4364.0.55.001 - National Health Survey: First Results, 2017-18  
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Appendix 3: Technical details explaining the accuracy of self-reported height and weight data


Sources
Data presented in the analysis of self-reported and measured data are from:
  • 2007-08 and 2017-18 National Health Surveys (NHS)
  • 1995 National Nutrition Survey (NNS) which is a sub-sample of the 1995 NHS.


Definitions

Adult
A respondent aged 18 years or over.

Body Mass Index
Body Mass Index (BMI) is a simple index of weight-for-height that is commonly used to classify underweight, normal weight, overweight and obesity. It is calculated from height and weight information, using the formula weight (kg) divided by the square of height (m) and is expressed in kilograms per metres squared; kg/m2.


Table 1. Body Mass Index, Adults

CategoryRange

UnderweightLess than 18.50
Normal range18.50 — 24.99
Overweight25.00 — 29.99
Obesity class I30.00 — 34.99
Obesity class II35.00 — 39.99
Obesity class III40.00 or more


Separate BMI classifications were produced for children. BMI scores were created in the same manner described above but also took into account the age and sex of the child. There are different cutoffs for BMI categories (underweight/normal combined, overweight or obese) for male and female children. These categories differ to the categories used in the adult BMI classification and follow the scale provided in Cole TJ, Bellizzi MC, Flegal KM and Dietz WH, Establishing a standard definition for child overweight and obesity worldwide: international survey, BMJ 2000; 320. See Appendix 4 in NHS 17-18 Users' Guide for more details.

Child
A person aged 0-17 years.


Outliers
Observations which appear inconsistent within the dataset.


Treatment of outliers
Routine processing of the NHS involved conservative editing of the most extreme outliers. However, such editing may not detect all of the inconsistent records (given the number of possible interrelated variables that could be examined). In the analysis of self-reported and measured height and weight data, further data ‘cleaning’ was applied to exclude any records with large disparities, which were likely to be a result of reporting or data entry errors. In the feature article analysis, if the difference between self-reported and measured height or weight (expressed as a percentage) was greater than 4 standard deviations from the group mean, then the record was excluded from analysis.


Respondent sample
In 2017-18, 57.5% of respondents agreed to provide both measured and self-reported data and were included in analysis. A comparison between those included and excluded (i.e. those who provided self-reported and measured height and weight versus those who did not) showed no substantial differences between the two groups (see table 1 below).Therefore, for the purpose of the report, the sample analysed was representative of the whole 2017-18 NHS respondent sample (see table 1 below).



Table 2. Sample characteristics from NHS and report subsample, proportion of persons 18 years and over(a)

Included in report analysis(b)Provided self-reported data only

Index of Relative Socio-Economic Disadvantage(c)
First Quintile 15.617.6
Second Quintile20.720.5
Third Quintile20.120.6
Fourth Quintile21.620.7
Fifth Quintile22.120.6

Self-perceived body mass(d)
Acceptable weight 57.853.9
Underweight3.83.5
Overweight38.538.2

Level of highest educational attainment
Bachelor/post-graduate degree32.029.2
Advanced diploma/Diploma/Certificate III/IV31.831.1
Year 12 or equivalent14.215.1
Below Year 1218.120.9

Smoking status
Current daily smoker(e)11.713.8

Self-assessed health status
Excellent21.120.2
Very Good37.335.4
Good28.429.1
Fair10.311.3
Poor2.93.9

Age group (years)
18-2411.412.0
25-3420.219.4
35-4417.817.3
45-5416.316.8
55-6415.515.1
65-7411.811.6
75 years and over7.07.8

Footnote(s): (a) Cells in this table containing data have been randomly adjusted to avoid the release of confidential data. Discrepancies may occur between sums of the component items and totals. (b) Adults providing both self-reported and measured height and weight. (c) A lower Index of Disadvantage quintile (e.g. the first quintile) indicates relatively greater disadvantage and a lack of advantage in general. A higher Index of Disadvantage (e.g. the fifth quintile) indicates a relative lack of disadvantage and greater advantage in general. See Index of Relative Socio-Economic Disadvantage in the Glossary. (d) Excludes pregnant persons. (e) In 2017-18, data from NHS and Survey of Income and Housing (SIH) have been combined to create a much larger sample which will allow for a more accurate smoker status estimate..
Source(s): NHS 2017-18, SIH 2017-18,


Imputed data
As standard practise, an imputation (editing) method was applied for respondents that opted not to be measured (33.8% in 2017-18) to derive height and weight data. However, since the feature article focused on measured and self-reported data only, respondents that had imputed height and weight values were excluded from the analysis. See Appendix 2 for more details about imputation methods.


Non-response
The feature article analysed reporting trends for the 57.5% of survey respondents who agreed to be measured and who self-reported their height and weight. Without data from the remaining survey respondents it is difficult to determine whether those who volunteered and those who did not would differ when it comes to BMI, or reporting accuracy. To this end, there was a third group of respondents of interest who provided self-reported height and weight, but no measured data. When comparing self-reported BMI between the group analysed in the feature article, and the self-reported only group, the latter had a higher average self-reported BMI (see table 2 below). More research is needed to determine potential differences between those who opt to provide self-reported data only, those who also agree to be measured, and those who provide neither.


Table 3. Self-reported BMI by respondent compliance, average BMI

Included in report analysis(a)Provided self-reported data only

Males26.628.2
Females25.627.5
Persons26.927.8

Footnote(s): (a) Persons who provided both measured and self-reported height and weight. (b) Persons who provided self-reported height and weight data but not measured.
Source(s): NHS 2017-18