FUNDAMENTALS OF EPIDEMIOLOGY by d9n1aQO

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									  Fundamentals of Epidemiology

Measures of morbidity and mortality used in
              Epidemiology


  Afolabi Olusegun MBChB (Ife), MPH (Hadassah), FMCPH
      Levels of measurement
Nominal
Ordinal
Interval
Ratio
Absolute and Relative measures
Absolute: age, weight, height

Relative: BMI, Ratio of total cholesterol
   usually relative to size and
characteristics of a population
           Different measures
Ratios express a quantity relative to another

Proportion : Numerator is part of the
denominator

Rate : classically is a ratio of a change in a
quantity to a change in another quantity
with denominator often being time
                   Ratio
Mortality: Death to case ratio

Morbidity: Risk ratio; Rate ratio; Odd ratio;
Period prevalence
               Proportion
Mortality: proportionate mortality

Morbidity: Attack rate, Point prevalence,
Attributable proportion
                     Rate
Mortality : Crude mortality rate, case fatality
rate, age specific mortality rate, infant
mortality rate

Morbidity: person time incidence rate
RATE: Measure of the frequency with which
an event occurs in a defined population during
a given length of time
(expressed in terms of some arbitrary constant
- %, /1000, 100,000, etc)
Rates may also relate special events to total events
eg. CFR (not time dimensioned)
          Comparison of Rates
           Advantages          Disadvantages
         Actual summary Differences in pop.
 Crude   rates;             compositions make
 Rates   Readily calculable interpretation difficult
         Addresses          Cumbersome if there
Specific homogeneous        are many subgroups;
 Rates subgroups;           No summary figure
         Useful for PH
         Summary            Fictional rates;
Adjusted measure;           Magnitude depends on
 Rates Permits unbiased     standard pop;
         comparisons        Hides subgroup diff’s
      Measures of health status
** Measures of frequency:

** Measures of association
** Measures of potential population impact


How do we measure dichotomies?--diseases,
  disorders, death …
Measures of frequency: eg morbidity—
  prevalence and incidence
Prevalence rate is defined as:
The proportion of a population-at-risk
 affected by a “disease” at a specific
             point in time
Prevalence rate (P) is calculated by:

    # of people with the disease
           at a specific time      x10n
    # of people in the population
     at risk at the specified time
    Types of Prevalence Rates
• Point Prevalence rate =
 # persons with the condition   at a point
    total number of persons      in time

• Period Prevalence rate =
  # persons with the condition in a specified
     total number of persons period of time
           Point Prevalence
• Calendar time (ex. Nov 12, 2001)
• Babies born between Jan 1 – Dec 30, 2001
  – Of 20,000 born, 60 had symptoms of malformation
• Women who enter a study when they show up
  at the clinic and are examined only once
        Prevalence question
 Interview question:    Type of measure:
Do you currently have • Point prevalence? 
 asthma?              • Period prevalence?
                      • Lifetime prevalence?
         Prevalence question
 Interview question:        Type of measure:
Have you had asthma      • Point prevalence?
 in the last 10 years?   • Period prevalence? 
                         • Lifetime prevalence?
       Prevalence question
Interview question:      Type of measure:
Have you ever had     • Point prevalence?
asthma?               • Period prevalence?
                      • Lifetime prevalence? 
Factors influencing observed prevalence rate




• Longer duration of the disease

• Prolongation of life without cure
• Increase in new cases (incidence / risk)

• Out-migration of non-diseased people
• In-migration of susceptible/diseased people

• Improved diagnostic &/or reporting
Factors influencing observed prevalence rate

 • Shorter duration of the disease
 • Increased case-fatality rate
 • Decrease in new cases (incidence / risk)
 • In-migration of “healthy” people
 • Out-migration of diseased people
 • Improved cure rate
                     Uses
•   Pretest probability
•   Community diagnosis
•   Basis for decision making and planning
•   Provide clues for etiology
                             Question
• A health centre needs information for use in planning a
  home care program for people who are too disabled to leave
  their houses: for example, how many cases can be expected
  to be under care at a given time, and what is the total
  number of cases that will be treated during a year? The
  following information is obtained from the agency that has a
  program in a similar neighborhood. At the beginning of
  1999 the population size was 24,000, and at the end of the
  year it was 26,000. At the beginning of 1999 there were 96
  house bound patients, 20 of these died during 1999, and 4
  were moved elsewhere. Another 40 people became house
  bound during 1999, and 8 of them died during the year.
Calculate the point prevalence rates at the beginning and end of 1999 and
  the period prevalence rate in 1999?
                    Solution
• Point prevalence rate was 96/24,000 *1000
At the beginning of 1999
• At the end of the year was [(96+40-20-4-
   8)/26000]*1000
• To calculate Period prevalence, we either use the
   mid year population if given or the average of the
   population at the beginning and the end.
 (24,000+26,000)/2 =25,000
Period prevalence = [(96+40)/25,000]*1000
                   Incidence
1. Counts
2. Risk [Cumulative incidence]
3. Rate [Incidence density, person-time incidence]
Cumulative incidence:

Is a proportion—
N of new cases (numerator) in a fixed
  group at risk (denominator) over
  defined time period i.e. risk
- attack rate (eg food poisoning)
- event rate—new + recurrent cases
- lifetime incidence, eg incidence to age
  75
Incidence density or person-time
          incidence:

a ratio, a true rate (not a proportion)
 N of new cases in a changing population
    observed for varying length of time
 expressed as N of cases / person-time

Eg NSAID-Alzh NEJM 2001;345:1022-9, HRT-CHD
                Ann Intern Med 2001;135:1-8
Rate analagous to speed of car (rate of
 change, eg 100 km/hr)
 --Instantaneous
 --Potential
 --Average

Risk analagous to distance covered
 --1/2 of the distance
 --1/10 of the distance
Incidence (cont’)
How do we calculate incidence?

*Cumulative incidence (CI)

 CI = I / N0
 CI = I / [N0 -(w/2)] ie survival analysis
 CI ~ 1 - e (-PTI* t ) ie calculated from rate
(based on relationship between rate and
 risk—see ‘sea’ of person-time)
 Incidence (cont’)
Incidence density (ID), person-time incidence
  (PTI)

PTI = I / ti             ti = PT
PTI = I / N (t)       ‘ordinary’ incidence

PTI = ID = average incidence density or
 average rate for a given period
Incidence (cont’)

ID (PTI) is a ratio not a proportion
CI is a proportion

Infant mortality example:

Is infant mortality a cumulative incidence
  or person-time incidence measure?
Incidence (cont’)

Is infant mortality a cumulative incidence or
  person-time incidence measure?

IM as usually calculated is a PTI rate:
  N of deaths within 12 months of birth divided by the N of
  live births in a calendar year—say 2003. Why is this a rate
  and not a proportion? Hint--What have we estimated in the
  denominator?                              PT



How would you determine infant mortality as
 a CI measure?
Incidence (cont’)


How would you determine infant mortality as
 a CI measure?

IM obtained by following a cohort of babies
  from birth to 1 year is a CI measure --a
  proportion ie the numerator is part of the denominator
 Calculations of incidence from example
So we can estimate risk in 3 ways:
1. Fixed cohort, no attrition   CI=I/N0

2.   Actuarial method       CI= I/(N0-w)
                                        2

3.   Incidence density
       CI  1 - e -ID * t = PTI·t / [(PTI · t/2)+1]
       CI ~ ID · t when this product is small
                     ~ < 0.10-- 0.20
       CI ~ ID in py when t (duration) is 1 yr
                Uses of incidence
- Prediction        individuals
- Clinical dx      

- Etiologic studies (cause) 
- Community dx                population
- Prevention/evaluation                 
      outcome evaluation of a program
      ie change in incidence of new or
      recurrent events
 Relationship between incidence
         and prevalence




Gordis
Relationship between incidence
        and prevalence
Relationship between incidence
        and prevalence
        Prevalence - Incidence
Screened Prevalence Annual Duration
population (per 1000) Incidence (years)
                      (per 1000)

Onetown      100


Twotown       60
       Prevalence - Incidence

Screened Prevalence Annual Duration
population (per 1000) Incidence (years)
                      (per 1000)

Onetown      100         4


Twotown       60        20
        Prevalence - Incidence
Screened Prevalence Annual Duration
population (per 1000) Incidence (years)
                      (per 1000)

Onetown      100         4        25

Twotown       60        20        3
          Prevalence - Incidence

Screened Prevalence Annual Duration
population (per 1000) Incidence (years)
                      (per 1000)

Onetown       100        4         25


Twotown        60        20        3

   Prevalence  Incidence x Duration
           Mortality studies
Basic and commonly available information
 for characterizing a population and
 community dx

Sources: *death notifications (DC)
  [numerator]
         *counts, census in defined areas
  [denominator]
             Mortality rates
1. Proportional mortality
       denominator - all death
2. Case fatality
        denominator - all cases of disease
   i. tells you how important - relatively
  ii. tells you how dangerous
3. Crude mortality—if ‘ordinary incidence’
  then the denominator is the average
  population, typically during a year
Mortality rates (cont’)


4. Stratum specific mortality
         eg age, sex specific - infant mortality
5. Disease specific mortality
        denominator - same as 3 (above)
6. Prevalence at death
         denominator - total number of
         autopsies
    Proportional mortality--examples
Mortality* from cancer in black mine workers
                 (S. Africa) and US blacks

  US Black            Bantu             Site
       3.0             12.7            liver
      61.5              1.3            other
      64.5             14.0               all

     * rates per 100,000 PY, not age adjusted
Proportional mortality (cont’)

               Proportional mortality ratio = 19
 (91% of cancer in Bantu due to PHC vs 4.7% in
                                  US blacks)

                     Rate ratio = 12.7 / 3 = 4.2
• Case Fatality rate (percent)=
No. of individuals dying during a specified period of time after disease onset or diagnosis *100
No. of individuals with a specified disease




• Crude Death Rate=
Total no. of deaths from all causes in 1 year *1000
No. of persons in the population at midyear
         Uses of Mortality data
•    Index of severity of disease
•    Index of risk of a disease
1.   When case fatality rate is high
2.   When duration of disease (survival) is
     short
               Problems
• Underlying cause of death
• Quality of data
• Cross comparability over time

								
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