Evidence-Based Approaches to Public Health

Question 1

Ratio (measure of disease frequency)

Answer 1

Ratios are calculated by dividing one number by another. Unlike proportions, the numerator does not need to be a subset of the denominator as they are two distinct quantities (e.g., the ratio of men to women).

Question 2

Proportion (measure of disease frequency)

Answer 2

Proportions are calculated by dividing one number by another, where the numerator is a subset of the denominator (e.g., the proportion of men in a population calculated as the number of men divided by the total population).

Question 3

Rate (measure of disease frequency)

Answer 3

Similar to ratios and proportions, rates are calculated by dividing one number by another, and additionally have a time component as a part of the denominator (e.g., the number of people who developed influenza in 2017, the birth rate per year in a population, the mortality rate per year in a population).

Question 4

Incidence

Answer 4

A measure of the number of new cases of a disease. Pre-existing cases of the disease are not counted. Incidence can be assessed as a proportion, in the form of “cumulative incidence,” or as a rate, in the form of “incidence rate.”

Question 5

Cumulative incidence

Answer 5

The number of new cases of disease in a population over a specified time period.

Question 6

Incidence rate

Answer 6

The number of new cases of the disease during person-time of observation. Time is measured as the amount of time people are followed or exposed ranging from before the onset of disease to the end of follow-up.

Question 7

Prevalence

Answer 7

The number of existing cases of a disease during a given time period. This includes cases that already existed as well as new cases that developed during the time.

Question 8

Point prevalence

Answer 8

The proportion of the population that is diseased at a single point in time (a calendar date, a point in life such as a college graduation).

Question 9

Period prevalence

Answer 9

The proportion of the population that is diseased during a secific duration of time, such as a year.

Question 10

Case studies and case reports (descriptive studies)

Answer 10

Studies used to alert people of a new illness or new association with illness. They usually are reports of only people with the condition of interest.

Question 11

Cross-sectional studies (descriptive studies)

Answer 11

Studies that include people who are representative of a given population. They are not selected on the basis of illness or exposure and can be used to determine initial associations and to identify the prevalence of either exposure or illness in a group.

Question 12

Ecological studies (descriptive studies)

Answer 12

Studies that are used to describe populations. The data are not analyzed on the individual level, but rather on the aggregate level. These studies may suffer from the ecological fallacy. The ecological fallacy occurs when group level data are used to report on individuals.

Question 13

Endemic

Answer 13

A situation in a community in which there is a consistent elevated rate of a certain disease.

Question 14

Epidemic

Answer 14

An increase in the number of cases of disease in a community, above what is expected.

Question 15

Pandemic

Answer 15

A worldwide epidemic.

Question 16

Case-control studies (analytic studies)

Answer 16

Studies that select people with or without disease and then proceed to look back over time to see if people had different rates of exposure. Case-control studies are good for rare diseases with long latency periods. A long latency period refers to diseases that take a long time to develop. For example, many cancers have a long latency period.

Question 17

Cohort studies (analytic studies)

Answer 17

Studies that select people on the basis of exposure and determine if people develop disease at different rates. Cohort studies are good for rare exposures and may follow individuals into the future (prospective) or look back in time (retrospective). Incidence can be calculated from such studies. People who have disease at the time point when the study period begins (prevalence cases) are excluded.

Question 18

Systematic reviews and meta-analyses (analytic studies)

Answer 18

These are studies that pool the results of multiple independent studies with established criteria to identify the evidence for associations.

Question 19

Randomized controlled trials (RCT) (analytic studies)

Answer 19

RCTs test an intervention that is given by the researcher to two or more groups. People are randomly assigned into groups (randomization), and some are given the active item (e.g., medicine, diet, educational program) while the other group(s) are given the usual treatment, nothing, or a placebo. The researcher then follows the resulting treatment groups over time and compares outcomes between the groups to determine the effects of the intervention. Research participants often do not know which group they are in, and at times researchers also do not know who is getting which treatment (single and double blinding, respectively).

Question 20

2x2 Tables (measure of association)

Answer 20

An association between an exposure amd an outcome is usually calculated in public health research with various epidemiologic meaures.

Question 21

Relative Risk (measure of association)

Answer 21

RR is a measure of the magnitude of an association between an exposure and a disease that is used in cohort studies. It is a ratio of the risk (incidence) of disease in the exposed to the risk in the nonexposed.
RR= risk (incidence) of outcome in the exposed/risk (incidence) of outcome in the nonexposed
RR= (A/ (A+B)) / (C/ (C+D))

Question 22

Odds ratio (measure of association)

Answer 22

OR is calculated in case-control studies or cross-sectional studies, as we do not have incidence data to calculate the RR for those studies. It is the odds of exposure among cases divided by the odds of exposure among controls, which equals the odds of disease among the exposed divided by the odds of disease among the nonexposed.
OR = odds of outcome in the exposed / odds of outcome in the nonexposed

OR = (A/B) / (C/D) = AD / BC

Question 23

RR or OR = 1

Answer 23

Means there is no association between the exposure and outcome. The risk or odds in the exposed equals the risk or odds in the nonexposed.

Question 24

RR or OR > 1

Answer 24

Means the exposure increases the risk of the outcome. The riskor odds in the exposed is greater than the risk or odds in the nonexposed.

Question 25

RR or OR < 1

Answer 25

Means the exposure decreases the risk of the outcome. The risk or odds in the exposed is less than the risk or odds in the nonexposed. This indicates that the exposure is a protective factor.

Question 26

Bias

Answer 26

A systematic error as compared to an error attributable to chance. Bias can cause an error in the estimation of an association between an exposure and an outcome.