Measures of Disease Frequency

Question 1

What are the two types of frequency measures?

Answer 1

Incidence (risk and rate) and Prevalence

Question 2

What is incidence?

Answer 2

Incidence concerns new cases of a disease or other health outcomes over a period of follow up

Question 3

What is prevalence?

Answer 3

Prevalence concerns existing cases of a disease at a point in time. It is the primary design feature of a cross-sectional study.

Question 4

What is the most useful aspect of incidence measures?

Answer 4

Incidence measures are useful for identifying risk factors and assessing disease etiology (cause)

Question 5

What is the most useful aspect of prevalence measures?

Answer 5

Prevalence measures are most useful in the planning of health services.

Question 6

How do you link prevalence to incidence?

Answer 6

Through duration! By using the formula P= I x D

Question 7

When does incidence usually tend to be higher than prevalence?

Answer 7

When the occurrence is common but the duration is short.

Question 8

When is prevalence likely to be higher than incidence?

Answer 8

When duration is very long but occurrence is not very frequent.

Question 9

What is risk?

Answer 9

Risk is the probability that an individual will develop or die from a given disease, or more generally, will experience a health status change over a specified follow up period. (5 yrs, 9 yrs, etc).

Question 10

How is risk represented?

Answer 10

Risk must be a value between 0 and 1, or correspondingly a percentage.

Question 11

What is Cumulative Incidence (CI)?

Answer 11

Cumulative incidence (CI) is a population-based estimate of individual risk. 
Cumulative incidence is always a proportion, determined by the following equation:
CI = I (incidence)/ N (size of disease free population)

Cumulative incidence and risk are often used interchangeably, although they are technically different.

Question 12

What is a shifted cohort?

Answer 12

Shifted cohort:

A dynamic cohort for which subjects progressively enter the study at different calendar times, but whose follow-up time is shifted to start at the time of initial entry into the study.

Question 13

How do you estimate risk for shifted cohort?

Answer 13

You use simple cumulative incidence. After shifting the cohort, we can compute cumulative incidence provided all subjects who remain disease free throughout the follow up are followed for the entire length of follow-up

Question 14

What are problems associated with assuming a fixed cohort when assessing risk?

Answer 14

1. The size of the cohort may reduce during follow up (due to death, withdrawal, etc) and we may not know if the disease occurs in the individual who may be lost upon follow up
2. Dynamic population rather than fixed cohort
3. Subjects followed for different time periods

Question 15

How does epidemiology define rate?

Answer 15

Rate is a measure of disease frequency that describes how rapidly health events (such as new diagnoses of cases or deaths) are occurring in a population. (Synonyms: hazard, incidence density)

Question 16

How do we calculate rate?

Answer 16

IR = I/PT
IR denotes incidence rate
I denotes the number of new cases developed over a study period
and PT denotes person-time information

This formula gives an average rate because it is difficult to estimate an instantaneous rate

Question 17

What is the difference between disease incidence rate and mortality incidence rate?

Answer 17

Disease incidence rate determines how rapidly new cases of a disease are developing.
Mortality incidence rate determine how rapidly persons with a disease are dying.

Question 18

How do we determine person-time denominator for the IR=I/PT?

Answer 18

Usually, it is the sum of the total follow up time for all persons in a study. When the individual follow-up time is not known:
PT = (avg size of disease free cohort) x (time length of study period)

Question 19

When is risk useful as a study objective?

Answer 19

To assess the change in individual health status.
You can use risk for:
1. Assessing a patient’s prognosis
2. Selecting treatment strategy
3. Making personal decisions about health behaviors.

Question 20

Is rate useful at the individual level?

Answer 20

No.

Question 21

What are key differences between risk and rate?

Answer 21

Risk is the probability that an individual will develop a given disease over a specified follow up period. Rate describes how rapidly new events are occurring in a population.

Risk must be between 0 and 1
Rate is always non-negative (greater than zero) and has no upper bound (can be up to infinity)

Risk is often the preferred measurement because it is easier to interpret.
Rate is most often the measure of choice due to problems estimating risk

Risk is better for acute outbreak (short follow-up) setting
Rate is better for studies with variable periods of follow-up.

Question 22

How do you choose between risk and rate?

Answer 22

The choice of risk versus rate depends on the study objective, the type of disease, the type of population, and the information available

Question 23

What does prevalence measure?

Answer 23

Prevalence measures existing cases of a health condition

Question 24

What are the two types of prevalence?

Answer 24

Point prevalence and period prevalence.

Question 25

What type of prevalence is most common?

Answer 25

Point Prevalence

Question 26

What is point prevalence?

Answer 26

Point prevalence is the proportion of people who have an observed case in a population at a particular point in time.

P = C/N
C is number of observed cases at time (t)
N is population size at time (t)

When stating a point prevalence you MUST denote the point in time (i.e. January 1, 2001 or Third post-operative day)

Question 27

What is period prevalence?

Answer 27

Period prevalence (PP) requires the assumption for a stable dynamic population for estimation. PP is estimated as the ratio of the number of persons (C*) who were observed to have the health condition anytime during a specified follow up period, to the size of N of the population for this same period.

PP = C*/N = C + I/N

Question 28

What is prevalence useful for?

Answer 28

1. Assessing the health status of a population

2. Planning health services by using the number of cases to determine the demand for healthcare

Question 29

Is prevalence good for identifying risk factors, and why?

Answer 29

Prevalence is not useful for determining risk factors because this measurement only concerns survivors. Cases who die are ignored.

Question 30

What is disease specific mortality risk?

Answer 30

CI = I/N
where I = number of deaths due to the specific disease of interest.
N= size of initial cohort regardless of disease status

Question 31

What is all-causes mortality risk?

Answer 31

CI = I/N
Where I = the number of deaths from all causes
N= size of initial cohort regardless of disease status

Question 32

What is case-fatality risk?

Answer 32

Proportion who die from a particular disease during the study period.
CI = I/N
Where I = number of deaths due to the specific disease of interest
N = Size of initial cohort with specific disease

Question 33

What is mortality rate?

Answer 33

IR = I/PT
I and PT vary according to the type of mortality rate being considered.
Disease-specific mortality rate
all-causes mortality rate
and case-fatality rate

Question 34

What is age-adjustment?

Answer 34

Age-adjustment is at type of rate-adjustment to address distortion of the crude data by age (a confounding variable). The corrected rates are therefore called age-adjusted rates