# Measures of Disease Frequency Flashcards

Define the term count. List examples.

The number of cases or individuals affected with a condition in a given population.

- Examples:
- 14 veterinary students get diagnosed with cryptosporidium. Don’t know class size.
- 300,000 cases of cryptosporidiosis reported in USA. Don’t know timeline, etc.
- 58 cows in a herd have mastitis
- Limited use because no denominator information

Define proportion. Give examples.

- A fraction in which the numerator is a subset of the denominator.
- A measure of risk or probability of a disease
- Can be measured at one point in time
- Example
- 300,000 cases cryptosporidiosis in 300 million Americans = 0.1%
- 58 cows in a herd of 100 = 58%

Define ratio. Give example.

- This is a number in which the numerator is NOT a subset of the denominator (denom has both numerator and rest combined)
- Defines the relative size of two quantities expressed by dividing one (numerator) by the

other (denominator) - Example
- Odds of disease (a ratio) in our herd of 100 cows is 58:42 (1.38 to 1)
- 10 males out of 100 students. 100 students includes males and females.

Define rate. Give examples.

- This is a fraction in which the numerator is a subset of the denominator and

the denominator includes a measure of *time *(e.g. animal-time or people-time

units) - Examples - Annual mortality rate, annual birth rate
- 12 students were diagnosed with influenza out of 600 students over a 10-month period

(12/(600*10) = 0.002 cases per student-month - Rate is often erroneously used in a general sense to refer to all types of measures of

disease frequency

Define prevalence (risk).

You want to know right at this moment have this disease. Don’t know when they got it though - have both NEW and OLD.

Define point prevalence.

Prevalence at a single point in time

Define period prevalence.

- Prevalence over a specific period in time
- Includes individuals with disease at start plus new cases during follow-up period

What is an example of prevalence (risk)?

Cryptorchidism in different dog breeds (Gubbels et al. 2009) examined 26,547 male dogs and found 405 were cryptorchid. What is the prevalence of cryptorchidism in their study population? Is this a Point or Period estimate?

405/26,547 = 0.0152 x 100 = 1.52% of male dogs in this pop are cryptorchid. This is a point prevalence. b/c you can’t re-develop this illness in particular

Define incidence. What is this expressed as?

Think NEW disease.

* Measures how frequently initially susceptible individuals become disease

cases as they are observed in time

* Incident case = new case

- Individual changes from susceptible to diseased!

* Expressed as:

* Incidence Risk

* Incidence Rate

Define incidence risk.

- Proportion of initially susceptible individuals in a population who become

new cases during a define follow-up period.

Risk = prevalence.

Example: Last year a herd of 121 cattle were tested for tuberculosis using the

tuberculin test and all tested negative. This year the same 121 cattle were

tested and 25 tested positive.

* The incidence risk of TB in this herd was 25/121 (0.207 = 20.7%) for the 12-month follow-

up period

Incidence risk in a close population?

- For a closed population, calculating incidence risk is straightforward!
- Closed population = no additions/removals during a define follow-up period

Incidence risk in an open population?

Define incidence rate.

days at risk = # of days you are susceptible to contracting disease.

When you have the disease, you are no longer contributing to at risk time and must remove those days from days contributed.

5 cows and you have 5 events of mastitis but not there for a whole year.

In 1 year, you expect cow to have 2.2 incidents of mastitis.

have disease = no longer at risk

all the time time they are orange = have disease = not at risk.

Case A = contributed to 4 months at risk and then the remainder of the year had disease.

Case B = never developed disease and were in study for all 12 months so contributed to 12 months at risk and not a new case.

Case C = only know about at risk prior to being withdrawn

0.044 cases per animal months

if you have 1 year period, you would expect to have half a case per one animal that is exposed for one year.