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1 
Presented by: 
reMAN dhaKAL 
CODSH-NMC 
FIRST BATCH
 Introduction 
 Types of epidemiology 
 Types of analytical epidemiology 
 Case control study 
 Cohort study 
 Compa...
 John M. last: "the study of the distribution 
and determinants of health-related states in 
specified populations, and t...
4 
EPIDEMIOLOGY
5 
Analytical epidemiology 
 Second major type of epidemiology. 
 Focus on individual within population unlike descripti...
 Retrospective or trohoc study 
 Distinct features: 
1. Both exposure and outcome have occurred 
before the start of dis...
7 
The basic study design 
Cases 
(those with condition) 
eg: cases with oral cancer 
characteristic or risk factor) 
Cont...
1. Selection of cases and controls 
2. Matching 
3. Measurement of exposure , and 
4. Analysis and interpretation. 
8
9 
A. Selection of case 
Definition of a case: 
I). diagnostic criteria: 
ii). Eligibility criteria 
Sources of cases: 
i)...
 Define as:”process by which we select controls in such a way 
that they are similar to cases with regard to certain pert...
 Definition and criteria about exposure are just as 
important as those used to define cases and controls. 
This may be o...
The final step is analysis, to find out: 
a) Exposure rates among cases and controls to suspected 
factors 
b) Estimation ...
Example of case control study of tobacco chewing and oral cancer 
Tobacco 
chewers 
Non-chewers 
Cases 
(with oral cancer)...
1. Relative risk = Incidence among exposed 
incidence among non exposed 
= a c 
(a+b) (c+d) 
2. Odds ratio (OR) = (a/b) 
(...
1. Selection bias : 
special types: 
a) Prevalence incidence (selective survival) 
b) Admission rate ( Berkson/Berkesonian...
16 
ADVANTAGES: 
CASE CONTROL STUDY 
1. Relatively easy to carry out. 
2. Rapid and inexpensive 
3. Require fewer subjects...
 Prospective ,longitudinal, incidence and 
forward-looking study 
 Distinguishing features: 
a) The cohorts are identifi...
time 
Study begins here 
Study 
population 
free of 
disease 
Factor 
present 
Factor 
absent 
disease 
no disease 
diseas...
When there is good association between 
exposure and disease. 
 When exposure is rare, but the incidence of 
disease is ...
20 
1. PROSPECTIVE COHORT STUDY 
1. RETROSPECTIVE COHORT STUDY 
2. A COMBINATION OF 
PROSPECTIVE AND 
RETROSPECTIVE COHORT...
 - Outcome has not yet occurred the time 
of investigation begins. 
Measure exposure 
and confounder 
variables 
Exposed ...
 Outcomes have all occurred before the start 
of the investigation. 
22 
Measure exposure 
and confounder 
variables 
Exp...
1. Selecting of study subject 
2. Obtaining data on exposure 
3. Selection of comparison group 
4. Follow up 
5. analysis ...
24 
1. Selecting of study 
subject 
When exposure or cause of 
death is fairly frequent in 
the population 
i. Select grou...
25 
Selection of comparison 
group 
a. Internal comparisons:- 
 Comparison groups are in built 
(eg. Smoking, bp etc.) wi...
Data are analysed in term of: 
a. Incidence rates of outcome among exposed 
and non-exposed: 
26
27 
Data are analysed in term of: 
a. Incidence rates of outcome among exposed 
and non-exposed 
RISK FACTOR 
(TOBACCO) 
5...
b. Estimation of risk 
A. Relative risk (RR): 
= incidence of disease among exposed 
incidence of disease among non-expose...
B. Attributable risk (AR) Or “risk difference”: 
Incidence of disease rate among 
exposed- incidence among non exposed 
In...
1. Selection bias: 
 Non consent bias 
 Missing data bias 
2. information bias: 
 Error in classification of individual...
1. Incidence can be calculated 
2. Several possible outcomes related to 
exposure can be studied simultaneously. 
3. Provi...
1. Unsuitable for investigating uncommon disease. 
2. Long time to complete study and obtain results. 
3. Administrative p...
 Start disease :effect 
cause 
 First approach to test 
hypothesis. 
 Involve fewer subject. 
 Yield result quickly. 
...
34
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Analytical epidemiology

  1. 1. 1 Presented by: reMAN dhaKAL CODSH-NMC FIRST BATCH
  2. 2.  Introduction  Types of epidemiology  Types of analytical epidemiology  Case control study  Cohort study  Comparison between case control and cohort study 2
  3. 3.  John M. last: "the study of the distribution and determinants of health-related states in specified populations, and the application of this study to control health problems. 3
  4. 4. 4 EPIDEMIOLOGY
  5. 5. 5 Analytical epidemiology  Second major type of epidemiology.  Focus on individual within population unlike descriptive epidemiology..  Objective not to formulate hypothesis but to test hypothesis. TYPES A. CASE CONTROL STUDY B. COHORT STUDY
  6. 6.  Retrospective or trohoc study  Distinct features: 1. Both exposure and outcome have occurred before the start of disease 2. Study proceed backward from effect to cause 3. Uses a control or comparison group to support or refute an inference. 6
  7. 7. 7 The basic study design Cases (those with condition) eg: cases with oral cancer characteristic or risk factor) Control Unexposed (without Eg. Non chewers Exposed (with characteristic (those without condition) eg: those free of oral cancer or risk factor) Eg. tobacoo chewers
  8. 8. 1. Selection of cases and controls 2. Matching 3. Measurement of exposure , and 4. Analysis and interpretation. 8
  9. 9. 9 A. Selection of case Definition of a case: I). diagnostic criteria: ii). Eligibility criteria Sources of cases: i). Hospital ii). General population B. Selection of control Sources of controls: i). Hospital controls(common source of selection bias) ii). Relatives iii). General population Number of controls/control groups
  10. 10.  Define as:”process by which we select controls in such a way that they are similar to cases with regard to certain pertinent selected variables(eg. Age) which are known to influence the outcome of disease and which, if not adequately matched for comparability, could distort or confounded the result”.  CONFOUNDING FACTOR 10 EXPOSURE (eg. Consumption of alcohol) DISEASE (eg. Oesophageal cancer) CONFOUNDING FACTOR (eg. smoking, age)
  11. 11.  Definition and criteria about exposure are just as important as those used to define cases and controls. This may be obtained by :  Interviews  Questionnaries  Studying past record of cases such as hospital records, employment records etc.  Clinical or laboratory examination. Investigator should not know whether a subject is in case or control group. 11
  12. 12. The final step is analysis, to find out: a) Exposure rates among cases and controls to suspected factors b) Estimation of disease risk associated with exposure (ODD RATIO) 12
  13. 13. Example of case control study of tobacco chewing and oral cancer Tobacco chewers Non-chewers Cases (with oral cancer) 33 (a) 2 (c) Controls (without oral cancer) 55(b) 27(d) 13 Total 35 (a +c) 82 (b +d) EXPOSURE RATES a. Cases =a/(a+c) = 33/35 = 94.2% b. Controls =b/(b+d) = 55/82 = 67.0%
  14. 14. 1. Relative risk = Incidence among exposed incidence among non exposed = a c (a+b) (c+d) 2. Odds ratio (OR) = (a/b) (c/d)  It measure strength of the association between risk factor and outcome. ,
  15. 15. 1. Selection bias : special types: a) Prevalence incidence (selective survival) b) Admission rate ( Berkson/Berkesonian) 2. Information bias: a) Memory or recall bias b) Telescopic bias c) Interviewer’s bias 3. Bias due to confounding 15
  16. 16. 16 ADVANTAGES: CASE CONTROL STUDY 1. Relatively easy to carry out. 2. Rapid and inexpensive 3. Require fewer subjects. 4. Suitable for investigation of rare diseases. 5. No risk of subject. 6. Allows the study of several different etiological factors. 7. Risk factor can be identify 8. No attrition problem because do not require follow up. 9. Minimal ethical problem. DISADVANTAGES: 1. Problem of bias since it relies on past memory or past records. 2.Difficulty in selection of appropriate control group. 3. Can not measure incidence only RR. 4. Doesn’t distinguish between cause and associated factors. 5.Not suited for the evaluation of therapy or prophylaxis of disease.
  17. 17.  Prospective ,longitudinal, incidence and forward-looking study  Distinguishing features: a) The cohorts are identified prior to the appearance of the disease b) The study groups, so defined, are observed over a period of time to determine the frequency of disease among them c) Study proceeds from cause to effect. 17
  18. 18. time Study begins here Study population free of disease Factor present Factor absent disease no disease disease no disease present future
  19. 19. When there is good association between exposure and disease.  When exposure is rare, but the incidence of disease is high among exposed. When attrition of study population can be minimized. 19
  20. 20. 20 1. PROSPECTIVE COHORT STUDY 1. RETROSPECTIVE COHORT STUDY 2. A COMBINATION OF PROSPECTIVE AND RETROSPECTIVE COHORT STUDY.
  21. 21.  - Outcome has not yet occurred the time of investigation begins. Measure exposure and confounder variables Exposed Non-exposed Outcome Outcome Baseline time Study begins here
  22. 22.  Outcomes have all occurred before the start of the investigation. 22 Measure exposure and confounder variables Exposed Non-exposed Outcome Outcome Baseline time Study begins here
  23. 23. 1. Selecting of study subject 2. Obtaining data on exposure 3. Selection of comparison group 4. Follow up 5. analysis 23
  24. 24. 24 1. Selecting of study subject When exposure or cause of death is fairly frequent in the population i. Select group – Professional group ( doctors,nurses ) ii. Exposure group- High risk situation (eg.radiologist exposed to x-ray) Obtaining data on exposure Information about exposure may be obtained directly from:-
  25. 25. 25 Selection of comparison group a. Internal comparisons:-  Comparison groups are in built (eg. Smoking, bp etc.) within same cohort group. b. External comparisons:-  Eg. Smoker and non smoker, radiologists with opthalmologists. c. With General population:-  If none is available, mortality of exposed group with general population Follow up  Main problem  Procedures to obtain data for assessing the outcome are: a. Periodic medical checkup b. Reviewing hospital records c. Routine surveillance of death records d. Mailed questionnaries, telephone calls, periodic home visits.
  26. 26. Data are analysed in term of: a. Incidence rates of outcome among exposed and non-exposed: 26
  27. 27. 27 Data are analysed in term of: a. Incidence rates of outcome among exposed and non-exposed RISK FACTOR (TOBACCO) 5. ANALYSIS DEVELOPED ORAL CANCER DID NOT DEVELOP TOTAL PRESENT (CHEWERS) 45 (a) 9955 (c) 10000 (a + c) ABSENT (NON CHEWERS) 5 (b) 9995 (d) 10000 (b + d) Incidence rates: 1. Among tobacco chewers: = 45/10000 =4.5 per 1000 2.Among non chewers = 5/10000 = 0.5 per 1000
  28. 28. b. Estimation of risk A. Relative risk (RR): = incidence of disease among exposed incidence of disease among non-exposed = 4.5/0.5 = 9  It implies 9 times higher risk of development of oral carcinoma in tobacco chewers compared to non chewers. 28
  29. 29. B. Attributable risk (AR) Or “risk difference”: Incidence of disease rate among exposed- incidence among non exposed Incidence rate among exposed = 4.5 – 0.5 4.5 = 88.9%  Indicates to what extent the disease under study can be attributed to the exposure. 29
  30. 30. 1. Selection bias:  Non consent bias  Missing data bias 2. information bias:  Error in classification of individual  Diagnostic bias 3. Confounding bias :  Due to confounding factors 4. Post hoc bias: 30
  31. 31. 1. Incidence can be calculated 2. Several possible outcomes related to exposure can be studied simultaneously. 3. Provide a direct estimate of RR. 4. Dose response ratios can be calculated. 5. Since comparison groups are formed before disease develops, certain forms of bias can be minimized like misclassification of individual. 31
  32. 32. 1. Unsuitable for investigating uncommon disease. 2. Long time to complete study and obtain results. 3. Administrative problem –  Extensive record keeping 4.Expensive 5. Alter people behavior  Stop or decrease smoking  Loss of interest  migration 5. Ethical problem of varying important 32
  33. 33.  Start disease :effect cause  First approach to test hypothesis.  Involve fewer subject.  Yield result quickly.  Suitable for studying rare disease.  Gives RR only.  Start people: cause effect.  Reserved for testing precisely formulated hypothesis.  Involve larger number of subjects.  Results are delayed due to long follow up.  Unsuitable for studying for rare diseases.  Yield RR and AR.  Relatively inexpensive.  Expensive  Does not give information  Can give information more about diseases other than that than one disease. selected for the disease. 33 1 2 3 4 5 6 7 8
  34. 34. 34
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