Validation of the NZW-HDI

5.4.2.1 Selection of a reference method

To ensure the NZW-HDI accurately reflected diet quality, dietary information obtained from the NZWFFQ was required to be validated against an independent reference method. The weighed food record was used as it has been recognised as the ‘gold standard’ for dietary assessment (Biro et al., 2002). The FR is a prospective method that is not memory dependent, and has less correlated errors with a FFQ when compared to a 24-hour recall (Cade et al., 2002). Of the studies that have validated

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DQIs using another dietary method, only two have used a food record (Roy et al., 2015; Wong et al., 2013), and the remainder have used food frequency questionnaires (Collins et al., 2015; Newby et al., 2003; Toft et al., 2007).

In the present study, participants were asked to maintain normal eating habits during the course of completing the FR. Participants were required to complete the food record for a duration of four days, which included three week days and one weekend day. This is a favourable duration of recording days, with four to five days recommended as an ‘ideal’ number of days to complete food records (Stram et al., 1995). The food record does however limitations that may influence agreement between the NZWFFQ derived NZW-HDI and the FR derived NZW-HDI. The use of a FR in this instance may be associated with a higher participant burden than other dietary assessment methods (Biro et al., 2002; Thompson & Subar, 2008). Despite the provision of audio-visual and written materials that were easy to comprehend (Grandjean, 2012), it is acknowledged that misreporting may still occur due to multiple-day recording that potentially discourages completion (Biro et al., 2002; Willet, 1998).

There were also limitations in regards to the time period in which the “test” and “reference” method covered. The two dietary methods should ideally cover the same period of time, but in this study, the NZWFFQ was completed first and therefore covered a different time period the FR. This implies that both methods did not assess the same period, which may have resulted in underestimation of the relative validity of the index (Wong et al., 2013). However, research suggests that the reference method (FR) be completed following the test method (NZWFFQ), to avoid the reference method influencing the test method (Gibson, 2005).

Similarly to the NZWFFQ, many researcher decisions and assumptions were made for the NZW-HDI score derived from the FR. In order to determine serving sizes (e.g. 1 serving of fruit) from the FR data, the weight in grams from each food item consumed was converted into volume (e.g. cups) over the four days, and then an average intake was obtained. This was deemed the most appropriate method for analysis. However, for some components this was not always possible. For example, takeaway consumption is based on the amount consumed per week. As the food record was only completed for four days, this meant the average obtained may not be as accurate. In addition, for two components - trimming of fat off meat/skin off chicken, and salt use – these could not fully be captured in the food composition database, therefore raw food record data was used as well. All of these factors may have in turn affected the validity of the NZW-HDI.

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5.4.2.2 Approaches to validation

Limited validation research has been conducted on diet quality indices (Guenther, Reedy, Krebs- Smith, et al., 2008; Kant, 1996; Woodruff & Hanning, 2010), particularly validating the index with an independent reference method (e.g. multiple day food record). Content validity was assessed by ensuring all dietary guidelines from the EAGNZA corresponded to components in the NZW-HDI. Both construct and relative validity were used to determine the validity of the NZW-HDI. Although there were many methodological issues surrounding the construction and validation of the index, the results suggest that the NZW-HDI is reasonably valid as a measure of diet quality and can adequately assess adherence to the EAGNZA.

Another approach used to assess relative validity of a newly developed index involves biomarkers, which are independent markers of dietary intake (Gibson, 2005). These have become widely used to overcome the inherent limitations associated with dietary assessment methods, such as misreporting (Gibson, 2005; Thompson & Subar, 2008). However, biomarkers are expensive, and the availability of biomarkers is limited to a number of nutrients (Cade et al., 2002; Gibson, 2005). Many dietary indices have used biomarkers in the validation process (Fung et al., 2005; Hann et al., 2001; Harnack et al., 2002). For example, the Healthy Eating Index was validated using plasma biomarkers, such as carotenoids, vitamin C, and folate that are markers of long term fruit and vegetable consumption (Hann et al., 2001). Further validation of the NZW-HDI against biomarkers should be completed.

Other studies have assessed construct validity by comparing diet quality index scores with demographic and health information (Fogli-Cawley et al., 2006; McNaughton et al., 2008) For example, a higher body mass index has been associated with lower diet quality scores (Hann et al., 2001; Harnack et al., 2002; Toft et al., 2007). These studies however, require larger population groups in order to find significant associations. In addition, the use of longitudinal studies as opposed to a cross-sectional study design is recommended (McNaughton et al., 2008).

5.4.2.3 Statistical methods

A range of statistical methods were used to validate the NZW-HDI including paired t-tests, correlation coefficients, cross classification, weighted kappa statistics, and linear contrast analysis. Due to the lack of agreement on the most suitable way to present results obtained from validation studies, it has been suggested that more than one statistical method be used (Cade et al., 2002). Another statistical technique that could have been utilised is the Bland and Altman method. This would measure the level of agreement between the NZWFFQ derived NZW-HDI and FR derived

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NZW-HDI, and also determine if there is any bias between the two dietary methods (Bland & Altman, 1999).