TECHNICAL NOTE DATA QUALITY
INTRODUCTION
1 Estimates in this publication are based on information obtained from occupants of a sample of dwellings, and are subject to sampling variability. That is, they may differ from those estimates that would have been produced if all dwellings had been included in the survey. One measure of the likely difference is given by the standard error (SE), which indicates the extent to which an estimate might have varied by chance because only a sample of dwellings was included. There are about two chances in three (67%) that a sample estimate will differ by less than one SE from the number that would have been obtained if all dwellings had been included, and about 19 chances in 20 (95%) that the difference will be less than two SEs. Another measure of the likely difference is the relative standard error (RSE), which is obtained by expressing the SE as a percentage of the estimate.
2 Due to space limitations, it is impractical to print the SE of each estimate in the publication. Instead, a table of SEs is provided to determine the SE for an estimate from the size of that estimate (see table T1). The SE table is derived from a mathematical model, referred to as the 'SE model', which is created using data from a number of past Labour Force Surveys. It should be noted that the SE model only gives an approximate value for the SE for any particular estimate, since there is some minor variation between SEs for different estimates of the same size.
CALCULATION OF STANDARD ERROR
3 An example of the calculation and the use of SEs in relation to estimates of persons is as follows. Table 11 shows that 366,600 people involuntarily ceased their last job during the year and their duration in that job was less than 12 months. Since this estimate is between 300,000 and 500,000, table T1 shows that the SE for Australia will lie between 7,150 and 9,000 and can be approximated by interpolation using the following general formula:
4 Therefore, there are about two chances in three that the value that would have been produced if all dwellings had been included in the survey will fall within the range 358,800 to 374,400 and about 19 chances in 20 that the value will fall within the range 351,000 to 382,200. This example is illustrated in the following diagram.
5 In general, the size of the SE increases as the size of the estimate increases. Conversely, the RSE decreases as the size of the estimate increases. Very small estimates are thus subject to such high RSEs that their value for most practical purposes is unreliable. In the tables in this publication, only estimates with RSEs of 25% or less are considered reliable for most purposes. Estimates with RSEs greater than 25% but less than or equal to 50% are preceded by an asterisk (e.g.*3.2) to indicate they are subject to high SEs and should be used with caution. Estimates with RSEs of greater than 50%, preceded by a double asterisk (e.g.**0.3), are considered too unreliable for general use and should only be used to aggregate with other estimates to provide derived estimates with RSEs of less than 25%.
PROPORTIONS AND PERCENTAGES
6 Proportions and percentages formed from the ratio of two estimates are also subject to sampling errors. The size of the error depends on the accuracy of both the numerator and the denominator. A formula to approximate the RSE of a proportion is given below. This formula is only valid when x is a subset of y.
7 Considering the previous example from Table 11, of the 366,600 people who ceased their last job involuntarily during the year ending February 2013, and their duration of last job was less than 12 months, 210,900 or 57.5% gave their reason as 'Job was temporary or seasonal'. The SE of 210,900 may be calculated by interpolation as 6,200. To convert this to an RSE we express the SE as a percentage of the estimate, or 6,200/210,900 = 2.9%. The SE for 366,600 was calculated previously as 7,800, which converted to an RSE 7,800/366,600=2.1%. Applying the above formula, the RSE of the proportion is:
8 Therefore, the SE for the proportion of people who reported their reason for ceasing their last job as 'Job was temporary or seasonal' and their duration of last job was less than 12 months is 1.2 percentage points (=(57.5/100)x2.0). Therefore, there are about two chances in three that the proportion of people who reported their reason for ceasing their last job as 'Job was temporary or seasonal' and their duration of last job was less than 12 months or more was between 56.3% and 58.7% and 19 chances in 20 that the proportion is within the range 55.1% to 59.9%.
DIFFERENCES
9 Published estimates may also be used to calculate the difference between two survey estimates (of numbers or percentages). Such an estimate is subject to sampling error. The sampling error of the difference between two estimates depends on their SEs and the relationship (correlation) between them. An approximate SE of the difference between two estimates (xy) may be calculated by the following formula:
10 While this formula will only be exact for differences between separate and uncorrelated characteristics or sub populations, it is expected to provide a good approximation for all differences likely to be of interest in this publication.
STANDARD ERRORS
T1 STANDARD ERRORS OF ESTIMATES 

 NSW  Vic.  Qld.  SA  WA  Tas.  NT  ACT  SE  RSE 
Size of Estimate (persons)  no.  no.  no.  no.  no.  no.  no.  no.  no.  % 

100  320  310  240  190  230  120  90  110  120  120.0 
200  430  410  340  260  310  170  130  180  210  105.0 
300  510  480  420  300  370  210  160  230  290  96.7 
500  620  580  540  370  450  260  210  290  410  82.0 
700  720  670  630  420  510  290  250  330  520  74.3 
1,000  830  770  740  490  590  340  290  360  660  66.0 
1,500  970  900  870  570  690  390  340  390  840  56.0 
2,000  1 090  1 000  980  630  770  430  380  420  990  49.5 
2,500  1 200  1 100  1 050  700  850  450  400  450  1 100  44.0 
3,000  1 250  1 150  1 150  750  900  500  450  450  1 250  41.7 
3,500  1 350  1 250  1 200  800  950  500  450  500  1 350  38.6 
4,000  1 400  1 300  1 250  800  1 000  550  450  500  1 450  36.3 
5,000  1 550  1 400  1 400  900  1 100  550  500  550  1 600  32.0 
7,000  1 750  1 600  1 550  1 000  1 250  650  600  650  1 850  26.4 
10,000  2 000  1 850  1 750  1 150  1 400  750  800  750  2 150  21.5 
15,000  2 350  2 150  1 950  1 300  1 600  900  1 100  900  2 500  16.7 
20,000  2 600  2 350  2 100  1 450  1 800  1 050  1 400  1 050  2 750  13.8 
30,000  3 000  2 750  2 450  1 700  2 050  1 350  1 950  1 350  3 150  10.5 
40,000  3 350  3 050  2 700  1 950  2 250  1 600  2 450  1 600  3 400  8.5 
50,000  3 600  3 300  2 950  2 150  2 500  1 800  2 950  1 800  3 650  7.3 
100,000  4 600  4 300  4 050  3 250  3 650  2 600  5 100  2 400  4 650  4.7 
150,000  5 400  5 200  4 950  4 150  4 800  3 100  7 000  2 700  5 400  3.6 
200,000  6 250  6 100  5 800  4 900  5 800  3 450  8 750  2 850  6 050  3.0 
300,000  7 850  7 800  7 400  6 000  7 300  4 000  11 950  3 000  7 150  2.4 
500,000  11 000  10 850  9 950  7 550  9 300  4 600  . .  3 000  9 000  1.8 
1,000,000  16 300  16 500  14 250  9 600  11 850  5 250  . .  . .  12 700  1.3 
2,000,000  21 950  24 350  19 150  11 450  13 700  . .  . .  . .  18 400  0.9 
5,000,000  28 000  39 000  25 850  12 900  14 300  . .  . .  . .  30 700  0.6 
10,000,000  30 000  53 750  30 200  . .  . .  . .  . .  . .  41 000  0.4 
15,000,000  . .  . .  . .  . .  . .  . .  . .  . .  46 250  0.3 

. . not applicable 
T2 LEVELS AT WHICH ESTIMATES HAVE RELATIVE STANDARD ERRORS OF 25% AND 50%(a) 

 NSW  Vic.  Qld  SA  WA  Tas.  NT  ACT  Aust. 
 no.  no.  no.  no.  no.  no.  no.  no.  no. 

Relative Standard Error of 25%  7 100  6 100  5 800  2 900  4 000  1 600  1 300  1 600  7 800 
Relative Standard Error of 50%  2 300  2 000  1 900  1 000  1 300  500  400  600  2 000 

(a) Refers to the number of people contributing to the estimate 
This page last updated 29 August 2013