Epidemiology (II) Rates

Question 1

Define incidence.

Answer 1

New rates of disease among the at-risk population

Question 2

Define prevalence.

Answer 2

Number of sick individuals in the population

Question 3

Define cumulative incidence.

Answer 3

of new cases of disease in a specified population at risk during a specified time period / at-risk population

Question 4

How is cumulative incidence often expressed?

Answer 4

As new cases per _____ at-risk individuals

OR

a percentage

Question 5

What do incidence rates allow us to infer that prevalence rates do not?

Answer 5

Risk of contracting disease

Question 6

What assumption must be met for cumulative disease rates to be most useful?

Answer 6

There must not be any attrition in the population follow-up

Question 7

What is incidence density (incidence rate)?

Answer 7

An individual’s risk per unit of person-time (e.g. person-month, person-year, etc.) (expressed as a number ‘per 100,000 person-years’)

Numerator = incidence of disease among at-risk individuals

Denominator = the sum of the units of time that each individual was at risk and was observed.

Question 8

Why would an individual no longer be tracked during a study of cumulative incidence?

Answer 8

If the individual is no longer at-risk (they die, stop being at risk, or are lost to follow-up)

Question 9

What is period prevalence (as opposed to point prevalence)?

Answer 9

Prevalence during a specified time period

(basically starting prevalence + incidence during that period)

Question 10

Cumulative incidence is directly interpretable as what?

Answer 10

Risk

Question 11

What are incidence rates interpretable as?

Answer 11

Indirect estimates of risk

Question 12

What is the relationship between incidence rate and cumulative incidence?

Answer 12

Cumulative incidence = incidence rate * time period

(assuming no secular trend in new disease incidence)

Question 13

What is the relationship between the incidence rate and prevalence?

Answer 13

Prevalence = incidence * duration of disease

(assuming low disease incidence and stable incidence over time)

Question 14

What are the steps for identifying a new case of disease?

Answer 14

Step 1: determine who has the disease and who does not.

Step 2: follow-up with those who did not have the disease at baseline (i.e. ignore baseline prevalent cases)

Question 15

Define mortality rate (basically an incidence rate, but with deaths instead of new cases).

Answer 15

(# of deaths in a specified population during a specified time period) / (Person-time contributed in the specified population during the specified time period)

Question 16

What person-time measurement is used for annual incidence rates?

Answer 16

The mid-year population estimate

Question 17

When might data need adjusting for comparison to other data sets?

Answer 17

Disease or mortality rates may differ between two populations merely because one is older or younger.

Adjustment is done to determine whether there are actual differences in incidence or mortality rates that cannot be attributed to differing age distributions

• In direct adjustment, this is done by artificially forcing the two populations to have the same age distribution by using a reference (standard) population

Question 18

What standard population can be used for direct age-adjustment of data sets?

Answer 18

• A combination of the two populations being compared
• A third-party reference population (e.g. the U.S. population)

Question 19

How would these two groups be combined/age-adjusted to reach a standard reference category?

Answer 19

Question 20

What is an indirect adjustment?

Answer 20

Comparing the observed number of events (death or disease) in a study population to the number of events expected if the study population had the same rates as a standard population.

Occupational example: Compare lung cancer rates in workers at an asbestos factory to lung cancer rates among people of the same age in the general U.S. population

Question 21

How can an indirect adjustment be calculated?

Answer 21

SMR/SIR = (observed # of deaths or disease per year) / (expected # of deaths or disease per Year)

(expressed as a standardized mortality ratio (SMR) or standardized incidence ratio (SIR))

Question 22

What is the crude mortality rate?

Answer 22

Annual Crude Mortality per 100,000 = (total number of deaths from all causes in one year) / (number of persons in the population at mid-year) x 100,000

Question 23

What is the case fatality rate?

Answer 23

The percentage of people with a given disease who die within a certain timeframe after their disease was diagnosed

Question 24

What is proportionate mortality?

Answer 24

The percent of all deaths in a population attributable to a specific cause of death.

E.g. (# of deaths due to CVD in 2019) / (total # of deaths in the population in 2019)

Question 25

How is the YPLL (years of potential life lost) calculated?

Answer 25

Measured by subtracting the age at death from an index date

Childhood deaths lead to a greater YPLL than deaths in the elderly.

Question 26

What is the purpose of DALYs (disability-adjusted life years)?

Answer 26

To assess the burden of disease. One DALY is equal to one year of healthy life that is lost.

Question 27

What are group I diseases?

Answer 27

Communicable diseases (infectious)

Question 28

What are group II diseases?

Answer 28

Noncommunicable (often chronic)

Question 29

What are group III diseases?

Answer 29

Injuries