An epidemiological investigation was begun on July 1, 2000 among a population of 1,000 individuals. Three individuals were found to have leukemia on July 1st. During the tenyear followup period, five new cases of leukemia were diagnosed. Among the eight leukemia cases, four deaths occurred during the tenyear followup period. Two additional individuals, neither of whom had leukemia, were lost to followup at some point during followup. None of the remaining 990 study participants became ill, died, or were lost to followup during the followup period.
The diagram below indicates the experience of the 10 study participants, using the following symbols:
*  :case recognition (diagnosis of leukemia) 
*<  :onset of leukemia was prior to start of study period 
  :observation period during followup 
X  :status at termination of observation (i.e., alive, dead, lost) 
case 

2000 
2001 
2002 
2003 
2004 
2005 
2006 
2007 
2008 
2009 
2010 

1 
 
* 
 
 
 
 
 
 
 
 
X 
alive 

2 
 
 
 
* 
 
 
 
 
 
 
X 
alive 

3 
*< 
 
 
 
 
 
 
 
X 
dead 

4 
 
 
 
 
X 
lost 

5 
 
 
 
 
 
 
* 
X 
dead 

6 
 
 
 
 
 
 
 
 
 
* 
X 
alive 

7 
 
* 
 
 
X 
dead 

8 
*< 
 
 
 
 
 
 
 
 
 
 
X 
alive 
9 
 
 
 
 
 
 
 
X 
lost 

10 
*< 
 
 
X 
dead 
What was the prevalence of leukemia on January 1, 2005?
What was the cumulative incidence of leukemia during the 10year followup period?
What was the incidence rate of leukemia during the 10year followup period?
What was the case fatality rate during the 10year followup period?
Based on the experience of 10 study participants,
1. By prevalence of leukemia on January 1,2005, one means point prevalence.It is given by,
Prevalence = No. of cases (both old and new) / Population size
= 6 / 10 = 0.6
(The count of cases obtained by adding the * and *< before 2005)
2.Cumulative incidence = No. of new cases during the 10 year period / No. initially at risk during the period
= 5 / 10 = 0.5
3. Incidence rate = No. of new cases / Total 'at risk' observation time (or disease free years)
= 5 / 31 = 0.1613
(The denominator here is obtained by count the no .of periods () before *, excluding the three (*<) old cases)
4. Fatality rate = No. of deaths / No. of cases
= 4 / 8 = 0.5
(Here numerator is the no.of 'X' s for * and *< cases before the 10 year period and the denominator is the no. of * and *< cases during the 10 year period.)
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