giving a standard error of approximately $118m (rounded). This implies that there are two chances in three that, if all units had been included in the survey, an estimate in the range of $39,397m to $39,635m would have been obtained. Similarly, it implies that there are 19 chances in 20 (i.e., a confidence interval of 95%) that the estimate would have been within the range of $39,278m to $39,754m.

6 Note that RSEs for ANZSIC Subdivisions 26 Electricity supply (and its constituent ANZSIC Groups) are generally very small. This is because there is very little sampling variability, due to the large number of units sampled from the population.

7 The size of the RSE may be a misleading indicator of the reliability of some of the estimates for trading profit, OPBT, EBITDA and IVA. Estimates of these variables may legitimately include positive and negative values, reflecting the financial performance of individual businesses. In these cases, the aggregated estimate can be small relative to the contribution of individual businesses, resulting in a standard error which is large relative to the estimate.


NON-SAMPLING ERROR

8 All data presented in this publication are subject to non-sampling error.

9 The imprecision due to sampling variability, which is measured by the standard error, should not be confused with inaccuracies that may occur because of inadequacies in available sources from which the population frame was compiled, imperfections in reporting by providers, errors made in collection such as in recording and coding data, and errors made in processing data. Inaccuracies of this kind are referred to collectively as non-sampling error and may occur in any enumeration, whether a full census or a sample.

10 For the purpose of compiling the estimates in this publication, businesses in the ATO maintained population (see Technical Note 1) are coded to ANZSIC industry classes on the basis of the activity reported to the ATO when they registered for an ABN. There are a number of reasons why a business classified to any given ANZSIC industry on the ABS Business Register may not have been mainly engaged in activities associated with that industry during the 2006-07 reference year. This may be because of inaccurate or incomplete information at the time the business was registered or it may be because the business has changed activity, either temporarily or permanently.

11 Although it is not possible to quantify non-sampling error, every effort is made to reduce it to a minimum. Collection forms are designed to be easy to complete and assist businesses to report accurately. Efficient and effective operating procedures and systems are used to compile the statistics. The ABS compares data from different ABS (and non-ABS) sources relating to the one industry, to ensure consistency and coherence.