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Evaluation of the Australian Wage Subsidy Special Youth ...

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220<br />

As both <strong>the</strong> Heckman bivariate probit and PSM are not statistically significant when<br />

weighted, <strong>the</strong>re would seem to be a substantial loss <strong>of</strong> efficiency in <strong>the</strong> application <strong>of</strong> <strong>the</strong><br />

weights.<br />

Table 6.9 Employment effects <strong>of</strong> Heckman versus PSM<br />

unweighted<br />

Weighted for attrition<br />

Heckman PSM<br />

Heckman PSM<br />

selection bivariate<br />

probit<br />

selection<br />

bivariate<br />

probit<br />

Employment effect 0.26 153 0.18 0.10 154 0.08<br />

t statistic (2.85)** (2.74)** (0.79) (1.50) 155<br />

PSM: one-to-one nearest-neighbour within-caliper (0.001) matching with replacement.<br />

The key assumption where weighting is applied is that <strong>the</strong> non-response has a missing at<br />

random [MAR] process. Violation <strong>of</strong> this assumption would invalidate <strong>the</strong> results. It<br />

should be noted that not treating for non-response, as in <strong>the</strong> unweighted results, adopts<br />

<strong>the</strong> stricter missing completely at random [MCAR] assumption. Thus, weighting <strong>the</strong> data<br />

is a relaxation <strong>of</strong> this assumption. Part <strong>of</strong> <strong>the</strong> MAR assumption for <strong>the</strong> weighting<br />

treatment <strong>of</strong> attrition and non-response is that <strong>the</strong> observed variables used for modelling<br />

can accurately represent <strong>the</strong> non-response and <strong>the</strong>re are no unobserved variables<br />

involved. 156<br />

This assumption is added to <strong>the</strong> modelling assumptions applicable to each <strong>of</strong> <strong>the</strong><br />

Heckman and PSM models, as discussed earlier. In <strong>the</strong> Heckman bivariate probit model,<br />

<strong>the</strong> unobservable information is assumed to be correlated suitably with <strong>the</strong> observed<br />

variables used, in order to solve <strong>the</strong> selection problem. For <strong>the</strong> PSM, CIA assumes that all<br />

observable variables that jointly influence employment and participation are both in <strong>the</strong><br />

data and in <strong>the</strong> model. Unobserved factors, such as lack <strong>of</strong> job search effort, could affect<br />

153 For SYETP in <strong>the</strong> employment equation: dy/dx =0.0325919 and mean for SYETP is 0.081060. The<br />

mean SYETP translates to 8.1 per cent. The marginal effect calculated at <strong>the</strong> mean is <strong>the</strong>n 0.26418. This is<br />

interpreted as a 26 per cent increase in employment. Estimated using <strong>the</strong> mfx command in STATA7.0.<br />

154 For SYETP in <strong>the</strong> employment equation: dy/dx =0.0126509 and mean for SYETP is 0.080747. The<br />

mean SYETP translates to 8.1 per cent. The marginal effect calculated at <strong>the</strong> mean is <strong>the</strong>n 0.10215. This is<br />

interpreted as a 10 per cent increase in employment. Estimated using <strong>the</strong> mfx command in STATA7.0.<br />

155 Probability <strong>of</strong> (0.22).<br />

156 In o<strong>the</strong>r words, MAR implies <strong>the</strong> absence <strong>of</strong> ‘attrition on unobservables’.

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