3.4  Health

The HWR survey includes an extensive series of questions related to health. The following extract describes the use of this information:

The measure of health used for the HWR study was the Australian and New Zealand version of the SF-36 which has become one of the most widely used (both in New Zealand and internationally) standard questionnaires for measuring physical and mental health status. The SF-36 includes 36 items measuring physical and mental health in relation to nine health scales: general health (self assessment of health overall), physical functioning, role physical (how much physical health has affected daily activities), mental health, role emotional (how much emotional health has affected daily activities), social functioning (how health has affected social activities), health transition (perceptions of health changes), bodily pain, and vitality. All scores have been weighted so that they may be interpreted in the same direction: higher scores mean better health. The subscales have also been combined to provide two summary scores for physical and mental health respectively. The general health status of this cohort is indicated by scores on the SF-36 which have been transformed using New Zealand population norms and standardised so that the population mean is 50. (Stephens and Noone, 2007: p3)

In the present study we focus on the summary measures for physical and mental health (see Figure 6).

The SF-36 also yields two psychometrically-based physical and mental health summary measures: the Physical Component Summary (PCS) and the Mental Component Summary (MCS). The PCS and MCS are computed following a three-step standardised procedure. First, all eight domains are standardised using a linear z-score transformation. Z-scores are calculated by subtracting the domain means for the general population (by age and sex) from each individual's domain score and dividing by the standard deviation of the NZ population (Ministry of Health, 1999). Second, the z-scores are multiplied by the domain factor score coefficients for PCS and MCS and summed over all eight domains. Finally t-scores are calculated by multiplying the PCS and MCS sums by 10 and adding 50 to the product to yield a mean of 50 and a standard deviation of 10 for the NZ population.[10]

Scott, Sarfati, Tobias and Haslett (2000) analyse the SF-36 health survey for New Zealand, to test whether the two-dimensional structure of physical and mental health is applicable to all major ethnic groups (European, Māori and Pacific). They find that while the structure applied to European and Māori aged less than 45, for Pacific peoples and older Māori the structure did not clearly differentiate between the physical and mental health components. For these groups the two components are not seen as independent. In subsequent analyses based on the HWR survey we have included ethnicity as an explanatory variable in an attempt to correct at least in part for this problem.

Table 3-4 summarises the pattern of the physical and mental health scores for the New Zealand population.

Source: Scott, Sarfati, Tobias and Haslett. (1999).

Figure 6: Structure of the SF-36 scheme for the measurement of health

Source:  Ware (2000).

Note: This figure does not include the category of health transitions referred to in the extract from Stephens and Noone (2007) cited above although these are in the summary measures used here.

A summary of the levels and distributions of the physical and mental health scores for both the working and retired groups is depicted in Figure 7. The average levels are similar for both groups although the ranges for the retired group are wider, especially in the case of the physical health measure.

As well as determining an objective measure of health, the survey asked respondents to self-assess their health. Table 3-5 reports this, broken down by sex and work status. The broad pattern of working aligning with better health is continued here.

Figure 7: Distribution of physical and mental health scores

Source:  Health, Work and Retirement Survey

Note:
¹ The estimates for those unemployed are based on a small sample of approximately 40 observations for each gender.