Healthcare and Medicine Reference

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That is, three quarters of cases in the exposed are due to the exposure

under study; this factor is predominant among others being part of the web

of causes.

Hence, strength and specificity (exclusivity) criteria of causality may

be estimated quantitatively by using ratios and other quantifications.

Dose-effect relationships (biological gradient) and concomitant varia-

tions may be studied by biostatistical methods, experimental studies like

clinical trials or field trials of vaccine protective effect (protective efficacy

ratios are one of the epidemiological tools used) require both epidemio-

logical and biostatistical methods. The rest relies mostly on qualitative

judgment. The magnitude and spread of health problems are quantified

by rates.

As the frequency of bad events following exposure to a bad factor may

be quantified and compared, so beneficial interventions (medicines, surger-

ies, care) can be related to their beneficial effects. In both cases, groups

with a higher frequency of results are in the numerator, those with a lower

frequency represent the denominator of such ratios in such comparative

expressions. Only the name changes in some instances:

Bad Events (Factors and Outcomes)

Good Events (Factors and Outcomes)

Individual risk (frequency, rate)

Individual risk (frequency, rate)

Relative risk

Relative risk of good events

Attributable risk

Attributable benefit increase

Etiological (attributable) fraction

Attributable benefit fraction

Relative risk increase (difference in

rates in exposed and unexposed)

Relative benefit increase (difference in

rates of outcomes in treated and

untreated groups)

Number needed to harm: Number

of individuals exposed to the

factor that would lead to one

additional person being harmed

compared with individuals who are

not exposed to this factor, that is,

1/attributable risk difference,

hence the reciprocal of the

attributable risk difference

Number needed to treat: Number of

patients who must receive the treatment

(beneficial factor) to create one

additional improved outcome in

comparison with the control treatment

group, that is, 1/absolute risk reduction,

hence the reciprocal of the attributable

risk reduction

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