# 3.1 - Disease Occurrence

## Outcomes Section

Typical outcomes for an epidemiologic study, (sometimes referred to as the 'D's of Epidemiology) are as follows:

### Outcomes of Epidemiology:

• Death Disease/Illness - Physical signs, laboratory abnormalities
• Discomfort - Symptoms (e.g., pain, nausea, dyspnea, itching, tinnitus)
• Disability - Impaired ability to do usual activities
• Dissatisfaction - Emotional reaction (e.g., sadness, anger)
• Destitution - Poverty, unemployment

## Calculations Section

In order to describe and compare measures of disease occurrence, these are the types of calculations most often used:

### Count

Definition: the number of individuals who meet the case definition

Example:

9188 cases of invasive colorectal cancer in Pennsylvania in 2005 (PA Cancer Registry data)

Notes: Calculating the magnitude of disease occurrence with a count is simple and useful for certain purposes, such as allocating health resources. For other purposes, it is more helpful to have a denominator under the count that indicates the size of the study population. The remaining measures address this.

### Proportion

Definition:  A/(A+B); a fraction in which the numerator (A) includes only individuals who meet the case definition and the denominator (A+B) totals the numbers of individuals who meet the case definition (A) plus those in the study population who do not meet the case definition and are at risk (B).

Example:

30% of persons over 50 years of age have been screened for colon cancer

Notes: A proportion is not dependent upon time. It can be expressed as a fraction or a percentage. A proportion indicates the fraction of the population that is affected by the disease or condition. It is linked to estimating risk.

### Ratio

Definition: A/B; a special fraction in which the numerator includes only individuals who meet one criterion (e.g. the case definition, A) and the denominator includes only individuals in the study population who meet another criterion (e.g. do not meet the case definition but are at risk, B). A ratio is not dependent upon time. If the ratio is a ratio of the number of individuals with the outcome to those without the outcome, the ratio is the odds. A ratio as a measure of disease frequency is used infrequently, in special situations. (not to be confused with an odds-ratio or risk-ratio)

Examples:
• 1 case of colon cancer for every 1 case of breast cancer.
• 2 female cases of major depression to 1 male case of major depression.

### Rate

Definition: a fraction in which the numerator includes only individuals who meet the case definition and the denominator includes individuals in the study population who do or do not meet the case definition but could meet the case definition (at-risk) and the total time at risk they contribute (person-time).  Person-time is defined as the sum of time that each at-risk individual contributes to the study.  If the study period is 2 years, person-time is as follows for certain groups:

1. For participants who develop the disease
1. time they spend on study before they developed the disease (< 2 years)
2. These participants count in the numerator, and denominator
2. For participants who drop out before 2 year period is over
1. time they spend on study before they developed the disease (< 2 years
2. These participants count only in the denominator
3. For participants who do not develop the disease (in the 2 year window)
1. 2 years
2. These participants count only in the denominator

The sum of all these times would be the denominator.

Example:

0.1 case/person-years indicates that, on average, for every 10 person-years (i.e.: 10 people each followed 1 year or 2 people followed for 5 years, etc.) contributed, 1 new case of the health outcome will develop

Notes: Rates differ from proportions in that there is always a time component. It is important to be intentional about the terminology that we use and to correctly differentiate proportions from rates.

### Risk

Definition: A measure of the probability of an unaffected individual developing a specified health outcome over a given period of time. Risk is calculated by dividing the number of new cases by the total number of individuals at risk during the specified time period.

Example:

A 5-year risk of 0.10 indicates that an individual at risk has a 10% chance of developing the given health outcome in a 5-year period

Notes: Risk is typically derived from a cohort study in which each at-risk person is followed over time until he/she is no longer at-risk