4715.0 - National Aboriginal and Torres Strait Islander Health Survey, 2018-19 Quality Declaration 
Latest ISSUE Released at 11:30 AM (CANBERRA TIME) 11/12/2019   
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PHYSICAL MEASUREMENTS


In the 2018–19 National Aboriginal and Torres Strait Islander Health Survey (NATSIHS), physical measurements of height, weight and waist circumference were collected from respondents aged two years and over, and was a voluntary component of the survey. Women who advised that they were pregnant were not measured. Voluntary blood pressure measurements were also collected from respondents aged 18 years and over. Persons with a high Body Mass Index (BMI) or blood pressure are at risk of developing chronic disease. BMI is a simple index of weight-for-height, which defines whether a person is underweight, normal weight, overweight or obese – see Assessing health risk factors (appendix).

NON-RESPONSE RATES

Physical measurements had a relatively high rate of non-response due to their voluntary and sensitive nature. To correct for the high rate of non-response, imputation of values for those that did not have measurements collected was used to achieve estimates of physical measurements for the whole population.


Non-response rates for physical measurements, by age


Total persons in sample
Body Mass Index(a)
Waist circumference
Blood pressure

no.
%
%
%

Children (2–17 years)
24
798
52.3
55.3
. .
57
789
54.8
56.4
. .
811
945
47.8
49.3
. .
1214
628
56.7
58.3
. .
1517
529
60.9
62.6
. .
Total 214 years
3 160
52.4
54.4
. .
Total 2–17 years
3 689
53.6
55.5
. .

Adults (18 years and over)
1824
931
34.0
36.2
34.3
2534
1 494
38.3
39.9
37.4
3544
1 167
40.0
42.2
41.4
4554
1 151
42.7
44.4
41.6
55 years and over
1 680
42.7
45.3
43.0
Total 18 years and over
6 423
39.9
42.0
39.9

. . not applicable
(a) Respondent's height and/or weight measurement not taken.


SELF-REPORTED HEIGHT AND WEIGHT

In addition to the voluntary measured items, respondents in the 2018–19 NATSIHS were also asked to self-report their height and weight measurements. Of those whose BMI was not measured, 55.8% of adults and 34.3% of children provided both self-reported height and weight measurements. This provided valuable information about the height and weight that was used in the imputation for people with missing values.

HOW IMPUTATION WORKS

In the 2018–19 NATSIHS and both the 2014–15 National Aboriginal and Torres Strait Islander Social Survey (NATSISS) and 2017–18 National Health Surveys (NHS), missing values were imputed using the 'hot decking' imputation method. In this method, a record with a missing response (the 'recipient') receives the response of another similar record (the 'donor').

A number of characteristics (imputation variables) were used to match recipients to donors.

For adults they were:
    • age group
    • sex
    • part of state (capital city and balance of state)
    • self-perceived body mass (underweight, acceptable, or overweight)
    • level of exercise (sedentary, low, moderate or high)
    • whether or not has high cholesterol (as a long-term health condition)
    • self-reported BMI category (calculated from self-reported height and weight).

For example, a female recipient aged 35–39 years who lives in a capital city, has a self-reported BMI category of overweight (calculated using self-reported height and weight), has a self-perceived body mass of healthy, has high cholesterol and lives a sedentary lifestyle will match to a donor record who has the same profile (female, 35–39 years, self-reports as overweight, etc).

For BMI, 80.4% of imputed records used all seven imputation variables to match to a donor record. The remaining 19.6% could not be matched using all seven variables and were therefore matched using fewer variables. For example, 7.4% of imputed records were matched to donors using all imputation variables except part of state.

For children aged 2–14 years, the following variables were used:
    • single year of age
    • sex
    • self-reported BMI
    • part of state.

For those aged 15–17 years, the same imputation variables were used as for those aged 2–14 years, in addition to level of exercise and self-perceived body mass (only if a person answered for themselves). Cholesterol data was not collected for persons under 18 years of age and so could not be used as an imputation variable.

Single year of age was used in the imputation method for the 2018–19 NATSIHS, which differs from the 2017–18 NHS. The change was made for the 2018–19 NATSIHS as children's height and weight can change rapidly within a short period of time and, to avoid over or under estimating BMI, donors that were of the same age as the recipient were used. A similar change to the imputation method will be investigated for the next iteration of the NHS.

IMPACT OF IMPUTATION

Physical measurement data (BMI, waist circumference and blood pressure) that includes imputed values are of suitable quality for comparisons with previous surveys; however, the Australian Bureau of Statistics (ABS) recommends using proportion comparisons only as imputation was not used on the physical measurement data in previous years.

The table below shows the impact of imputation on BMI estimates.


Measured and imputed Body Mass Index (BMI) results, by age(a)

Measured only
Measured and imputed

Body Mass Index category
no.
%
no.
%

2–17 years
Underweight
169
9.9
357
9.7
Normal
942
55.1
2 026
54.9
Overweight
374
21.9
823
22.3
Obese
225
13.2
483
13.1
Total overweight/obese
599
35.0
1 306
35.4
Total 2–17 years
1 710
100.0
3 689
100.0

18 years and over
Underweight
130
3.4
221
3.4
Normal
872
22.6
1 414
22.0
Overweight
1 062
27.5
1 791
27.9
Obese
1 794
46.5
2 997
46.7
Total overweight/obese
2 856
74.0
4 788
74.5
Total 18 years and over
3 858
100.0
6 423
100.0

(a) Using National Aboriginal and Torres Strait Islander Health Survey 2018–19 unweighted sample counts.


Whether Body Mass Index (BMI) measured or imputed(a)

2–17 years
18 years and over

Body Mass Index
no.
%
no.
%

Measured
1 710
46.4
3 858
60.1
Imputed
1 979
53.6
2 565
39.9
Total
3 689
100.0
6 423
100.0

(a) Using National Aboriginal and Torres Strait Islander Health Survey 2018–19 unweighted sample counts.