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Absolute and Relative Risks

Shifting to an absolute risk provides a better understanding of risk


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Relative vs Absolute Risk

Relative Risk - Mathematically correct but very deceptive

Why use Relative Risk instead of Absolute Risk

  • Relative risk provides a big number and this makes for big headlines or a big impact on the person receiving the information.
  • In the example above, looking at mortality rates of 10,000 individuals, a 50 percent reduction could either mean a decreased mortality from 200 individuals to 100 individuals a very significant result or it could just as well mean a decreased mortality from a much less significant, 2 patients to 1 patient1.

Why absolute risk?

  • They take into account background rates
  • Given the absolute risk, the relative risk can be derived but not vice versa.

Medical Statistics at a Glance

  • This is a wonderful and easy-to-understand book on medical statistics with worked examples.
  • Please click on the graphic opposite to purchase this informative book through NutriDesk.

An example of increased risk of pancreatitis with tetracyclines

  • A large population-based case controlled study has shown a 60% increased risk for acute pancreatitis among current users adujsted for relevant confounding factors.

  • So if we know that the incidence in any given country for acute pancreatitis is say 100 cases per 100,000 people (1 per 1,000) then taking tetracyclines may increase the risk for acute pancreatitis from 1 per 1,000 to 1.6 per 1,000 and this then puts this increased risk into perspective.

  • The 60% increase which seems like a highly significant increased risk turns out to have an increased risk of 0.6 per 1,000 people taking these medications.

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Blood Clots with oral contraceptive pills

2nd Generation vs 3rd Generation oral contraception in women

Panic for no reason

  • In October 1995, the Committee on Safety of Medicines in the UK warned that the risk of getting a life-threatening blood clot increased twofold --- in other words an increased risk of 100%1.
  • Looking at the graphic above, it was shown that one woman in 7.000 who took the 2nd generation oral contraceptive pill developed a blood clot.
  • In 3rd generation oral contraceptive pill users, the risk went up by only 1 to 2 women developing a clot with the newer pills.
  • Thus the absolute risk was only 1 in 7,000 --- i.e. only 1 extra case in the 3rd generation pill user. This was a 100% increased relative risk.

What does this say about the use of relative risk?

  • Putting information like this in terms of a large relative risk increases anxiety amongst the lay population and is deceptive.
  • It was estimated that 13,000 extra abortions occurred in the UK in 1996, about 800 conceptions in teenage girls and for every abortion there was one extra birth1.
  • How many teenage lives did this have a negative impact on? There are a multitude of  negatives attached to unwanted pregnancies including child neglect.
  • It is a known fact that abortions and pregnancies are associated with an increased risk of thrombosis [blood clots] that in fact exceeds the blood clotting rate of 3rd generation oral contraceptive pills1.

Link to statistics references

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Statistics References