Aucun titre de diapositive - PowerPoint PPT Presentation

Loading...

PPT – Aucun titre de diapositive PowerPoint presentation | free to download - id: 6f0966-MTJhY



Loading


The Adobe Flash plugin is needed to view this content

Get the plugin now

View by Category
About This Presentation
Title:

Aucun titre de diapositive

Description:

Title: Aucun titre de diapositive Author: ANCELLE Last modified by: usuari08 Created Date: 9/18/1998 9:38:39 AM Document presentation format: Presentaci n en pantalla – PowerPoint PPT presentation

Number of Views:16
Avg rating:3.0/5.0
Slides: 63
Provided by: ANC63
Learn more at: http://ecdc.europa.eu
Category:

less

Write a Comment
User Comments (0)
Transcript and Presenter's Notes

Title: Aucun titre de diapositive


1
Epidemiological analytical studies
17th EPIET introductory course Lazareto, Menorca,
Spain September 2011
Alicia Barrasa Biagio Pedalino Based on
previous EPIET intro courses
2
Epidemiological studies
who where when
hypothesis
descriptive
analytical
population
observational
individual
experimental
Clinical trials
cohort
cross-seccional
case-control
ecological
3
Experiment
Exposed
Disease occurrence
Exposure assigned
Not exposed
Unethical to perform experiments on people if
exposure is harmful
4
If exposure not harmful Treatment Preventive
measure (vaccination)
Blinded Doses Time period Risk - effect No bias
Randomised Controlled Trial
If RCT not possible Left with observation of
experiments designed by Nature
5
Epidemiological studies
who where when
hypothesis
descriptive
analytical
population
observational
individual
experimental
Clinical trials
cohort
cross-seccional
case-control
ecological
6
(No Transcript)
7
Cohort studies marching towards outcomes
8
What is a cohort?
  • One of 10 divisions of a Roman legion
  • Group of individuals
  • sharing same experience
  • followed up for specified period of time
  • Examples
  • birth cohort
  • cohort of guests at barbecue
  • occupational cohort of chemical plant workers
  • EPIET cohort 17

9
follow-up period
10
  • Calculate measure of frequency
  • Cumulative incidence
  • Attack rate (outbreak)
  • Incidence rate

end of follow-up
11
Cohort studies
  • Purpose
  • Study if an exposure is associated with
    outcome(s)?
  • Estimate risk of outcome in exposed and
    unexposed cohort
  • Compare risk of outcome in two cohorts
  • Cohort membership
  • Being at risk of outcome(s) studied
  • Being alive and
  • Being free of outcome at start of follow-up

12
Cohort studies
13
Cohort studies
exposed
unexposed
14
Presentation of cohort data 2x2 table
ill
total
49 98 ?
ate ham
did not eat ham
4 10 ?
15
Presentation of cohort data Population at risk
Does HIV infection increase risk of developing TB
among a population of drug users?
Population Cases (f/u 2 years)

HIV 215 8
HIV - 289 1
Source Selwyn et al., New York, 1989
16
Presentation of cohort data Person-years at risk
Tobacco smoking and lung cancer, England
Wales, 1951
Person-years Cases
Smoke 102,600 133
Do not smoke 42,800 3
Source Doll Hill
17
Presentation of data Various exposure levels
18
Prospective cohort study
Disease occurrence
Exposure
time
19
Retrospective cohort study
Disease occurrence
Exposure
time
20
Recipe Cohort study
  • Identify group of
  • exposed subjects
  • unexposed subjects
  • Follow up for disease occurrence
  • Measure incidence of disease
  • Compare incidence between exposed and unexposed
    group

21
(No Transcript)
22
  • Our objective is to compare
  • the incidence rate in the exposed population
  • to the rate that would have been observed
  • in the same population, at the same time
  • if it had not been exposed

23
Principle of case control studies
24
Source population
Exposed
Unexposed
25
Source population
Cases
Exposed
Unexposed
26
Source population
Cases
Exposed
Sample
Unexposed
Controls
27
Source population
Cases
Exposed
Sample
Unexposed
Controls Sample of the population
Representative with regard to exposure
Controls
28
Intuitively if the frequency of exposure is
higher among cases than controls then the
incidence rate will probably be higher among
exposed than non-exposed
29
Case control study
Exposure
? ?
Disease Controls
Retrospective nature
30
Distribution of cases and controls according to
exposure in a case control study
Cases Controls Exposed a b Not
exposed c d Total a c b d
exposed a/(ac) b/(bd)
31
Distribution of myocardial infarction by oral
contraceptive use in cases and controls
Oral Myocardial contraceptives Infarction
Controls Yes 693 320 No 307
680 Total 1000 1000 exposed 69.3
32
32
Distribution of myocardial infarction by amount
of physical activity in cases and
controls
Physical Myocardial activity Infarction
Controls gt 2500 Kcal 190 230 lt 2500
Kcal 176 136 Total 366 366
exposed 51.9 62.8
33
Two types of case control studies
  • Exploratory New disease Several
    exposures "Fishing expedition"
  • Analytical Define a single
    hypothesis Dose response

34
(No Transcript)
35
Cohort studies
exposed
unexposed
36
Effect measures in cohort studies
  • Absolute measures
  • Risk difference (RD) Ie - Iue
  • Relative measures
  • Relative risk (RR)
  • Rate ratio
  • Risk ratio

Ie incidence in exposed Iue incidence in
unexposed
37
ill
total Incidence
ate ham
49 98 50
did not eat ham
4 10 40
Risk difference 50 - 40
10 Relative risk 50 / 40 1.25
38
Does HIV infection increase risk of developing TB
among drug users?
39
Vaccine effectiveness (VE)
VE 1 - RR 1 - 0.28 72
40
Various exposure levels
Population Cases Incidence
Exposure level
at risk
41
Various exposure levels
Population Cases Incidence RR
Exposure level
at risk
RR1
RR2
RR3
Reference
42
Cohort study Tobacco smoking and lung cancer,
England Wales, 1951
Source Doll Hill
43
A cohort study allows to calculate indicators
which have a clear, precise meaning. The
results are immediately understandable.
44
(No Transcript)
45
Cohort studies
  • Rate
  • Rate difference
  • Rate Ratio (strength of association)

Case control studies
  • No calculation of rates
  • Proportion of exposure

Any way of estimating Rate ratio ?
46
Source population
Cases
E
a
P1

I1 a / P1
a/P1 I1/ I0 ------
c/P0
c
P0
I0 c /P0
E
a I1 -------- P1/10

a/P1 I1/ I0 ------ c/P0
c I0 -------- P0/10
47
Source population
Pop.
Cases
E
a
P1
I1 a / P1

a/P1 I1/ I0 ------
c/P0
c
P0
I0 c /P0
E
sample

Cases
Controls
E
a
b
P1 b --- ---- P0 d
c
d
E
48
Source population
Pop.
Cases
E
a
P1
I1 a / P1

c
P0
I0 c /P0
E
a/P1 a . P0 a . d I1 / I0 ------
------- ----- c/P0 c . P1 c . b
a / c ------ b / d
sample

Cases
Controls
E
Since d/b P0 / P1
a
b

P1 b --- ---- P0 d
c
d
E
49
Case control study design
Cases
Controls
Odds ratio
E
a
b
a b a x d ---- --- --- ---- c d b x c
c
d
E

50
Distribution of myocardial infarction by recent
oral contraceptive use in cases and
controls
Oral Myocardial contraceptives Infarction Cont
rols OR Yes 693 320 4.8 No 307
680 Ref. Total 1000 1000
exposed 69.3 32
51
Distribution of myocardial infarction by amount
of physical activity in cases and controls
Physical Myocardial activity Infarction Control
s OR gt 2500 Kca l 190 230 0.64 lt 2500
Kcal 176 136 Ref. Total 366 366
exposed 51.9 62.8
52
Distribution of cases of endometrial cancer by
oestrogen use in cases and controls
Oestrogen use
Cases
Controls
Odds ratio
High
a1
b1
a1d/b1c
Low
a2
b2
a2d/b2c
None
c
d
Reference
53
Relation of hepatocellular adenoma to duration
of oral contraceptive use in 79 cases and 220
controls
54
(No Transcript)
55
Limitations of case-control studies
  • Cannot compute directly relative risk
  • Not suitable for rare exposure
  • Temporal relationship exposure-disease difficult
    to establish
  • Biases
  • control selection
  • recall biases when collecting data
  • Loss of precision due to sampling

56
(No Transcript)
57
Disadvantages of cohort studies
  • Large sample size
  • Latency period
  • Cost
  • Time-consuming
  • Loss to follow-up
  • Exposure can change
  • Multiple exposure difficult
  • Ethical considerations

58
Advantages of case control studies
  • Rare diseases
  • Several exposures
  • Long latency
  • Rapidity
  • Low cost
  • Small sample size
  • Available data
  • No ethical problem

59
(No Transcript)
60
Strengths of cohort studies
  • Can directly measure
  • incidence in exposed and unexposed groups
  • true relative risk
  • Well suited for rare exposure
  • Temporal relationship exposure-disease is clear
  • Less subject to selection biases
  • outcome not known (prospective)

61
Strengths of cohort studies
  • Can examine multiple effects for a single exposure

62
The cohort study is the gold standard of
analytical epidemiology
CASE-CONTROL STUDIES HAVE THEIR PLACE IN
EPIDEMIOLOGY but if cohort study possible, do not
settle for second best
About PowerShow.com