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TECHNICAL NOTE 1


SAMPLING AND NON-SAMPLING ERROR


INTRODUCTION

1 When interpreting the results of a survey it is important to take into account factors that may affect the reliability of estimates. Such factors can be classified as either sampling error or non-sampling error.

2 Estimates for the road sector in this publication are based on information collected from a sample of registered articulated vehicles, rather than a full enumeration, and are therefore subject to sampling error. The road freight estimates may differ from the figures that would have been produced if the information had been obtained for all registered articulated vehicles. Examples of the sampling error for selected estimates from the FMS 2000 are included in the tables below.

3 It should be noted that although the rail, sea and air sectors of this collection are fully enumerated and are not subject to sampling error, they are still subject to non-sampling error. Non-sampling error is discussed in more detail below.


SAMPLING ERROR

4 The sampling error associated with any estimate can be calculated from the sample results. One measure of sampling error is given by the standard error, which indicates the extent to which an estimate might have varied by chance because only a sample of articulated vehicles was included. 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 all articulated vehicles had been included, and about 19 chances in 20 that the difference will be less than two standard errors.

5 Another measure of sampling variability is the relative standard error (RSE), which is obtained by expressing the standard error as a percentage of the estimate to which it refers. The RSE is a useful measure as it provides an immediate indication of the percentage error likely to have occurred due to sampling.

6 In this publication, only estimates with a RSE of less than 25% are considered sufficiently reliable for most purposes. Estimates with a RSE between 25% and 50% are preceded by a single asterisk (*) and should be used with caution, while those with a RSE of greater than 50% are preceded by two asterisks (**) and are considered too unreliable for general use (see T1.1 and T1.2 for more detail).

7 The FMS 2000 was designed primarily to minimise relative standard errors for estimates of total tonnes carried, total distance travelled and total tonne-kilometres at the state/territory of registration level.

8 The RSEs relating to estimates contained in tables 3 and 4 of the publication are shown in T1.1.

									Destination
Origin	NSW	Vic.	Qld	SA	WA	Tas.	NT	ACT	Aust.
TONNES (%)
NSW	4.51	4.62	5.31	12.06	29.22	. .	45.09	12.57	4.01
Vic.	4.85	3.89	7.25	8.14	26.09	. .	. .	21.13	3.40
Qld	5.80	8.38	5.91	18.74	36.36	. .	25.33	37.86	5.55
SA	8.35	9.47	15.67	7.14	17.37	. .	25.19	40.83	5.98
WA	26.18	29.6	30.86	19.94	6.57	. .	23.90	-	6.52
Tas.	. .	. .	. .	. .	. .	8.25	. .	. .	8.25
NT	74.66	99.59	29.19	22.53	27.31	. .	17.92	-	17.16
ACT	14.92	36.09	52.96	84.36	-	. .	-	30.94	18.17
Aust.	4.06	3.38	5.53	6.10	6.52	8.25	16.37	12.18	2.16
TONNE-KILOMETRES (%)
NSW	2.93	4.01	4.99	9.59	29.60	. .	44.71	12.93	2.09
Vic.	4.18	3.59	7.31	6.79	26.13	. .	-	21.39	2.25
Qld	5.07	9.05	3.84	18.87	37.56	. .	27.33	37.61	2.81
SA	9.44	6.38	16.13	8.27	17.06	. .	21.92	39.47	4.29
WA	26.37	30.44	30.61	18.34	4.56	. .	27.50	-	4.39
Tas.	. .	. .	. .	. .	. .	7.09	. .	. .	7.09
NT	74.66	99.59	35.29	24.13	30.89	. .	13.95	-	10.50
ACT	15.69	36.06	52.15	82.38	-	. .	-	28.20	13.52
Aust.	2.09	2.29	2.65	4.38	4.35	7.09	10.73	10.62	1.39
(a) Data are for freight carried by articulated vehicles only, freight movements made by rigid and light commerical vehicles are excluded from the survey. (b) These relative standard errors relate to the estimates of road freight in tables 3 and 4 of the publication.

		Tonnes	Tonne-kilometres
Commodity		%	%
Food and live animals
	Cereal grains	8.54	5.88
	Food (for human and animal consumption)	5.04	3.3
	Live animals	6.61	7.25
	Total	4.02	2.63
Beverages and tobacco		11.53	8.43
Crude materials, inedible, except fuels
	Crude materials	10.48	6.08
	Metalliferous ores and metal scrap	16.12	12.2
	Sand, stone and gravel	6.87	6.55
	Cork and wood	6.27	5.51
	Total	4.85	3.45
Mineral fuels, lubricants and related materials
	Coal	12.35	11.85
	Gases, natural and manufactured	17.88	21.2
	Petroleum and petroleum products	7.7	7.89
	Total	7.96	6.38
Animal and vegetable oils, fats and waxes		26.49	27.79
Chemical and related products n.e.s.
	Chemicals	9.68	10.04
	Fertilisers, manufactured	20.98	9.03
	Total	11.68	6.83
Manufactured goods classified chiefly by material(c)
	Cement	11.99	12.01
	Iron and steel	11.3	8.2
	Other manufactured goods	7.86	4.97
	Total	5.76	4.06
Machinery and transport equipment		5.29	6.71
Miscellaneous manufactured articles(d)		9.24	9.69
Commodities and transactions n.e.s.
	General freight(e)	5.49	4.63
	Other commodities n.e.s.(f)	8.34	10.69
	Total	4.67	4.22
Total		2.16	1.39
(a) (b)	Data are for freight carried by articulated vehicles only, freight movements made by rigid and light commerical vehicles are excluded from the survey. These relative standard errors relate to the estimates of road freight in tables 10 and 11 of the publication.
(c)	Manufactured goods used in the production of other manufactured goods and/or made mainly of one material e.g. clay
	products, glass and glassware.
(d)	Manufactured goods for final consumption and/or made of more than one material.
(e)	Consignments not classified by commodity.
(f)	All other commodities not elsewhere specified (n.e.s.) including empty used containers, personal effects and furniture not
	for sale.