Development of the NZW-HDI

5.4.1.1 Index components, cut-off points, and scoring

There are many researcher decisions involved in the development of a diet quality index. These include choices related to the selection of dietary components, grouping of food items, cut-off points and the scoring approach used. Unfortunately, for indices previously developed there is a lack of detailed information in the literature regarding the rationale behind decisions and assumptions made.

The index components of the NZW-HDI were fundamentally based on the Eating and Activity Guidelines for New Zealand Adults (EAGNZA). A qualitative check of the index components against the dietary guidelines used ensures aspects of diet quality specified in the guidelines are captured (content validity) (Guenther, Reedy, Krebs-Smith, et al., 2008). The NZW-HDI was reviewed and it

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was found that all components reflected recommendations within the EAGNZA. As there are numerous diet quality indices available, it has been suggested that researchers model newly developed indices on existing DQIs (Wirt & Collins, 2009). For the NZW-HDI, a comprehensive literature review was conducted prior to development. Based on the findings, the NZW-HDI components selected were modelled on the Dietary Guideline Index developed for the Australian population by McNaughton et al (2008).

To further limit subjectivity, cut-off values were based on the dietary guidelines where possible. Cut- off points were established based on recommendations specified in the EAGNZA, such as number of servings or types of foods to consume. Instead of using dichotomous values (e.g. a score of 0 for not adhering to guidelines, and a score of 1 for adhering to guidelines) which can be a somewhat black and white approach (Bazelmans et al., 2006; Wong et al., 2013), this study used an alternative method. A minimum and maximum score was attributed to each index component, and where possible, intermediate scores were assigned proportional to intake (e.g. number of servings consumed). This was to eliminate arbitrary adjustments and also ensured each serving was recognised and contributed to the final score. For example, for individuals who ate two servings of vegetables (and not the recommended three as in the EAGNZA, this was still recognised and contributed to the final score. In some cases, however, quantitative criteria could not be used to establish cut-off points as there was no explicit recommendation in the EAGNZA. An example in the guidelines is ‘choose foods low in salt’, which is not quantified. In this case, cut-off points were based on previous indices such as the DGI (McNaughton et al., 2008). It has been suggested that population specific dietary guidelines should be formulated in quantitative terms, to assist in the interpretations of dietary guidelines and to reduce the amount of arbitrary decisions in the development of an index (Wong et al., 2013).

Due to the number of index components and scoring method applied, the total score was out of 115 points. Unfortunately, this meant that the diet quality score may not be as easy to interpret. It has been suggested that a maximum score of 100 that has been used in some indices provides more meaningful interpretation, as results can be expressed as percentages (Dubois et al., 2000). However, there is no reason why the score out of 115 cannot be converted to a percentage value.

Difficulty also occurred when attributing weights for components towards the total score. For index such as the main food groups (e.g. fruits, vegetables, grain foods, milk and milk products, meat and meat products), a higher weighting of 10 points was given to toward the total score. Index components such as takeaway consumption and alcohol intake contributed only 5 points to the total score. This is based on the assumption that components can be either favourable or unfavourable to

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health. However, it is still unclear as to whether index components have equal or unequal effects on diet, health, and disease (Woodruff & Hanning, 2010). This method of unequal weighting is similar to those found in previous DQIs (Bazelmans et al., 2006; Lee et al., 2008).

5.4.1.2 Use of NZWFFQ data

The dietary data from the NZWFFQ was also problematic as it was not specifically developed to construct the index. As the index was developed within the boundaries of the NZWFFQ data; the types of questions on food items may have restricted the variation in dietary variety and serving intake responses. For example, the NZWFFQ data does not distinguish between whole grains and refined grains, and there were a limited number of whole grain options that could be used compared to total grain foods. Other arbitrary decisions needed to be made regarding the NZWFFQ (see Appendix 1). Overall, data from the NZWFFQ may have misclassified participants in a way which may have affected the total NZW-HDI score. It is suggested that further validation work be undertaken using an alternative dietary assessment method, for example a short questionnaire which reflects the EAGNZA.

5.4.1.3 Adjustment for energy intake

In addition, as nutrient intakes are positively correlated with energy intake, DQIs have the potential to overrate high energy diets. This may lead to participants who consume higher amounts of energy having a better chance at adhering to the dietary guidelines, and therefore receiving a higher NZW- HDI score. Some studies have taken this into account and adjusted for energy intake by using gender and age specific recommendations (Kennedy et al., 1995). However, findings from the Healthy Eating Index 2005 have shown that index components were independent of energy intake. This is because no consensus has been reached on whether adjusting for energy is necessary (Guenther, 2008). Energy adjustment was initially attempted in the NZW-HDI, but made no further differences to the nutrients across the tertiles (data not shown). For this reason, the NZW-HDI did not adjust for energy intake.