What is Epidemiology?

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Tuesday, March 1, 2011

Morbidity vs. Mortality

"Morbidity" and "Mortality" are the two most important and basic concepts in epidemiology, because both of them could be used for measuring the occurrence of disease.

Firstly, what is morbidity measurement? Normally, we use incidence rate and prevalence to express the extent of a disease in populations. Here, 

Incidence rate per 1,000 =                                             
  No. of NEW cases of a disease occurring                  
              in the population during                                    
            a specified period of time               * 1,000        
       No. of persons who are at risk of                                        
          developing the disease during
                   that period of time

Prevalence per 1,000 =
 No. of cases of a disease present
in the population at a specified time * 1,000
   No. of persons in the population
           at that specified time

The relationship between incidence and prevalence could be demonstrate further by figure 1. [1] Assume that the flask is a community, and the beads in the flask represent the prevalent cases of a disease in this community. The first pic shows the baseline of prevalence, then the three pics below show three different ways that incidence could affect prevalence.

Figure 1. Relationship between incidence and prevalence.

Secondly, what is mortality measurement? Normally, mortality could be used for measurement of disease severity and how effective a treatment for a diseases is over time. There are two different types of mortality rates:

Annual mortality rate for all causes                                 
(per 1.000 population) =                                                    
  Total # of deaths from all causes in 1 year  * 1000    
  # of persons in the population at midyear                    

Disease-specific/cause-specific rate
(per 1,000 population) =
     # of deaths from a disease in 1 year     * 1000
  # of persons in the population at midyear

Case-Fatality rate (percent) =                                       
    # of individuals hying during a                                     
specified period of time after disease                                      
             onset or diagnosis                          * 100        
# of individuals with the specified disease                      
Proportionate mortality (percent) =
   # of deaths from certain disease
             in a population during
              certain time period              * 100
  Total # of deaths in the population
      during the same time period

Those are some basic concepts and formulas on how to measure the occurrence of a disease that I learned from the textbook. After knowing those formulas, I think you would have more sense on how epidemiological event is measured.

1. Gordis L. Epidemiology. 4th ed. Elsevier. 2008. ISBN: 9781416040026

1 comment:

  1. Great way to compare prevalence and incidence. I will use it with my Oral Pathology residents. Thanks
    J.E. Bouquot, DDS, MSD; WVU School of Dentistry