Lecture 3: measuring disease Flashcards
What are the measures of disease frequency?
Incidence
Prevalence
What is incidence rate?
The number of new events in a defined population within a specified period of time
What is prevalence?
The number of cases of disease in a given population
How is point prevalence measured?
Point prevalence = n of cases in a defined pop at one point in time / n of persons in a defined pop at same point in time
X 10,000
X 100,000
How do you measure Incidence rate?
N of new cases of a diseases during a given period
Incidence rate= ————————————————————————————
Population at risk during a giving period / total person-time at risk
X1,000
X 10,0000
Pop at risk= total pop - n of cases
What is Period Prevalence?
Period prevalence is a variation which represents the number of persons who were a case at any time during a specified (short) period as a proportion of the total number of persons in that population.
What are Person years and person months?
Person years and person months are types of measurement take into account both the number of people in the study and the amount of time each person spends in the study. For example, a study that followed 1000 people for 1 year would contain 1000 person years of data.
What makes measurement of incidence complicated?
Sometimes measurement of incidence is complicated by changes in the population at risk during the period when cases are ascertained, for example, through births, deaths, or migrations. This difficulty is overcome by relating the numbers of new cases to the person years at risk, calculated by adding together the periods during which each individual member of the population is at risk during the measurement period.
What is a case definition in epidemiology?
A set of criteria used in making a decision as to whether an individual has a disease or health event of interest.
What are the characteristics of a case definition?
Clear
Simple
Concise
Beeasily applied to all individuals in the population of interest.
What does a case definiton comb8ne?
Clinical symptoms and laboratory information
What is sensitivity?
A sensitive definition will identify as many of the actual cases as possible, so will avoid false negatives
AVOIDS FALSE NEGATIVE but may include false positives
What’s is specificity?
A specific definition will avoid false positives, it will maximise the proportion of non-cases that are correctly identified as such, but it is highly likely that some true cases will be ‘missed’
AVOID FAKSE POSITIVE but may include false negatives
How Are Prevalence and Incidence typically Measured?
Routine surveillance
Population samples
What designs are used to measure prevalence?
Cross sectional designs
