# Measures of Disease Frequency Flashcards

1
Q

Define the term count. List examples.

A

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
2
Q

Define proportion. Give examples.

A
• 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%
3
Q

Define ratio. Give example.

A
• 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.
4
Q

Define rate. Give examples.

A
• 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
5
Q

Define prevalence (risk).

A

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

6
Q

Define point prevalence.

A

Prevalence at a single point in time

7
Q

Define period prevalence.

A
• Prevalence over a specific period in time
• Includes individuals with disease at start plus new cases during follow-up period
8
Q

What is an example of prevalence (risk)?

A

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

9
Q

Define incidence. What is this expressed as?

A

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

10
Q

Define incidence risk.

A
• 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

11
Q

Incidence risk in a close population?

A
• For a closed population, calculating incidence risk is straightforward!
• Closed population = no additions/removals during a define follow-up period
12
Q

Incidence risk in an open population?

A
13
Q

Define incidence rate.

A
14
Q
A

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.

15
Q
A

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.

16
Q
A
17
Q
A

point prevalence is very common

18
Q

Define attack rate. Give examples.

A
• Proportion of population developing illness during a finite period of time.
• Equivalent to incidence risk
• ‘Attack risk’ would be a better term for this statistic.
• Typically used to measure average risk during a common-source disease outbreak
19
Q

Define case fatality rate. Give examples.

A
• Proportion of individuals with a disease who die from it
• Equivalent to incidence risk
• ‘Case fatality risk’ would be a better term for this statistic
20
Q

Example of ebola outbreak.

A

This is how they reported the case fatality rate.

21
Q

Define crude mortality rate.

A
• Incidence risk of fatal cases of a particular disease in a population at risk of
death from that disease
• Denominator includes both:
• Cases of the disease (who haven’t died yet)
• As well as individuals who are at risk of disease

Crude means unadjusted, raw. Has to do with denom = everone both cases and suceptibels. How many people ar dying out of pop regardless of whetehr you do or do not have disease.

22
Q

Define proportional mortality rate.

A
23
Q
A