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### 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
- Demographic year book,World health statistics annual
- US cencus
- Palestinian census

Health data

- Death certificate—cause/s
- National notifiable disease surveillance system
- disease registries—Palestine?
- Data from third party

Epidemiological measurements

- Frequency
- Incidence/t (case)frequency of new occurrence of dse injury or death during specified period
- Prevalence/t (case)=point prevalence--# of persons in defined pop who have specific dse at a point in time
- Point prev /period prev(time interval)
- Period prev=point prev +incidence

Frequency Measures

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

Rates

- Rate—frequency (#) of events that occur in defined period of time devided by avarege population at risk
- Rate=numinator/denominator x constant multiplier
- Denominator =midperiod population
- Risks and rate –less than 1

Risk

- Defin:proportion of persons who are unaffected at the beginning of a study period but who undergo the risk event during the study period
- is simply an expression of probability. It is usually, but not invariably, the probability of an adverse event such as disease, injury, or death.
- Risk event may be—death,dse ,injury
- Cohort –persons at risk for event,clearly defined group of persons studied over time

Ratios, Proportions, and Rates Compared

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

Rates

- Incidence rate: # of incident cases over defined study period /population at risk at midpoint of study period
- Incidence is the number of new cases of a given disease at a specified time divided by the population at risk for that disease at that time.
- Prevalence rate: proportion of pop that have a defined dse or condition at point in time
- Prevalence is the number of existing cases of a given disease at a specified time divided by the population at risk for that disease at that given time.

Attack Rate

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.

Secondary Attack Rate

is a measure of the frequency of new cases of a disease among thecontacts of known cases.

Rate Ratio

compares two groups in terms of incidence rates, person-time rates, or mortalityrates.

Example

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.

Risk

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

Risk

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.

Risk Ratio(relative risk)

compares the risk of some health-related event such as diseaseor death in two groups.

Impossible to be done from case control study

Comparative risk

relative risk, odds ratio, and attributable risk

Odds

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).

measure of association which quantifies the relationship between anexposure and health outcome from a comparative study(case control study)

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 RatioExample

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

Crude rates

- Rates that apply to entire pop without ref to any charecteristic of the individual in it
- Crude birth rate=# of live births(defined place & time period)/mid period pop(same place & time period)x1000
- Crude death rate= =# of deaths(defined place & time period)/mid period pop(same place & time period)x1000

Specific rates

- Rates that are calculated after pop has been categorized into groups with particular charecteristics
- Age specific death rate =# of deaths in particular age group(defined place & time period)/mid period pop(same age group,place & time period)x1000
- Cause –specific death rate =# of deaths due to particular cause(defined place & time period)/mid period pop(same place & time period)x1000

Rates

- Infant mortality rate=# of deaths to infants under 1 year of age(defined place & time period)/# of live births (same place & time period)x1000
- Neonatal mortality rate =# of deaths to infants under 28 days of age(defined place & time period)/# of live births (same place & time period)x1000
- Postneonatal mortality rate=# of deaths to infants between28-365 days of age(defined place & time period)/# of live births (same place & time period)-# of neonatal deaths (same place & time period)x1000
- Approximate postneonatal mortality rate=Infant mortality rate-Neonatal mortality rate

Rates

- Perinatal mortality rate= # of stillbirths

(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

- Maternal mortality rate=# of pregnancy- related deaths (defined place & time period)/# of live births (same place & time period)x100,000

Standardized (adjusted)Rates

- Direct
- Indirect
- Crude rate adjusted to control the effect of age and other characteristics
- Direct—ASDR –2 pop to compare and standardize
- Standard pop is real or realstic e.g whole conutry ASDR
- Indirect—ASDR unknown
- SMR—standardized mortality ratio=observed death rates /expected death rates for same groupx100

Direct method for rate adjustment

- Applies observed age-specific rates to a standard population.
- appropriate when each of the populations being compared is large enough to yield stable age specific rates
- For example, the direct method is used for comparison of cancer rates over time.

DIRECT STANDARDIZATION OF CRUDE DEATH RATES OF TWO POPULATIONS, USING THE AVERAGED WEIGHTS AS THE STANDARD POPULATION(FICTITIOUS DATA)

Indirect method of rate adjustment

- is used if ASDRs are unavailable in the population whose crude death rate is to be adjusted.
- It also is used if the population to be standardized is small, which makes the ASDRs statistically unstable.
- The indirect method uses standard rates and applies them to the known age (or other stratum) groups in the population to be standardized.

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

How do relative risk and odds ratio differ?

- relative risk can be calculated from a cohort study but not from a case-control study
- odds ratio can be calculated from either type of study
- The odds ratio closely approximates the relative risk when the outcome of interest is rare and the groups are representative of the populations from which they are drawn
- Neither relative risk nor odds ratio gives any clue to the magnitude of risk in the background population or control group.

What is attributable risk?

- Attributable risk is the risk in the study group less the risk in the control group. It can be calculated from the equation:Rs-Rc
- This is the most appropriate measure of change in risk when making a decision for an individual, such as whether the risk of side effects from taking a drug is tolerable or not
- If a study finds that the risks of dying in middle age are 0.4 and 0.22 for a smoker and a nonsmoker, respectively, then the attributable risk for smoking is 0.4 - 0.22 = 0.18.

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