# 2.2 - Measures of Disease Frequency

There are several means by which the occurrence of disease may be measured. The commonly used measures of incidence and prevalence can be distinguished by differences in the time of disease onset. **Incidence** is a count of *new cases* of the disease (or outcome). **Prevalence**, on the other hand, counts *both new and existing cases* of the disease.

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, tinnitis)
- Disability - Impaired ability to do usual activities
- Dissatisfaction - Emotional reaction (e.g., sadness, anger)
- Destitution - Poverty, unemployment

The first two, death and disease are the most commonly used.

#### Measures of Disease Frequency

Epidemiologic measures of disease frequency are of 5 types:

**Count**: the number of individuals who meet the case definition;-
**Proportion**: A/(A+B); a fraction in which the numerator (A) includes only individuals who meet the case definition and the denominator totals the numbers of individuals who meet the case definition plus those in the study population who do not meet the case definition and are at risk. -
**Ratio**: A/B; a special fraction in which the numerator includes only individuals who meeting one criterion (e.g. the case definition) 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). A ratio is not dependent upon time.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) -
**Rate**: 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).A rate is dependent upon time. In other words, a proportion over a particular period of time. An epidemiologic rate will contain the following: disease frequency (numerator), unit of population size, and the time period during which the event occurred -
**Risk**: the probability of an individual meeting the case definition (person-time rate). Risk is dependent upon time.

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

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 four measures address this.

A proportion is not dependent upon time. It may 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.

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

Ex: Calculating the proportion of women with cervical cancer requires a special consideration. Cervical cancer only occurs in women with a cervix. A woman who has had a complete hysterectomy is no longer are at risk for developing cervical cancer. This is a large segment of the population of older women. The National Women’s Health Information Center of the US Department of Health and Human Services reports that 1 in 3 women have had a hysterectomy by age 60. Thus, women with hysterectomies are not included in the denominator when calculating the proportion of women with cervical cancer as part of the population at risk.

Ex: 1 case of colon cancer for every 1 case of breast cancer.

Ex: 2 female cases of major depression to 1 male case of major depression.

The next two measures also have a time factor.

Ex: 44 cases of colon cancer per 100,000 population in Pennsylvania during 2000.

Ex: 0.00044 colon cancer cases per person-year (typically derived from a cohort study in which each at-risk person is followed over time until he/she is no longer at-risk).

What is the difference between a proportion and a ratio in these definitions? How do you interpret a risk of 0.00044?

(if these are not clear to you, post a question in the General Discussion for Week 2..let's make sure we have our definitions down.)

As part of your homework for this week, you will find an example of a proportion, rate, count or risk estimate related to colon cancer.

Continue reading in Week 2 Lesson 2 by clicking on Disease Occurrence below.