Epidemiologic data sources and measurements. Samar Musmar,MD,FAAFP. Sources. Rates: denominators—pop at risk ,nominators—events or conditions Census—denominators International census—periodeically, annually Vital statistics—national responsibility
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Dichotomousvariables:many nominal variables have only two possible categories: alive or dead;case or control; exposed or unexposed; and so forth
frequency measures we use with dichotomous variables are ratios, proportions,and rates.
Ratio, proportion, rate =
The size of 10n may equal 1, 10, 100,1000 and so on depending upon the value of n.
For example, 100 = 1
101 = 10
102 = 10 × 10 = 100
103 = 10 × 10 × 10 = 1000
Ratio--values of x and y may be completely independent, or x may be included in y.e.g—
Proportion--is aratio in which x is included in y
Rate, is often aproportion, with an added dimension:it measures the occurrence of an event in a population overtime
is a variant of an incidence rate, applied to a narrowly defined populationobserved for a limited timesuch as during an epidemic.
is usually expressed as apercent, so 10n equals 100.
is a measure of the frequency of new cases of a disease among thecontacts of known cases.
compares two groups in terms of incidence rates, person-time rates, or mortalityrates.
Rate ratioof smokers of 1-14 cigarettes per day to nonsmokers = 0.57 / 0.07 = 8.1
The rate of lung cancer among smokers of 1-14 cigarettes is 8.1 times higher than the rate oflung cancer in nonsmokers.
What is absolute risk? Is the probability of that event occurring. Only two pieces of information are required to calculate absolute risk, given by the equation:
# of observations in which the occurrence of interest is found /total # of observations
If a denotes the total number of observations in which the occurrence of interest is found, and c denotes the total number of observations in which the occurrence of interest is not found, then the total number of observations is a + c
absolute risk of the occurrence of interest is : a/a+c
What is the difference between a risk and a rate?
a rate has a denominator with units that include time. For instance, 25 deaths per 100,000 population per year. A risk does not require time within the denominator; for instance, 2.6 deaths per 100 open-heart operations.
What are high and low values for absolute risk?
A single value of absolute risk in isolation does not tell you whether it is high or low. The only way that this can be done using absolute risk is by comparing findings in two or more sets of observations.
compares the risk of some health-related event such as diseaseor death in two groups.
Impossible to be done from case control study
relative risk, odds ratio, and attributable risk
The probability of an event expressed as a proportion of the probability that it will not occur is known as the odds of an event
The probability of an event in a study population is Rs
because the sum of all probabilities must equal 1, the probability that the event will not occur is (1 - Rs)
Odds are calculated by dividing the probability of the event by the probability that it will not occur
The odds of the event occurring in the study population= Rs / (1 - Rs).
a = number of persons with disease and with exposure of interest
b = number of persons without disease, but with exposure of interest
c = number of persons with disease, but without exposure of interest
d = number of persons without disease and without exposure of interest
a + c = total number of persons with disease (“cases”)
b + d = total number of persons without disease (“controls”)Odds Ratio
The odds ratio is sometimes called the cross-productratio
numerator is the product of cell a and cell d
denominator is theproduct of cell b and cell c
(defined place & time period) +# of deaths to infants under 7 days of age (defined place & time period) / # of stillbirths
(same place & time period) + # of live births
(same place & time period) x1000
Indirect Standardization of Crude Death Rate for Men in a Company, Using the Age-Specific Death Rates for Men in a Standard Population (Fictitious Data)Part 1 Beginning Data
Part 2 Calculation of Expected Death Rate, Using Indirect Standardization of Above Rates and Applying Age-Specific Death Rates from the Standard Population to the Numbers of Workers in the Company