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To understand risks that are smaller than 1% (or 1 in 100) you may find it helpful to compare these risks to other risks in life. Some people use words like ‘high’ or ‘low’ to talk about risk. So Calman (1996), an expert in risk communication, has produced a ‘risk classification’ scale that looks at particular risks and suggests words that the public and health care professionals can use to describe them. An outline of the scale is given in Table 3.8.

Table 3.8 Risk of an individual dying (D) in one year or developing an adverse response (A)

(Source: Calman 1996).

Term used	Risk range	Example	Risk estimate
High	>1:100	(A) Transmission to susceptible household contacts of measles and chickenpox (A) Transmission of HIV from mother to child (Europe)	1:1–1:2 1:6
Moderate	1:100–1:1000	(D) Smoking 10 cigarettes per day (D) All natural causes, age 40	1:200 1:850
Low	1:1000–1:10 000	(D) All kinds of violence (D) Influenza (D) Accident on road	1: 300 1:5000 1:8000
Very low	1:10 000–1:100 000	(D) Leukaemia (D) Playing soccer (D) Accident at work	1:12 000 1:25 000 1:43 000
Minimal	1:100 000–1:1 000 000	(D) Accident on railway	1:500 000
Negligible	<1:1 000 000	(D) Hit by lightning (D) Release of radiation by nuclear power station	1:10 000 000 1:10 000 000

Table 3.9 Results of randomised controlled trial in primary care in patients with venous leg ulcers to compare a new specially impregnated bandage, called ‘Band aid’, with usual care.

Leg ulcer completely healed	Group
Band‐aid intervention	Usual care control
Yes, healed	147	123
No, not healed	63	82
Total	210	205

From the data in Table 3.9: