Measure of Disease Frequency Flashcards
What is a count? Why does it have limited statistical use?
number of cases or individuals affected with a condition in a given population
no denominator information - it is simply a number giving no information about the population
EXAMPLES:
- 14 veterinary students get diagnosed with cryptosporidiosis
- 300,000 cases of cryptosporidiosis in USA
- 58 cows in a herd have mastitis
What is a proportion?
a fraction in which the numerator is a subset (%) of the denominator and measures the risk or probability of disease at one point in time
EXAMPLES:
- 300,000 cases of cryptosporidosis in 300 million Americans = 0.1%
- 58 cows with mastitis in a herd of 100 = 58%
What is a ratio? What can it be considered?
number in which the numerator is NOT a subset of the denominator and defines the relative size of 2 quantities expressed by dividing the numerator by the denominator
ODDS of disease
EXAMPLE:
- 58 cows with mastitis in a herd of 100 = 58:42 = 1.38:1
What is a rate?
fraction in which the numerator is a subset of the denominator and the denominator includes a measure of time - how long someone is susceptible to a disease for a certain amount of time
EXAMPLE:
- annual mortality rate, annual birth rate
- 12 students diagnosed with influenza out of 600 over a 10-month period = (12/(600 x 10)) = 0.002 cases per student-month
When is rate commonly erroneously used?
in a general sense to refer to all types of measures of disease frequency
What is prevalence? How is it calculated?
proportion of new and old cases at a specific point in time - ALL CASES not just new cases
(number of existing cases)/(size of population at risk)
What is point prevalence? Period prevalence?
POINT - prevalence at a single point
PERIOD - prevalence over a specific period in time including individuals with disease at the start + new cases during follow-up period
Cryptorchidism in different dog breeds examined 26547 male dogs and found 405 were cryptorchid. What is the prevalence of cryptorchidism in their study population? What kind of prevalence is this?
405/26547 = 0.01526 = 1.526%
point prevalence
What is incidence? In what 2 ways can it be expressed?
measure of how frequently initially susceptible individuals become disease cases as they are observed in time - incident case = NEW CASES
- incidence risk - proportion, no time
- incidence rate - follow population over time to follow disease cases
What is an incidence risk? How is it calculated?
proportion of initially susceptible individuals in a population who become new cases during a defined follow-up period - new case prevalence
(number of incident NEW risk)/(number initially at risk)
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 where tested and 25 tested positive. What is the incidence risk?
25/121 = 0.207 = 20.7% for the 12-month follow-up period
Why is calculating an incidence risk for a closed population more straightforward?
there are no additions/removals during the defined follow-up period - the same herd will be present at the follow-up
Calculating the incidence risk for an open population is complicated. What 3 things need to be accounted for?
herd dynamics will be different at the follow-up period (death, birth, movement)
- individuals at the start of the period (start)
- add half the number that entered the population (new)
- subtract half the number that were lost from the population (lost), assuming there is only one case per individual
What is an incidence rate? How is it calculated?
proportion of initially susceptible indivudals in a population who become new cases during a follow-up period
(number of incident NEW cases)/(amount of at-risk experience)
denominator = animal or person-time at risk
How is the denominator for incidence rate expressed? What are 2 reasons why it is done like this?
units of animal/person-time at risk
- easier to account for individuals in the study - keep record for each individual for exact number of at-risk experience
- accounts for re-infections each treated as a new incidence case
There is a study of 5 cows over a 12-month period. 5 clinical cases of mastitis (in 3 cows) occurred over 825 cow-days at risk. What is the incidence rate per day? Year?
5 cases/825 cow-day = 0.006061 cases/cow-day
(0.006061 cases/cow-days x 365 days/year) = 2.21 cases/cow-year
Comparison of measures of frequency:
PREVALENCE = % of population with disease (not just new cases)
INCIDENCE = new cases
Case fatality risk calculation:
MORBIDITY = 294/378 = 77.8%
no, this study was dependent on samples collected by a private lab
What is the incidence rate in this population? Prevalence?
4 NEW cases / 91 animal-months = 0.044 cases per animal-months
0.044 x 12 month = 0.53 cases/year
5 diseased animals / 12 = 0.417 = 41.7%
DISEASED = no longer at risk
incidence is only interested in new cases, so J does not count
What is the incidence rate for boredom in this Epidemiology class? If the class was checked at the 21st and 43rd minute, what would risk (prevalence) be for a student to be bored at each of these 2 times?
INCIDENCE RATE = new cases / student minutes at risk
(7 boredom events) / (345 student mins) = 0.0203 bored/student mins
(0.0203 bored/student mins) x (50 mins/lecture) = 1.015 bored/lecture
21st min - 8 awake, 1 asleep = 0/8
43rd min - 2 left, 2 bored, 5 awake = 2/7
