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sedentary lifestyle and risk of death

This analysis accompanied a roundup which can be viewed here.


Title, Date of Publication & Journal

Sedentary behaviour and life expectancy in the USA: a cause-deleted life table analysis’ – BMJ Open, Monday 9th July

Claim supported by evidence?

  • The paper does not prove a causal link between reducing sitting time and extending life.
  • The link between the two could be due to confounders (the paper only adjusts for sex and age) – see below.
  • The link between TV and extending life could be entirely due to the risk of sitting – i.e. not specific to TV at all.


  • The data is based on a few studies, 5 in total: 2 for sitting and 3 for watching TV.
  • The 5 papers show similar relative risks. The evidence is consistent across the studies.
  • The effects of watching TV and of sitting are similar across the studies.
  • The study is observational and of a suitable size. But observational studies are limited in their ability to make causal associations.

Study Conclusions

  • The results for watching TV are in line with the results for sitting. (i.e. watching TV has about the same effect as sitting).
  • The authors clearly state that their conclusions assume a causal relationship, and that their results do not determine one.


  • The study is well conducted and well described in the paper.
  • The authors clearly flag the main limitation that there is no adjustment for confounders other than sex and age. This is not clear in the press release. For example, people who are less healthy to begin with might spend more time sitting down. 
  • There are only 2 studies for the effect of sitting and only 3 studies for the effect of watching TV.
  • The NHANES data does not contribute to the relative risks; i.e. it does not contribute to the evidence for a relationship between sitting / watching TV and life extension
  • It is not clear how much sitting is reduced to achieve <3 hours per day, nor how much TV is reduced to achieve <2 hours per day.


NHANES data: National Health and Nutrition Examination Survey – a survey of selected participants who are representative of the US civilian population.

Any specific expertise relevant to studied paper (beyond statistical)?



‘Before the headlines’ is a service provided to the SMC by volunteer statisticians: members of the Royal Statistical Society (RSS), Statisticians in the Pharmaceutical Industry (PSI) and experienced statisticians in academia and research.  A list of contributors, including affiliations, is available here.

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