Biases in Self-Rated Heights: Part Deux

I am re-posting on the issue of biases in self-rated heights after some skepticism over the content and theories raised in my last post. The graphs above now contain 95% confidence intervals. These results are consistent with the shrinking misrememberment hypothesis.

The biases for the post-1940's year of birth cohorts are not significant. However, the reason I suspect these figures are biased is because people are rounding up by small amounts. Personally speaking I am 5ft 11.5 inches (181.6 cm) although I always round this up to an even 6ft (182.88cm) when asked my height. I propose this measurement error occurs systematically over large sample sizes. For example, if there are 100 5ft 11.5inches in a sample, I would assume that the majority declare themselves to be 6ft based on convenience. Researchers should be wary of the rounding up hypothesis in subjective heights - despite the insignificant results obtained above.

Biases in Self-Rated Heights

The purpose of this post is to alert researchers towards biases that arise when people self-report their height. I will contrast the way height is reported between two different surveys, the Health Survey for England and The British Household Panel Survey. The 2005 Health Survey for England reports heights objectively – as measured by a visiting nurse using specifically designed apparatus. Wave 14 of the British Household Panel Survey asks participants their heights subjectively. Both surveys specifically want measured height from participants not wearing shoes. In addition, both surveys are nationally representative when compared to the UK census. For the purposes I only look at adults aged over twenty born in England of white race or “British,” ethnicity. Results are split by gender and into year of birth cohorts.

Table 1.

Table 1 and the accompanying graphs display the degree of biases which take place when people self report their heights. In almost all cases individuals have higher self reported heights in comparison to their subjective heights. There are a number of potential reasons for this. Firstly, people may be rounding their heights. Subjective height from the 2005 BHPS is mostly measured in feet and inches (although participants were given the option to answer in centimeters). The results displayed above are consistent with the hypothesis that people round their heights up.

Examining the frequency distributions of both samples and sexes illustrate this “rounding,” effect. Objectively measured height from the Health Survey for England clearly follows a more normal distribution. The BHPS data is quite staggered and is more discrete than continuous. It also appear that males are more likely to round than females.

The rounding up hypothesis does not explain why the heights diverge for the later year of birth cohorts. This effect seems indicate that the over 60 age category systematically mis-remember their heights. It is widely known and observed that people shrink with age (Diacinti et al. (1995)). The results in table one are consistent with this hypothesis. They also suggest that the divergence between subjective and objective measures of heights are caused by older age cohorts recalling their heights from the early adult years and not taking into account that they may have shrunk over time.

The implications of this post are clear. Self-reported height is biased and researchers should be extremely careful in their analyses and interpretation of it.

Reference:
Diacinti, D., Acca, M., D'Erasmo, E., Tomei, E., Mazzuoli, G.F, “Aging Changes in Vertebral Morphometry,” Calcified Tissue International, 57 (1995), 426—429.