Epidemiological Estimate General Approach

1
Epidemiological Estimate General Approach

• Chawetsan Namwat
• MD., MPH., Cert. FETP
• Bureau of Epidemiology, DDC, Thailand

2
Outline

• Measurement in epidemiology
• Epidemiological estimation
• Steps
• Activity, output, and important questions

3
Epidemiology

• Study of distribution and determinants of disease
  occurrence in population and use results for
  prevention and control health problems
• Descriptive v.s. Analytic epidemiology

4
Epidemiological measurements

• Measure of magnitude number or rate of
  morbidity, mortality, disability, year life loss,
  etc..
• Measure of association Oods ratio, Risk ratio
• Measure of impact Attributable fraction, etc.

5
Definitions

• incidence new cases
• prevalence existing case (new cases old but
  currently ill)

Cure
Died
6
Definitions

• RR Risk Ratio ... Relative Risk (range 0 - 8)
• Interpret RR How many times higher risk of the
  exposed compare to the non-exposed
• RR1 (no relationship)
• RRgt1 (Risk factor)
• RRlt1 (Protective factor)
• OR Odds Ratio ( estimation of RR and interpret
  in the same way)

7
Disease prevention and control
Affected, no symptom
Suscep-tibility
Symp-toma-tic
Natural history of Disease
...
Level
Secondary
Tertiary
Primary
?????????????
????????????????????
???????????
Impact
8
Steps of Epidemiological Estimation

• 1. Research Current Knowledge of the Disease
• 2. Construct Diagram of the Natural History of
  the Disease
• 3. Identify the Epidemiological Indicators to be
  Estimated
• 4. Review the Published and Non-Published Data
  Available
• 5. Check Data Consistency and Quality (DisMod)
• 6. Apply Data to Calculate YLD

9
Goal

• A set of epidemiological parameters prevalence,
  incidence, complication rate, death rate,
  remission rate, etc.
• Then it will be use for calculation of DALYs

10
From input to .. output
11
DisMod
12
Miss Motor Show
13
(No Transcript)
14
Research Current Knowledge of the Disease

• Actions
• Focus on the definition, natural history,
  classification, severity and epidemiology of the
  condition studied and its disabling sequelae
• Acquire a general familiarity with the current
  knowledge on the condition, from major textbooks
  in the specialty, an authoritative review paper
  or scrutiny of attempts by GBD or other NBD
  studies to describe the disease

15
Research Current Knowledge of the Disease(cont.)

• Output
• A clear knowledge of the state-of-art concerning
  the epidemiology of the disease and potential
  sources of data
• Enough information to construct a diagram of the
  natural history of the disease. If there is not
  enough information, consult experts (preferably
  opinion leaders) to put up assumptions necessary
  to fill in the gaps

16
Research Current Knowledge of the Disease(cont.)

• Important questions
• What is the current knowledge on the disease
  being studied? What are the limitations? Is there
  any controversy ?
• What are the relevant data available on the
  natural history of the disease and its disabling
  sequelae? (prevalence, incidence, duration, age
  of onset, remission rate, and mortality rate,
  relative risk, level of severity and duration
  from disease onset to disabling sequela)
• If there is no clear consensus, do you think a
  panel of experts will be needed? If yes, how
  would you choose the expert panel

17
(No Transcript)
18
2. Construct Diagram of the Natural History of
the Disease

• Define the population for which the estimates are
  being developed
• Describe case definitions and relationships
  between the diseases
• Intersect? Relate? with other diseases
• Define and draw the natural history of the disease

19
Define the population
Sub-Saharan Africa
South Africa
Cape Town
Describe case definitions relation
Asthma?
South Africa
Bronchitis?
Hepatitis
20
dynamic disease model
Dead
Risk factor 1
7
1
Susceptible and Exposed
7
Risk factor 2
1
Immune
9
Risk factor 3
1
5
2
7
8
3
6
7
Affected
10
4
Sequelae
21
dynamic disease model for hepatitis B
Dead
Risk factor 1
7
1
Susceptible and Exposed
7
Risk factor 2
1
Immune
9
Risk factor 3
1
5
2
7
8
3
6
7
Acute or Asymtomatic Infection
10
4
Carrier Cirrhosis Liver cancer
22
Disease diagram for breast cancer with tumor
larger than 5 cm at diagnosis
Clinically disease-free
Cured
5 years 8 months
p
Initial diagnosis Treatment
1-p
In remission
Terminal
Disseminated disease
Death
Remainder of time
1 year
1 month
23
Simple disease model
24
2. Construct Diagram of the Natural History of
the Disease

• Output
• A clear delineation of the case-definition, as
  well as epidemiological relationships between
  disease determinants, outcomes, and sequelae.
• A clear delineation of the epidemiological
  parameters needed to describe the dynamic of the
  disease.
• A simple disease model which captures the
  important components of the disease (in terms of
  pathways, sequelae, stages etc) but is not too
  complex to estimate from available data

25
2. Construct Diagram of the Natural History of
the Disease

• Questions
• Is the disease well represented by the diagram
  created?
• Is there any other factor not represented in the
  diagram that could be affecting the accuracy of
  this representation?
• Is the marginal effort associated with
  introducing a new factor in the diagram much
  bigger than the marginal benefit? Does the
  perfection matter here?

26
3 Identify the Epidemiological Indicators to
be Estimated

• Epidemiological Data
• Incidence
• Prevalence
• Remission
• Case fatality or RR of mortality
• Duration
• Mortality

• - by age sex group
• - Disability Weight

27
Epidemiological data

• Usually not complete
• Prevalence available, but incidence
• Not consistency
• Mortality may be higher than incidence?
• Incidence and prevalence

28
Epidemiological data

• Complete data desirable
• Allows to calculate BoD in a comparable way
  between diseases
• Consistent data desirable
• Inconsistent data means something is wrong

29
3 Identify the Epidemiological Indicators to
be Estimated

• Output A list of the most important
  epidemiological indicators needed to construct
  the diagram of the natural history of the disease
  
• Incidence ???????????
• Prevalence ???????
• Remission ????????????
• Duration ????????????/?????
• Case fatality ???????????????
• Mortality ????????
• RR on total mortality ???????????????????

30
4 Review the Published and Non-Published Data
Available

• Quality of data?
• How could we use them?
• Registrations have some limitations, please try
  looking another sources to confirm

31
4 Review the Published and Non-Published Data
Available

• Data sources
• Registration
• Population surveys
• Epidemiological study
• International source

32
4 Review the Published and Non-Published Data
Available

• Actions
• Organize the study results in a way that
  facilitates easy comparison of the results. For
  example, Table 7.2
• From the summary table choose the highest quality
  studies from populations similar to the target
  population.

33
4 Review the Published and Non-Published Data
Available

• Output
• A set of estimates coming from the most valid
  and representative sources
• Questions
• Are you sure that the data selected are the most
  reliable and representative?
• Is the reason why other papers/data sources have
  been excluded clearly explained?
• Which estimates are missing? Why could they not
  be found in Steps 1 and 4?

34
5. Check Data Consistency and Quality (use DisMod)

• Use DisMod checking data consistency quality
• Adjust for representativeness
• Estimated the reasonable indicators
• Consult experts
• Everytime adjusting indicators, use DisMod to
  confirm the consistency

35
Relationship between DisMod input and output data
Output
Prevalence
Mortality
Duration
Input
Ý
ÝÝÝ
Ý
Incidence
Ý
ßß
ß
ßß
Remission rate
Ý
ß
ÝÝÝ
ß
Case-fatality rate
36
0.04
Incidence
0.035
0.03
Mortality
0.025
0.02
Rate
0.015
0.01
0.005
0
25-29
30-34
35-39
40-44
45-49
50-54
55-59
60-64
65-69
70-74
75-79
80-84
85-89
90-94
95
Age group
37
0.03
0.025
Incidence
0.02
0.015
Rate
0.01
0.005
0
25-29
30-34
35-39
40-44
45-49
50-54
55-59
60-64
65-69
70-74
75-79
80-84
85-89
90-94
95
Age group
38
5. Check Data Consistency and Quality (use DisMod)

• Output
• A consistent set of epidemiological data.
• All the necessary data to calculate YLD for the
  burden of disease and injury.
• Question
• If data from the same source are not consistent,
  the question should be how reliable is the data
  source?

39
5. Check Data Consistency and Quality (use DisMod)

• Questions
• If DisMod provides right away a consistent set
  Is the disease so homogeneously distributed in
  the population
• Do the numbers make sense? Are results consistent
  with the published literature? Does the expert
  panel agree with these numbers?

40
6 Apply Data to Calculate YLD (use template)

• Output
• Burden of disease in term of DALY
• Questions
• Was the epidemiological assessment well done?
• Does the uncertainty of the assumptions made
  result in large variability in the burden
  outputs? (Sensitivity/uncertainty analysis)
• Are experts and opinion leaders supporting this
  epidemiological assessment
• make sense?

41
Conclusion
42
Steps of Epidemiological Estimation

• 1. Research Current Knowledge of the Disease
• 2. Construct Diagram of the Natural History of
  the Disease
• 3. Identify the Epidemiological Indicators to be
  Estimated
• 4. Review the Published and Non-Published Data
  Available
• 5. Check Data Consistency and Quality (DisMod)
• 6. Apply Data to Calculate YLD

43
Goal

• A set of epidemiological parameters prevalence,
  incidence, complication rate, death rate,
  remission rate, etc.
• Then it will be use for calculation of DALYs

44
How to get complete consistent data?

• More and better measurement
• Expert knowledge
• Disease modelling

45
Thank you
46
(No Transcript)