Health Survey for England, 2021 part 1

Self-reported height and weight

Participants were asked to report how tall they were and how much they weighed, both without shoes. They could give measurements in either imperial or metric units, and these were converted to metric units if necessary (metres for height, kilograms for weight).

Estimating overweight and obesity in 2021

Empirical studies (Connor Gorber et al, 2007; Maukonen, Männistö and Tolonen, 2018) have shown that adults typically overestimate height and underestimate weight compared with measured values. The prevalence of obesity is thus underestimated when based only on self-reported height and weight. 

A methodology study using HSE data from 2011 to 2016 developed a set of prediction equations that adjusted self-reported values of height and weight so that they more accurately predicted measured values of height and weight. The correction factors (prediction equations) used in this report are presented in the Technical Appendix.

A fuller description of the methods and findings of the methodology study is provided here in Health Survey for England predicting height, weight and body mass index from self-reported data and in the academic paper (Scholes et al, 2022).

Limitations

It cannot be assumed that the correction factors, developed using HSE 2011-16 data, produced precise estimates when applied to the self-reported heights and weights recorded in HSE 2021.

First, it is likely that participants in the HSE 2011-2016 might have anticipated that interviewers would take direct measurements of height and weight, resulting in more ‘truthful’ reporting compared, for example, with a telephone interview where participants would not anticipate being measured. Misreporting of weight has been found to be lower for face-to-face versus telephone interviews (Ezzati et al, 2006). Estimating a set of correction factors through surveys such as the HSE that often combine face-to-face interviewing with direct measurements of height and weight may therefore capture a lower bound of bias associated with self-reported data on height and weight. Applying these correction factors to HSE 2021, which collected participants’ reports of their height and weight by telephone rather than by face-to-face interviewing, likely underestimates measured values of height and weight to a greater extent than shown in the methodology study  Health Survey for England predicting height, weight and body mass index from self-reported data.

Second, although the methodology study showed no steady rate of increase or decrease in misreporting between 2011 and 2016, the correction factors developed using combined HSE 2011-2016 data might not be entirely applicable to the more recent data. This might be the case for example, if misreporting of weight has increased in recent years in tandem with any increase in obesity prevalence during the COVID-19 pandemic (due at least partly to fewer opportunities for outdoor physical activity). Such factors are not taken into account by the prediction equations used in this report.

In conclusion, based on previous studies, all things being equal, estimates of overweight (including obesity) and obesity based on BMI from corrected height and weight will approximate estimates based on BMI from measured height and weight more closely than those based only on self-reported data. However, corrected data will not completely adjust for the differences between self-reported and measured height and weight (and BMI and BMI status derived from these). In-home measurements of height and weight by trained interviewers are required to continue accurate monitoring of adults’ overweight and obesity.  

Body mass index

To define overweight or obesity, a measurement is required that allows for differences in weight due to height. A widely accepted measure of weight for height, BMI, defined as weight in kilograms divided by the square of the height in metres (kg/m²), is used for this purpose in the HSE series. For this report, BMI was calculated using a set of correction factors that adjust self-reported height and weight to predict measured height and weight more accurately.

Participants were classified into the following three mutually exclusive BMI groups according to the World Health Organization’s BMI classification (WHO, 2010). BMI status is presented only for these groups due to corrected values likely being less accurate at the low and high ends of the BMI scale.

BMI categories of overweight and obese were combined to show the proportion of participants who are either overweight or obese (BMI 25kg/m² or more). In this report, a BMI threshold of 30kg/m² has been used to define participants as obese. These definitions were applied to all participants aged 16 and over, as this is how adults are defined in the HSE series.