University of Bristol
Wellcome Trust
Recommended by:
Society of Biology
PEEP for Physics & Ethics at GCSE

Fractions, Percentages and Expressing Risks

Work it out

Embryo screening or pre-implantation genetic diagnosis (PGD) has been used during IVF since 1990 to identify embryos with genetic defects, to ensure healthy ones are implanted in the womb.
A study in 2004 of 754 babies born after PGD showed that abnormalities were no more common than in the general population, at about 0.4% each for Down's syndrome and neural tube defects, and up to 4% for minor medical conditions such as cleft palates.

How many babies in 1,000 born after PGD will suffer from a minor medical condition?

  • Show Answer
    • Answer = 40

      Why 40?     4% = 4/100 = 40/1000

How many babies in every 754 born after PGD are likely to have Down’s syndrome?

  • Show Answer
    • Answer = 3

      Why 3 in every 754?     

      0.4 hundreths of 754 is roughly 3.

      Note this is equivalent to the level of these problems in the general population.

Thus fractions and percentages are two different ways of expressing the size of a risk. To translate between them we need to remember that a fraction is part of a whole where the whole is 1 and a percentage is part of a whole where the whole is 100.

It’s not easy

In two studies in 1999 and 2001, an American doctor, David Grimes found that women understood rates (e.g. 4/1000) better than they understood proportions (e.g. 1/250). Seventy-three percent accurately identified the higher risk when rates were used (2.6/1000 versus 8.9/1,000), but only 56% could do so when proportions were used (1/384 versus 1/112).


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