Measures of Disease frequency survey design Flashcards
What are counts, and fractions?
Counts: Number of individuals infected, diseased, dead.
Fractions: Number of individuals infected diseased or death divided by the number of individuals capable of experiencing infection disease or death
What are fractions used for?
Used to express the probability of disease in populations of different size (different number of individuals at risk)
How to calculate ratio and proportion?
Ratio: a/b
Proportion: a/a+b
In epidemiology terms, What is rates?
Rates are used to represent magnitude of change in the occurrence of an event of intrests with respect to a population at risk over time.
What is prevelence?
The total number of individuals who have a disease at a particular time divided by the population at risk of having the disase
SOOOO basically just probability
DOES NOT DIFFERENTIATE BETWEEN OLD AND NEW CASES OF DISEASE.
What is incidence?
The number of new cases of disease that occur in a defined population within a specified period of time.
Relates to new cases of disease
What is cumulative incidence? (Incidence risk)
Proportion of disease-free individuals developing a given disease over a specified time
Needs to have a period of time
How to calculate Incidence density? (Incidence rate)
incidence density: Number of new cases / Population time at risk.
Total individual’s time at risk=population time at risk
What are the interpretations of Incidence rate, Incidence risk, and Prevelence?
Incidence rate: Average number of cases per unit time
Incidence risk: Risk of developing disease over given time period
Prevelence: Probability of having disease at a particular point in time
What is simple random sampling?
Fixed % of the population is sampled using a formal random process (number generator/numbers out of the hat etc.) Each unit in the population has an equal chance of being selected?
What is Systemic Random Sampling?
The first unit is selected randomly and then further units are selceted at regular intervals in the sampling frame
What is Statified Random Sampling?
Before selection, the sampling frame is divided into based on factors likely to influence the level of the characterisitc being estimated, next simple or systematic random sampling is performed within each group
What is multistaged random sampling?
Cluster or groups of animals are randomly selected, then within these groups a further sample of units is chosen
What are teh advantages and disadvantages of Simple random sampling?
A: Simple to conduct
D:can only be conducted when all the unuitsin population are identified
What are the advantages and disadvantages of systematic random sampling?
A: Does not require identification of all units in the population
D: Effect of periodicity (bias, caused by particular characteristics arising in the sampling frame at regular intervals, e.g every 3rd batch of animals and there is some type of sequencing of types of batches, i.e. larger batches always followed by few smaller ones)
Lack of precision when population is heterogeneous
What are the advantages and disadvantages of stratified radom sampling?
A: can give higher precision that a simple random sample of the same size
We can ensure that we obtain sufficient observations per group to allow seperate analysis of each group
D: the status of sampling units with respect to the factor must be known
More complex methods to obtain varience estimates
What are the advantages and disadvantages of Multistage sampling?
A: Flexibility: the number of primary and secondat units may be varied to account for differences in cost of sampling
Does not require complete list of units (flocks if cluster are villages)
D: can reduce precision since animals within clusters tend to be more similar to each other than to animals in other clusters
Require more dificult analysis
