Putting a figure on risk

Risk = probability that an event will occur during a specified time (a quantified uncertainty)

Reasons to be interested in risk:

- There isn't much certainty about it in medicine.

- We generally believe that quantifying uncertainty is important.

- e.g. will this operation make me better / will I have a healthy baby.

Options for 'quantifying' risk:

Risk

1 in 85 (natural frequency)

1/85 = 0.012 (probability)

1.2% (percentage)

Odds

Ratio of probability that will to the probability that won't

1/85 / 85/85

= 1 to 84 or 1/84 or 0.012.

Hence, in a group of people:

Risk of thing = number who get thing / number in group In a specified time period.

How to compare and contrast risk

Study population who are 'free of disease'

Then measure and classify 'participants on the pill, participants not on the pill'

Then measure outcome/compare rates of cervical cancer on participants with or without pillage.

(Watch this lecture the bit on calculating risk ratio is important)

Risk difference tells us something about 'impact'

Rist ratio tells us something about 'strength' of relationship.

There are some circumstances where we can't calculate a risk ratio - in particular case-control study.

In this case we are able to use odds ratio.

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