4 Empirical Results
4.1 Estimation results
This subsection presents the main empirical results. The results for the joint models of selection equations and wage equations are reported for each demographic group in Tables 1, 2 and 3. Tables A.4 to A.6, containing the results using the two-step Heckman approach, can be found in the appendix. The sample sizes are, for married women, married men, single women, single men and sole parents respectively 12294, 10528, 4279, 4691 and 2166. The inverse Mill’s ratio has the expected sign for all groups except sole parents, that is, for most groups the parameter is significant and positive. The inverse Mill’s ratio is negative for sole parents. However, its value is very small and insignificant. The interpretation of negative inverse Mill’s ratios in this context was discussed by Ermisch and Wright (1994)[9].
| Wage equation | Selection equation | |||||||
|---|---|---|---|---|---|---|---|---|
| Married men | Married women | Married men | Married women | |||||
| Coef. | z-value | Coef. | z-value | Coef. | z-value | Coef. | z-value | |
| Coef. | z-value | Coef. | z-value | Coef. | z-value | Coef. | z-value | |
| Married men | Married women | Married men | Married women | |||||
| Wage equation | Selection equation | |||||||
| age/10 | 0.5104 | 16.74 | 0.2569 | 8.73 | 1.2803 | 10.33 | 1.1491 | 11.13 |
| Age sq./100 | -0.0592 | -16.16 | -0.0297 | -8.34 | -0.1717 | -11.44 | -0.1671 | -12.95 |
| Education (reference group is less than school cert.) | ||||||||
| School certific. | 0.0879 | 6.32 | 0.0668 | 5.44 | 0.3402 | 6.54 | 0.3179 | 8.57 |
| bursary | 0.1539 | 4.09 | 0.1974 | 5.37 | 0.8469 | 5.90 | 0.4941 | 4.45 |
| Voc./trade cert. | 0.1041 | 2.79 | 0.2469 | 6.52 | 0.8526 | 6.00 | 0.6449 | 5.59 |
| Bach. Deg/dipl. | 0.1382 | 2.32 | 0.2629 | 4.46 | 1.0607 | 4.01 | 0.4246 | 2.10 |
| Post-grad. qual. | -0.1135 | -1.23 | 0.4187 | 4.16 | 0.3450 | 0.83 | 1.1105 | 2.88 |
| part deg/other | 0.1062 | 2.30 | 0.2770 | 6.16 | 0.6298 | 3.56 | 0.7459 | 5.15 |
| Pgrad*age/10 | 0.1205 | 5.68 | -0.0170 | -0.70 | 0.0995 | 1.08 | -0.0504 | -0.56 |
| Bach*age/10 | 0.0297 | 2.12 | 0.0000 | 0.00 | -0.1033 | -1.80 | 0.0629 | 1.27 |
| Voc/med*age/10 | 0.0015 | 0.18 | -0.0269 | -3.18 | -0.0939 | -3.19 | -0.0300 | -1.19 |
| Nr of children | -0.0195 | -1.00 | -0.0646 | -3.97 | ||||
| Age of youngest child | ||||||||
| 0 | -0.0920 | -1.18 | -1.5844 | -25.35 | ||||
| 1 to 3 | -0.1083 | -1.64 | -1.1239 | -21.85 | ||||
| 4 to 5 | -0.1486 | -1.84 | -0.8134 | -12.84 | ||||
| 6 to 9 | -0.1479 | -2.10 | -0.4217 | -7.66 | ||||
| Over 9 | -0.1677 | -2.26 | -0.1593 | -2.68 | ||||
| Ethnicity (reference group is European) | ||||||||
| Maori/Pacif.Isl. | -0.0178 | -0.64 | -0.0223 | -0.84 | -0.3525 | -3.50 | 0.1168 | 1.34 |
| Maori/Pac.only | -0.0754 | -3.55 | -0.0502 | -2.44 | -0.2891 | -4.16 | 0.0277 | 0.45 |
| Other non-Eur. | -0.2015 | -3.54 | -0.2732 | -2.12 | -0.2997 | -1.44 | -0.4296 | -1.19 |
| Other n.E. only | -0.1248 | -2.11 | 0.1926 | 1.07 | ||||
| Pgr/bac*ma/pa | 0.0049 | 0.08 | -0.2013 | -3.26 | 0.1808 | 0.61 | -0.1238 | -0.56 |
| Pgr/bac*other | 0.1435 | 2.49 | 0.0966 | 1.57 | -0.3044 | -1.56 | -0.1005 | -0.58 |
| voc/med*ma/pa | 0.0088 | 0.31 | -0.0878 | -2.88 | -0.0509 | -0.49 | -0.1778 | -1.90 |
| Voc/med*other | 0.0807 | 1.30 | 0.1552 | 2.45 | -0.0288 | -0.13 | 0.0951 | 0.52 |
| Industry (reference group is services) | ||||||||
| agriculture | -0.0672 | -2.77 | 0.0279 | 0.87 | ||||
| Mining/quarry | 0.1707 | 3.56 | 0.1681 | 0.88 | ||||
| Manufacturing | 0.0639 | 4.88 | 0.0478 | 3.10 | ||||
| Elec./gas/water | 0.1354 | 4.88 | 0.0577 | 0.92 | ||||
| construction | -0.0135 | -0.72 | -0.0007 | -0.02 |
|
|||
| trade | -0.1111 | -7.86 | -0.0617 | -5.09 | ||||
| transport | 0.0416 | 2.00 | 0.0353 | 1.43 | ||||
| finance/real est. | 0.1355 | 8.93 | 0.0707 | 5.35 | ||||
| communication | 0.0617 | 2.96 | 0.0526 | 1.67 | ||||
| other | 0.0093 | 0.29 | 0.0622 | 1.83 | ||||
| Occupation (reference group is elementary occupations) | ||||||||
| manager | 0.2930 | 15.96 | 0.3493 | 17.17 | ||||
| professional | 0.2587 | 12.74 | 0.3833 | 19.56 | ||||
| associate prof. | 0.2643 | 13.33 | 0.3191 | 15.09 | ||||
| clerks | 0.1274 | 5.37 | 0.2574 | 14.40 | ||||
| Sales workers | 0.1139 | 5.35 | 0.0680 | 3.75 | ||||
| agr./fish. wrk. | -0.0280 | -0.95 | -0.0069 | -0.18 | ||||
| Trades workers | 0.1173 | 6.19 | 0.1083 | 3.30 | ||||
| plant/mach.wrk | 0.0604 | 3.28 | 0.0316 | 1.30 | ||||
| Elig. NZ super | -0.7402 | -8.17 | -0.3099 | -3.26 | ||||
| Other inc./1000 | -0.1614 | -4.98 | -0.1388 | -4.89 | ||||
| Characteristics of partner | ||||||||
| wage inc./1000 | -0.0739 | -1.20 | -0.0511 | -3.17 | ||||
| employed | 0.6492 | 15.01 | 0.6954 | 18.77 | ||||
| School certific. | 0.1991 | 4.41 | 0.0300 | 0.69 | ||||
| bursary | 0.2432 | 3.98 | 0.0701 | 1.29 | ||||
| Voc./trade cert. | 0.1232 | 2.54 | 0.0019 | 0.05 | ||||
| Bach. Deg/dipl. | 0.0818 | 1.05 | -0.1912 | -3.67 | ||||
| Post-grad. qual. | -0.2657 | -2.27 | -0.4025 | -5.52 | ||||
| part deg/other | 0.1079 | 0.87 | -0.0514 | -0.52 | ||||
| Elig. NZ super | -0.0046 | -0.04 | -0.1480 | -2.44 | ||||
| Maori/Pacif.Isl. | -0.0407 | -1.57 | 0.0304 | 1.09 | -0.1078 | -1.06 | 0.0308 | 0.34 |
| Maori/Pac.only | -0.0771 | -4.35 | 0.0058 | 0.33 | -0.2287 | -3.65 | 0.0413 | 0.73 |
| Other non-Eur. | 0.0856 | 0.84 | 0.0372 | 0.19 | -0.0775 | -0.19 | -0.6884 | -1.17 |
| Other n.E. only | -0.1442 | -3.26 | -0.1411 | -2.83 | -0.1501 | -0.80 | -0.6351 | -4.00 |
| Region (reference group is Auckland) | ||||||||
| North North isl | -0.0835 | -6.64 | -0.0931 | -7.49 | -0.2345 | -4.86 | -0.1111 | -2.86 |
| Central N. isl | -0.1103 | -8.21 | -0.0744 | -5.62 | -0.2333 | -4.44 | -0.0286 | -0.68 |
| Wellington | 0.0423 | 3.31 | 0.0325 | 2.58 | -0.0984 | -1.81 | 0.1269 | 3.01 |
| Canterbury | -0.0859 | -6.37 | -0.0712 | -5.26 | -0.1249 | -2.20 | -0.0274 | -0.63 |
| South island | -0.0911 | -6.78 | -0.0954 | -7.14 | -0.1712 | -3.06 | -0.0324 | -0.75 |
| Year (trend) | 0.0018 | 0.69 | 0.0079 | 3.11 | 0.0223 | 2.09 | 0.0123 | 1.51 |
| Unempl. rate | -0.0061 | -1.70 | 0.0008 | 0.16 | -0.0351 | -2.49 | -0.0212 | -1.43 |
| constant | 1.5735 | 20.29 | 1.7486 | 21.66 | -1.3547 | -4.56 | -1.6161 | -6.42 |
| Correlationa | 0.3120 | 0.22,0.40 | 0.2807 | 0.21,0.35 | Nr of obs. | 10528 | 12294 | |
| Sigmaa | 0.3639 | 0.36,0.37 | 0.3352 | 0.33,0.34 | Nr of | |||
| Mill’s ratioa | 0.1135 | 0.08,0.15 | 0.0941 | 0.07,0.12 | cens.obs | 2081 | 5076 | |
| Log likelihood | -7245.30 | -8779.27 | ||||||
| Restricted Log likelihood b | -10128.46 | -11872.76 | ||||||
| Pseudo-R2 c | 0.28 | 0.25 | ||||||
Notes a: Instead of a z-value a 95% confidence interval is given.
b: The restricted model consists of a wage equation with a constant and sigma and a selection equation with a constant only.
c: Pseudo R2 is calculated by 1 – 
Notes
- [9]Miller and Rummery (1991) found a positive value for women and a negative value for men. They also review results found in previous Australian studies. They do not distinguish between single and married men and women. Murray (1996) found a positive value for mothers (single and married) and Kalb (2000) found a positive value for married women and a negative (insignificant) value for married men.
