Measures of Disease Frequency

Ratio

A fraction in which the numerator is not part of the denominator

Eg 1 Students of the Dental Faculty/ Students of the Colombo University

2 I.U.D/Live birth

Proportion

Numerator is a part of the denominator

Eg 1.Students of the Dental Faculty/ Students of Peradeniya University

2. I.U.D/All birth

Rate

Proportion in which change over time is considered. (but in practice often used interchangeably with proportion)

Prevalence-number of cases

1. Point prevalence

Proportion of individuals in a specified population at risk who have the disease of interest at a given point in time.

2. Period prevalence

Over a specified period of time (year,lifetime)

Incidence- number of new cases arising in a given period in a specified population

1. Incidence rate/density- concept of person

time. Eg- Ref example

2. Cumulative incidence- No of new cases of disease occurring over a specified period of time in a population at risk at the beginning of the interval.

Eg New cases of flu, bet March 09 and Mar 10/All

students as at 17/3/2009

Features of above measurements

Incidence-

Aetiology

Unit if analysis an event

Actively acquired diseases

May exceed 100%

Duration

Conveys information about the risk of contracting the disease

Prevalence-

More permanent states/conditions

Societal burden

Unit of analysis is person

Never exceed 100%

Indicate how widespread the disease is

Prevalence=Incidence x disease duration

Short duration conditions Eg Flu- Low prevalence, High incidence

Incidence Rate/Density

Best estimation of the true risk of acquiring a disease at any moment in time.Cumulative Incidence

Best estimation of how many people will eventually get the disease in an given population.

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