Download - Epidemiological Studies
![Page 1: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/1.jpg)
Basic steps in a survey –An 8-step Model
1
Formulating the research problem
2
Conceptualizing the research
design
3
Constructing an instrument for data collection
4
Selecting a sample
5
Writing the survey proposal
6
Collecting data
7
Processing data
8
Writing the survey report
![Page 2: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/2.jpg)
2
Conceptualizing the research
design
![Page 3: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/3.jpg)
![Page 4: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/4.jpg)
Observational Studies• A DESCRIPTIVE STUDY is limited to a
description of the occurrence of a disease in a population and is often the first step in an epidemiological investigation.
• An ANALYTICAL STUDY goes further by analyzing relationship between health status and other variables.
![Page 5: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/5.jpg)
![Page 6: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/6.jpg)
Some Examples of Descriptive Studies
• Case Report
• Case Series
• Surveys
Pure descriptive studies make noattempt to analyze the links betweenoutcome and exposure
![Page 7: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/7.jpg)
![Page 8: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/8.jpg)
Ecological Studies
• In an ecological study, the units of analysis are groups of people rather than individuals.
![Page 9: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/9.jpg)
![Page 10: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/10.jpg)
![Page 11: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/11.jpg)
Cross-sectional studies
• Because they measure the prevalence of a disease, they are sometimes also called ‘prevalence studies’.
• Exposure and effect (outcome) are measured at the same time.
![Page 12: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/12.jpg)
Uses of cross-sectional studies
• Prevalence
• Disease outbreaks
• Assessing healthcare needs of populations.
• Trends in diseases (repeated c/s studies)
• Risk factors for diseases (e.g., NCDs)
![Page 13: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/13.jpg)
Advantages and Disadvantages
• Easy and relatively inexpensive
• Less time consuming
• The temporal relationship between exposure and effect is difficult to establish.
![Page 14: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/14.jpg)
CASE-CONTROL STUDIES
![Page 15: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/15.jpg)
Key Features
• The exposure experience of a group of people who have the disease [CASES] is compared to the exposure experience of a similar (matched) group who do not have the disease [CONTROLS]
• Suitable for rare diseases or diseases with long latency periods
• The study proceeds backwards from “EFFECT to CAUSE’
![Page 16: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/16.jpg)
Basic Design
![Page 17: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/17.jpg)
• Selection of cases
• Selection of controls
• Measurement of exposure
• Analysis
Steps
![Page 18: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/18.jpg)
Selection of Cases
• Define a ‘case’HospitalGeneral population
‘Prevalent cases’???TemporalityDisease severity (Those who are exposed survive longer)
• Sources of cases
• ‘Incident’ cases
![Page 19: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/19.jpg)
Selection of Controls
• Free from disease
• As similar to the cases (matched) as possible, except for the absence of disease under study
• Sources of controls• General population
• Relatives/Friends/Neighbours
• Hospital controls
• How many controls per case?• One to four
![Page 20: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/20.jpg)
“MATCHING” is the process of selectingcontrols in a case-control study so that thecontrols are similar to the cases with regardto certain key characteristics-such as age, sexand race.
Group matching(Frequency matching)
Individual matching(Pair matching)
![Page 21: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/21.jpg)
Measurement of exposure
• Definition and criteria
• Done in the same way for both cases and controls
• How to measure exposures?
– Interviews/Questionnaires
– Past records (Hospital, Employment)
– Laboratory measurements
![Page 22: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/22.jpg)
Analysis
• Find exposure rate in cases
• Find exposure rate in controls
• Calculate “Odds Ratio”
![Page 23: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/23.jpg)
ExposureDisease
Yes No
Yes a b
No c d
Total a+c b+d
Odds Ratio=ad/bc
![Page 24: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/24.jpg)
Use of Oral Contraceptive
Thromboembolism
Yes No
Yes 26 10
No 32 106
Total 58 116
Exposure rate among cases=(26/58)x100=45%
Exposure rate among controls=(10/116)x100=9%
Odds ratio= (26x106)/(10x32)=2756/320=8.6
![Page 25: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/25.jpg)
People who use oral contraceptives have an8.6 times higher risk of developingthromboembolism compared to those whodo not use oral contraceptives
How to interpret the OR?
![Page 26: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/26.jpg)
Cigarette smokingLung cancer
Yes NoYes 85 160No 15 240
Total 100 400
Odds Ratio=(85x240)/(160x15)=8.5
![Page 27: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/27.jpg)
Q. An investigator selected 40 cases ofgastric carcinoma and an equal number ofcontrols matched for age, sex andsocioeconomic status. It was found thatamong cases 30 had an evidence of H pyloriinfection and among controls 15 had anevidence of H pylori infection. Is there anevidence of association between H pyloriinfection and gastric carcinoma?
5
![Page 28: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/28.jpg)
![Page 29: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/29.jpg)
COHORT STUDIES
![Page 30: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/30.jpg)
Key Features
• The study proceeds from “CAUSE to EFFECT”
• At the start of the study, all participants are free from disease.
• A group of people who are ‘exposed’ to some factor and another ‘not-exposed’ group are followed up for a certain time. The disease rate (incidence of disease) among ‘exposed’ is compared to the disease rate among the ‘not-exposed’ group.
![Page 31: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/31.jpg)
Basic Design
![Page 32: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/32.jpg)
• Selection of study subjects
• Obtaining data on exposure
• Follow-up
• Analysis
Steps
![Page 33: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/33.jpg)
Selection of Study Subjects
• General population
• Special groups
– Occupation group
– Professional group
• Radiologists
• Nurses
• Doctors
• Teachers, etc.
Free from study disease
![Page 34: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/34.jpg)
Obtaining data on exposure
Cohort
Not-exposed
Exposed1
Exposed Cohort Not-Exposed Cohort
2
Exposed CohortHigh Exposure
Medium Exposure
Low Exposure3
![Page 35: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/35.jpg)
Follow-up
• The follow-up procedures should be similar for both the exposed and the non-exposed groups.
• Clear, and valid definitions for disease status.
![Page 36: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/36.jpg)
Analysis
• Find disease incidence rate in exposed
• Find disease incidence rate in non-exposed
• Calculate “Relative Risk” (Risk Ratio)
![Page 37: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/37.jpg)
ExposureDisease
TotalYes No
Yes a b a+b
No c d c+d
Incidence rate among exposed=(a/a+b)*10x
Incidence rate among non-exposed=(c/c+d)*10x
Relative Risk = (a/a+b)
(c/c+d)
![Page 38: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/38.jpg)
Exposure to prolonged heat stress
Kidney diseaseTotal
Yes No
Yes 67 4458 4525
No 39 5443 5482
Incidence rate among exposed= 14.8
Incidence rate among non-exposed= 7.1
Relative Risk= 14.8/7.1 = 2.1
![Page 39: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/39.jpg)
People exposed to prolonged heat stresshave a 2.1 times higher risk of developingkidney disease compared to those who arenot exposed to prolonged heat stress
How to interpret the RR?
![Page 40: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/40.jpg)
THREE types of Cohort studies
• Prospective
• Retrospective (Historical)
• Retrospective-Prospective (Ambispective)
![Page 41: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/41.jpg)
Retrospective cohort studies
2014
1990
Data about oral contraceptive intake
in a cohort of women
How many of the women have
thromboembolicdisease, exposed
versus not-exposed
Study starts
![Page 42: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/42.jpg)
Case-control studies start with outcome/diseaseCohort studies start with exposure
Case-control studies compare exposure rates among cases and controlsCohort studies compare disease rates among exposed and not-exposed
![Page 43: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/43.jpg)
Case control CohortA
dva
nta
ges
Excellent way to study rare diseases with long latency
Better for studying rare exposures
Relatively quickProvides complete data on cases, stages
Relatively inexpensiveAllows study of more than one effect of exposure
Requires relatively few study subjects
Can calculate and compare rates in exposed and unexposed
Can often use existing records
Choice of factors available for study
Can study many possible causes of a disease
Quality control of data
![Page 44: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/44.jpg)
Case control CohortD
isad
van
tage
sRelies on recall or existing records about past exposure
Need to study large numbers
Difficult or impossible to validate data
May take many years
Control of extraneous factors incomplete
Circumstances may change during study
Difficult to select suitable comparison group
Expensive
Cannot calculate rates Control of extraneous factors may be incomplete
Cannot study mechanism of disease
Rarely possible to study mechanism of disease
![Page 45: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/45.jpg)
![Page 46: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/46.jpg)
![Page 47: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/47.jpg)
Experimental epidemiology
![Page 48: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/48.jpg)
• Intervention or experimentation involves attempting to change a variable in one or more groups of people.
• The effects of an intervention are measured by comparing the outcome in the experimental group with that in a control group.
• Ethical considerations
• Informed consent
![Page 49: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/49.jpg)
RANDOMIZED CONTROLLED TRIALS
![Page 50: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/50.jpg)
A randomized controlled trial is an
epidemiological experiment designed to study
the effects of a particular intervention, usually a
treatment for a specific disease (clinical trial).
Subjects in the study population are randomly
allocated to intervention and control groups, and
the results are assessed by comparing outcomes.
![Page 51: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/51.jpg)
“Randomization” is a statisticalprocedure wherein patients areallocated randomly to either theintervention group or the controlgroup. The purpose of randomization isto ensure that the patients in the twogroups are similar and hencecomparable.
![Page 52: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/52.jpg)
FIELD TRIALS
![Page 53: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/53.jpg)
Field trials, in contrast to clinical trials,
involve people who are healthy.
Data collection takes place “in the field,”
usually among people in the general
population
![Page 54: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/54.jpg)
![Page 55: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/55.jpg)
COMMUNITY TRIALS
![Page 56: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/56.jpg)
Treatment groups are communities, rather than individuals.
This is particularly appropriate for diseases that are influenced by social conditions, and for which prevention efforts target group behavior.
![Page 57: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/57.jpg)
ERRORSIN
EPIDEMIOLOGICAL STUDIES
![Page 58: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/58.jpg)
RANDOM ERROR
SYSTEMATIC ERROR
![Page 59: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/59.jpg)
RANDOM ERROR
• When a value of the sample measurement diverges-due to chance alone-from that of the true population value.
• Random error is “random”; therefore cannot be predicted.
![Page 60: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/60.jpg)
• THREE major sources of random error
– Individual biological variation
– Sampling error
• Increase the size of the sample
–Measurement error
• Stringent protocols
• Systematic quality control measures
![Page 61: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/61.jpg)
SYSTEMATIC ERROR
• Also called “BIAS”
• Systematic deviation of results or inferences from truth.
• Bias is defined as ‘any systematic error in the design, conduct or analysis of a study that results in a mistaken estimate of an exposures effect on the risk of disease’.
![Page 62: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/62.jpg)
TWO major types of bias
SELECTION BIAS
INFORMATION BIAS
![Page 63: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/63.jpg)
• A systematic difference between the characteristics of the people selected for a study and the characteristics of those who are not.
• Examples
– Bias due to non-response
– Exclusion bias
– Berkson’s bias (Berksonian bias)
SELECTION BIAS
![Page 64: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/64.jpg)
• Bias due to non-response
– Those who volunteer to take part in a study are different from those who don’t
• Exclusion bias
– Different eligibility criteria for cases and controls
•Berkson’s bias or Berksonian bias
![Page 65: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/65.jpg)
• Berkson’s bias (Berksonian bias)
–When both exposure and disease understudy affect selection. This occurs when thecombination of exposure and disease understudy increases the chance of admission tohospital, leading to a higher exposure rateamong hospital cases as compared tohospital controls. Thus it causes hospitalcases and controls in a case-control study tobe systematically different from oneanother.
![Page 66: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/66.jpg)
• A flaw in measuring exposure or outcome variables that results in different accuracy of information between comparison groups.
• This usually arises when the way of obtaining information about exposure or outcome is inadequate and hence may lead to incorrect information about exposure or outcome.
INFORMATION BIAS
![Page 67: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/67.jpg)
Examples of Information bias
• Misclassification bias
– Wrongly classify exposure or outcome
• Recall bias
– Differential recall about exposure among cases and controls. Case are more likely to remember exposure as compared to controls
• Reporting bias
– Cases may be reluctant to report exposure
![Page 68: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/68.jpg)
• Surveillance bias
– If exposed group is monitored more closely compared to the unexposed group
• Interviewer bias
– Knowledge about exposure status may consciously or subconsciously influence the interviewers measurements biased. Observer bias is a related bias.
• Single-blinding
• Double-blinding
• Triple blinding
![Page 69: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/69.jpg)
CONFOUNDING
![Page 70: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/70.jpg)
• In a study of whether factor A is a cause of
disease B, we say that a third factor, factor X is
a confounder if the following are true:
– Factor X is a known risk factor for disease B
– Factor X is associated with factor A, but is
not a result of factor A
![Page 71: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/71.jpg)
![Page 72: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/72.jpg)
![Page 73: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/73.jpg)
![Page 74: Epidemiological Studies](https://reader034.vdocuments.site/reader034/viewer/2022052700/55a205a21a28abdf648b46a0/html5/thumbnails/74.jpg)
• In designing and carrying out the study
– Matching
– Exclusion
• In the analysis of data
– Stratification
– Adjustment
Approaches to handling confounding