MEASURING THE DISEASE
Nina Ratnaningsih
Count diseased people
ILLNESS CASES
Ratio, proportion, rate
Add dimension of
time ?
Measuring the disease
Frequency EFFECT
● How common the disease
● How is the risk of
diseases in an subject
● How fast the diseases develop
● How strong the effect of ex in one group comparing
another group
● How much diseases attributable to ex
● How much the cases would be eliminated if exposure were also eliminated
How common the disease
● Proportion → prevalence
● Related to burden of disease
● Help the decision maker to determine the investment/ planning
● In specific point of time/time period
● Cross sectional design
How big is the disease risk
● Risk = incidence cumulative
● proportion of an initially disease-free population that develops disease, becomes injured, or dies during a specified (usually limited) period of time.
● To know the etiology/risk of the disease
● Cohort design with fixed population and short period of follow up
How fast the disease develop
● Person time rate = incidence rate = risk rate
● cohort design with dynamic population, and fixed population with long period of Follow Up
● The rate provides the capacity to anticipate future incidents and plan accordingly
● Simply said “ there’s new cases__ /Persons- year”
Formulas
How strong the effect :
Comparing the groups with/out exp
RISK RATIO RATE RATIO ODDS RATIO
RISK Ex/
RISK Uex
RATE Ex/
RATE Uex
Odds ex / Odds uex
Formulas
RISK RATIO
a/(a+b) : c/(c+d)
RATE RATIO → RISK RATIO
ODDS RATIO a/b : c/d
Ratio = 1, ex=uex Ratio > 1, risk>
Ratio < 1, risk <
Simply said….
● the exposed group has ____ times the risk of developing illness as the unexposed group.
● illness among exposed group is developing at ___times the rate that illness is developing among unxposed group
● the odds of ilnes is ___ times greater in exposed group
Note : OR always overestimate than RR OR = RR if outcome less than 10%
Disease Proportion that atributable to exposure
● Atributtable risk = risk difference = excess risk
● Atributtable risk %
● Estimate the cases would be eliminated if exposure were also eliminated.
● Cohort design
● AR = Iex - Iuex AR%= AR/Iex x 100
● Simply said “ if exposure were eliminated, the illness will decrease _____ which would represent ____%
reduction of the ilness incidence
How about in population?
● Population Atributtable risk =PAR
● Population Atributtable risk % = PAR%
● Estimate the incidence of disease in population that would be eliminated if exposure were also eliminated.
● Cohort design
● PAR = Ip - Iuex PAR%= PAR/Ip x 100
● Simply said “ the reduction of___new cases of illness is expected if the exposure were eliminated, which would represent ____% reduction of the ilness incidence
Number Need To Treat
It is used to see how many individuals need to take the treatment in order to prevent one bad event
NNT = 1/ AR
Simply said “To eliminate 1 case of illness, we
need to prevent exposure from ______ people who would been exposed
Example
The preventive advantages of eating fish have been reported in numerous studies. A recent
cohort study1 reported that not eating fish increased the risk for stroke. The table below shows
the results of this study:
A recent cohort study1 reported that not eating fish increased the risk for stroke. The table
below shows the results of this study:
Ie = a/(a+b) =0,0505 or 5,03/100
Iu = c/(c+d) =0,287 or 2.87/100
Ip = (a+c)/(a+b+c+d) = 0,0432
or 4,32 per 100
RR = (Ie) / (Iu) = 5,04 / 2.87 = 1.75 per 100 AR = (Ie) - (Iu) = 5,04 / 2.87
= 0.0216 = 2.16 per 100 AR % = (AR/Ie ) x 100
= 2.16/5.03 x100 = 43%
PAR = (Ip) - (Iu)
= 4.32-2.87 =1.45 per 100 PAR %= (PAR/ Ip) x100
= (1.45/4.32) x 100 = 33.6%
NNT = 1/ AR = 1/ 0,0216 = 46
● Those who never eat fish have 1.75 times as much risk
(higher incidence) as those who eat fish almost daily (RR = 1.75)
● If those who do not eat fish change their eating habits and begin to eat fish almost daily, their incidence of strokes will decrease by 2.16 per 100 individuals (AR = 2.16 per 100), which 43% reduction of their stroke incidence (AR% =
43%).
● A reduction of 1.45 new cases of stroke per 100 population (exposed and unexposed) is expected if everybody eats fish almost daily (PAR = 1.45 per 100), with 33.6%
reduction of the incidence in the population (PAR% = 33.6%).
● To eliminate 1 case of stroke, we need to prevent “never eat fish behaviour” from 46 people who have the risk of it
Number Need To Treat
20.000 people 2.500 people
