Evaluation of the Australian Wage Subsidy Special Youth ...

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5.5.4 Attrition: natural attrition or analytical sample reduction?
Table 5.5 draws a distinction between the sources of sample reduction. Sample reduction
resulting from attrition, where the respondent did not take part in later surveys, is here
termed natural attrition. As outlined earlier, further sample reduction through analytical
selection also takes place before the final sample used for analysis is reached. Zabel
(1998) points out such analytical selection can in theory introduce biases of a similar
nature to attrition bias.
Indeed, Table 5.5 shows that both forms of sample reduction result in statistically
significant differences to the means observed of those remaining in the final sample. This
hints that both sources of sample loss may be suspected of altering the sample
characteristics. Also, the pattern of differences varies by the form of sample reduction
being natural attrition or analytical selection. This signals the added value of treating each
source separately in order to allow for differential impacts.
The first two columns of Table 5.5 show the mean and standard deviation in the final
sample, to which the two sources of sample reduction are compared. The next four
columns make up the first panel of the table which relates to the sample lost due to
natural attrition, and they show the mean, standard deviation, absolute difference in the
means and the t statistic, in a similar fashion to the former tables. The second panel gives
the statistics for those lost though analytical selection. Only differences that are
statistically significant at normally adopted confidence levels are discussed.
The outcome variables ‘ever employed in 1986’ and ‘ever in a government programme in
1986’ are not observable for those lost through natural attrition. It is useful to compare
the statistically significant differences in the profile of those remaining in the sample to
those lost to natural attrition. Of those not interviewed again: fewer had taken part in
SYETP, more had year nine of school but less had year 11, more had held jobs of 1-2
years while less had held jobs of 2-3 years, the proportion of unemployment in 1984 had