In most surveys quantitative data are gathered with questionnaires and interviews. A comparison of such self reported data with measured equivalents shows that people systematically underestimate or overestimate frequencies (cigarettes smoked
daily, age of onset, time to pregnancy) or clinical parameters (height, weight, blood pressure). This may occur because participants intentionally or unintentionally round figures to a preferred end digit. Bopp et al analysed the preference for the end digits zero and five when reporting height. Since height is overestimated in almost all cultures (with variable magnitude) people who round to zero and five probably overestimate rather than underestimate their height.
Their results show shows similarities between languages belonging to the same family. People speaking Germanic (and possibly Slavic) languages indicated the end digits zero or five consistently less frequently than did people speaking a Romance, Greek, or Semitic language. In a large and representative Swiss sample, people kept the end digit preference characteristic for their native language even when living in a region using a different language, suggesting that such preferences are inherent in culture.
This cultural bias might be particularly important when analysing trends in countries with high cultural preferences for rounding numbers and this could mask or exaggerate real differences between populations and could also explain why differences between measured and self reported estimates vary between cultures.
This is a real lesson for international surveys where countries are compared.
Bopp et Faeh 2008. Who gives me fives? BMJ vol 337, 1463

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