Healthcare and Medicine Reference

In-Depth Information

medical understanding and decision making are relative risks, attributable

risks, and etiological fractions (attributable risk percent).

In analytical studies,
ratios
, as already mentioned above, are fractions

relating two entities (two different health events or their related character-

istics) in which the entity in the numerator is not reproduced (with some-

thing else added) in the denominator.
Odds
(cross product ratios, relative

odds) are ratios of the probability of occurrence of an event to that of

nonoccurrence, or the ratio of the probability that something is one way

to the probability than it is another way: In our case of a case control

study, ad/cb.

Relative risk
(relative benefit increase or reduction) is a fraction relat-

ing incidence of events in one group of subjects, such as those exposed to

some factor, to another group or groups nonexposed to it. Relative risks are

quantifications of the
strength of the causal relationship
. In the case of a

study of noxious factors, the higher the relative risk, the stronger the causal

association. If the relative risk of a presumably noxious factor is lower than

1.0, the factor (agent) has a protective effect, not a noxious one. Cohort

studies (observations of new events over a certain period of time) of groups

(cohorts) to be compared serve this purpose.

In our example:

80 /100,000

20 /100,000

=

4.0

Hence, relative risk may be called also a ratio of rates or rate ratio.

Odds ratios
most often exposures/non-exposure frequencies in cases

to exposures/non-exposures frequencies in non-cases are another kind of

estimation of the
strength of a causal association
derived from case-control

studies.

In our example:

a/d

b/c

80

×
=

50

4,000

1,000

=

×
=

4.0

20

50

N.B. If case-control studies are based on incident cases and if the

proportion of subjects exposed is similar as in the cohort study, both cohort

and case-control studies yield comparable estimations of the strength of

causal association.

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