# Australian Bureau of Statistics

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 Understanding statistics

Module 2: Describing, Clarifying and Presenting Data

2. Characteristic, Variable and Measurement

2.1. The relationship between characteristic and variable

Let’s suppose you have the question, “Are Wollongong teenagers obese?” You need to define your
population, your characteristic, and your variables.

Let’s say that you decide that the population for the study is residents from 13 to 19 years of age who live in the Wollongong Local Government area and that the units of the study will be individual Wollongong teenagers.

That was the easy part. How are you going to define a characteristic such as ‘obesity’; i.e. how will you define what you mean by ‘obese’? You soon realise that there are different concepts of what it means for different people to be obese depending on their sex, age, and height, as well as on other variables. You need to use a variable to define and measure the characteristic ‘obesity’.
What variable will you use to measure obesity? You might decide to use Body Mass Index (BMI) as an indicator of obesity. BMI is a constructed variable because it cannot be observed directly but must be calculated from observable variables, namely mass (weight) and height. Body Mass Index can be calculated by dividing a person's weight (in kilograms) by the square of their height (in metres). You can write this as the following equation:

For your study, you decide to use the standard provided by the World Health Organisation (WHO) in which a BMI of less than 25 kg/m2 indicates normal weight, more than 25kg/m2 indicates a person is overweight, and more than 30kg/m2 indicates that a person is obese.

Let’s look at an example.

 Scenario Consider two teenagers, both 1.5 metres tall, one of whom weighs 45kg and the other 67.5kg. The first teenager has a BMI of 20kg/m2 and the second a BMI of 30kg/m2. Thus, according to the WHO specifications for this measure, the second teenager is obese.

 Test your knowledge Question Fill in the blanks The characteristic you are testing is: ________ The variables you are measuring are _______ and _______ The variable you are using to describe the characteristic is __________ The population you are testing is _________ body mass index Wollongong teenagers weight obesity height Answer Click here for answers

Is BMI necessarily the best variable for obesity?

A good researcher is flexible enough to take on board new information. You might realise as you conduct your research that although BMI is a convenient and simple measure of obesity, it is by no means the only or the best measure. Two people with the same BMI are not necessarily equally obese. Gender and age might influence BMI but, more importantly in terms of indicating obesity, so do skeletal and musculature factors (A big-boned, muscular, very fit athlete may have the same BMI as an obese person!). Other variables that can be used include:

• the Ponderal Index, which is calculated by dividing a person's height by her/his weight cubed, (height/weight3);
• skin fold thickness measured using calipers; and
• body density using a CAT scan.

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