Researchers from Uppsala University in Sweden set up a project called UK Biobank, and collected data from nearly 500,000 participants in the United Kingdom from 2006 to 2010. They took blood samples and other physical measurements from participants, who ranged in age from 40 to 70, and also gave them questionnaires. They followed up throughout the period to figure out which participants had died, and their causes of death.

After crunching the numbers, they realized the questionnaire was a more accurate predictor of death than any medical measurements. Self-reported assessments of overall health and how fast participants walked were the strongest predictors when it came to risk of death.