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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