Epidemiology Made Easy Poornima Tiwari, Shashank Tiwari
Adenocarcinoma of vagina 159
Association and causation 21
Attack rate 50
Bias in epidemiological studies 155
Biological plausibility of the association 29
Blinding 221
double blinding 221
single blinding 221
triple blinding 221
Blonde hair and blue eyes 19
Case control study 138
advantages 157
easy of conduct 157
faster results 157
inexpensive due 157
more than one RFs 157
no attrition 157
no ethical problems 158
data analysis 151
determining exposure levels 151
disadvantages 158
information on exposure 147
sources of information on exposure status 148
matching 144
group matching 147
paired matching 147
selection of controls 147
selection of cases 140
sources of cases 141
sources of controls 142
Causative association 24
Chance variation 14
Chi-square (χ2) test 243
special situation 251
steps 245
Coherence of association 30, 42
Cohort studies 161
advantages 179
disadvantages 180
steps 166
data analysis 173
getting data on exposure to risk factor 168
identification of exposure cohort 166
regular follow-up 171
selection of control 169
types 163
combination of retrospective and prospective cohort design 165
concurrent 163
non-concurrent 163
prospective cohort design 165
Community trial 223
Concurrence by chance 18
Consistency of association 28
Coronary heart disease 10
Descriptive study 102
steps 102
Design of a randomized clinical trial 217
Design of a randomized controlled trial 233
steps 233
choosing the reference population 233
choosing the study or experimental population 233
data analysis to determine the effect 234
fullow-up 233
intervention to the members of treatment groups 233
random allocation to study and control group 233
Differences between case control and cohort study 183
Epidemic 119
types 119
common source epidemics 119
propagated epidemics 119
Epidemic curve 118
Epidemiological studies 87
experimental studies 92
observational studies 89
analytical studies 90
descriptive study 89
usual sequence 93
Epidemiological techniques 278
compose 278
Epidemiology 2, 213
aims 5
causation of disease 6
community diagnosis 5
data for 7
evaluation 7
implementation 7
planning 7
Evaluation 305
Experimental studies 226
advantages 226
limitations 226
Exposed cohort 186
Field trial 222
Formulation of hypothesis 113
Hemophilia 22
High risk group 36
Incidence 48
Indirect evidence of causation 25
Investigation of epidemic 287
steps 287
analysis of the initial data 290
confirmation of diagnosis 287
confirmation of epidemic existence 288
formulation of early or tentative hypotheses 291
rapid search for cases 289
study of etiological factors 290
total population 289
writing the report 291
Longitudinal studies 116
Male sex and hemophilia 19
Measurements 45, 46, 57
epidemiology 45
morbidity 46
mortality 57
limitations 57
Measuring disease frequency 47
Migration studies 124
limitation 126
critical point 126
long incubation period 126
Monitoring 298
air quality 298
functioning of a health facility 299
health program 298
intrauterine growth monitoring 298
nutritional status 299
water quality monitoring 298
Mortality rates 58
case fatality rate 66
crude death rate 58
group specific death rates 64
age and sex specific death rates 65
infant mortality rate 64
neonatal mortality rate 64
proportional mortality rate 67
cause 69
specific death rates 62
use of cause 63
standardised rates 71
indirect standardization 72
survival rate 70
Non-exposed cohort 186
Non-randomized trials 215
Null hypothesis 15
Oral contraceptives 160
P value 15
Paired ‘t’ test 255
Person distribution 109
Phases of vaccine trial 224
Place distribution 108
Population-attributable risk 176
Prevalence 51
period prevalence 52
point prevalence 51
Propagated epidemics 122
herd immunity 123
opportunities for contact 123
secondary attack rate 124
Qualitative data 10
Quantitative data 11
Randomization 215
Randomized controlled field trial 222
steps 222
Relative risk 33, 38
Reye's syndrome 159
Secondary attack rate 50
Significance of incidence and prevalence 53
Smoking and lung carcinoma 20
Sources of bias 156
case control studies 156
Berkesonian bias 156
bias due to a confounding factor 156
bias in obtaining information 156
recall bias 156
selection bias 156
experimental studies 220
evaluation bias 220
observer bias 220
subject variation 220
Specificity of the association 27, 41
Standard error of difference 240
Standardization 265
direct 265
indirect 266
community diagnosis 279
determining risk to individual 281
evaluation of new therapy 281
filling in the gaps in the natural history of a disease 282
identification of syndromes 282
planning and evaluation 280
searching for the causes of a disease 282
Strength of association 26
Surveillance 299
types 301
active surveillance 301
passive surveillance 302
sentinel surveillance 302
Temporal association 25
Tests of significance 240
qualitative data 240
quantitative data 252
Thalidomide tragedy 160
Thromboembolic phenomenon 160
Time distribution 107
Unpaired ‘t’ test 252
Z test 254
Chapter Notes

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Principles of Epidemiology2Chapter 1

Epidemiology is a basic skill for every healthcare personnel. Moreover, it is very interesting to learn and apply the skills of epidemiology.
This book will introduce the topic of epidemiology in a very simple, logical and interesting manner and is like reading a story book.
As the student will read through the pages, the foundations of Epidemiology will gradually build up and get strengthened, so that one doesn't have to cram up anything.
For the purpose of a quick revision before an exam, one can go through the ‘Review’ given at the end of each chapter.
So let's begin……………
Epi= Among, Demos= People, Logos= Study
Epidemiology is concerned with the study of disease in populations of people.
An epidemiologist strives to determine if there has been an increase or decrease of the disease over the years, whether one geographical region has a higher rate of the disease compared to another and also if the characteristics of persons diagnosed with the disease (under study) are different from persons who are free of the same disease.
Note that epidemiology is concerned with the study of distribution and causation not only of ‘disease’ but also of ‘health’ and ‘health related events’, as also ‘Facts of Life’ (e.g. Health facility use by people, impact of health services etc.)
It is more concerned with well being of the society as a whole rather than that of an individual.3
Definition (as given by John M. Last, in 1988)
“The study of distribution and determinants of health related states or events in specified populations and the application of this study to control the health problems.”
Note that the important terms in the above definition have been highlighted. Let us consider each one separately.
1. Frequency: Frequency measures of the amount of disease, disability etc. The information is expressed in the form of ‘Rates’ and ‘Ratios’.
Only when you calculate the rate/ratio in different populations, can you compare the disease amount among them.
By knowing the rates of disease in different populations or subgroups of the same population you can have a clue to the etiology, e.g. it was observed that the frequency of AIDS was more in the groups of commercial sex workers, their customers, I/V drug addicts, etc. This pointed finger at an infectious etiology and finally HIV was isolated.
2. Distribution: Disease occurs in patterns, i.e. time pattern, place pattern and person pattern.
Let us take the example of diabetes. By measuring the frequency in the Indian population we came to know that it is more common in the urban areas than in the rural area (Place distribution). Also we found out that it is more prevalent in the affluent society as compared to slums in the urban area (Person distribution). Measurements made in the same population year after year has brought out that the prevalence has been increasing. So we know that the disease frequency was lesser in the past and is rising with time (Time distribution).4
Recognition of these patterns is known as ‘studying the distribution of the disease’.
This is also known as describing the disease in Time, Place and Person. In the above example, it gives us a clue to the etiology (Hypothesis) that it has something to do with the modern lifestyle.
A study which thus describes the disease is called a ‘Descriptive study’.
3. Determinants of the disease: How are the affected eople different from the non-affected? What are the special characteristics of the population which has a higher rate of the disease? These characteristics may be the cause of the disease in question or they may increase the chances of exposure to the cause (in which case these characteristics can be called as ‘Risk Factors’ as they increase the risk of getting exposed to the cause of the disease).
E.g. in the case of poliomyelitis, the disease was found to have a higher incidence in under-5 population, population living in unhygienic conditions, with low coverage of routine immunization, high rate of illiteracy and more.
It was found to be having a lower incidence in the adults, the educated, population with high immunization coverage, etc.
These identified characteristics of the population with a higher incidence are the determinants of polio, i.e. these determine the distribution of polio.
4. Specified population: The distribution and the determinants discussed above apply strictly to the population which has been studied. They may or may not be fit for extrapolation to other populations.5
5. Application: The ultimate aim of epidemiology is to eliminate or reduce the health problem under study. Once we know of the determinants of the disease, we can modify them to achieve this objective, e.g. in the above case of poliomyelitis, if levels of hygiene are improved, safe water and sanitation provided, immunization coverage improved in the affected population, the incidence of the disease will come down.
Unless we use the knowledge gathered from the epidemiological studies to reduce human suffering, the knowledge remains incomplete.
Remember that the above discussion relates not only to diseases but also to disability, death, health and health-related events. But for convenience we will mostly use the term ‘disease’ henceforth.
These are enumerated below and will be explained one by one.
  1. Describing the disease and health related events in a specific population.
  2. Understanding causation of disease
  3. To provide data for planning and implementation and evaluation of health services
Describing the Disease and Health Related Events in a Specific Population (Community Diagnosis)
To know about the local disease patterns—
Suppose you are practicing in your country but suddenly you are asked to provide health care in another country.6
Wouldn't you like to know about the type of diseases prevalent there and what is each disease's burden in the population, before you take your position there? You would like to check the various sources of such information. How has the information which is there, generated? How do we know that PEM is more common in India and Sarcoidosis in the West?
This type of information is generated out of surveys done on that population. This survey is a type of Epidemiological Study.
Also, finding out the disease patterns, risk factors, environmental hygiene, factors responsible for the prevalence of various diseases, etc. in a particular community is known as ‘Community Diagnosis’. It means that you are diagnosing the community regarding health related factors.
It means to describe the distribution and magnitude of health and disease problems in specified human populations.
Understanding Causation of Disease
By comparing the incidence/prevalence of a particular disease in different groups of people we may get a clue to the etiology of the disease.
We may note that the disease is more common in a particular group of people with a particular behavior, a particular exposure or a particular characteristic which is absent or less seen in other groups which are having a lower incidence of the disease.
This gives us a clue that the particular characteristic is the cause or is somehow related to the cause of the disease under study.7
To Provide Data for Planning, Implementation and Evaluation of Health Services
Suppose you and two of your friends are given the responsibility of improving the health of villagers in one remote village each. Your performance will be judged against each other and the best performer will be given some incentive.
What is the next logical step? How can the authorities judge your performance and even more difficult, compare the three of you?
First of all the three of you would like to do a basic survey to provide yourself with basic information regarding the prevalent diseases in the community and other health related factors, so that you will establish the clinic equipped to handle mostly these. No clinic can be expected to keep facilities for ALL the diseases affecting mankind.
Thus, based on the local disease pattern you will PLAN your clinic and the run it to achieve your goal.
After some time of running the clinic (Let's say 6 months), you would like to do another survey to know if your clinic has made any impact. Thus you would look for any reduction in the disease load (incidence/prevalence).
If you have made an impact, you would happily continue your clinic till the time you are evaluated by the authorities.
If the desired reduction is not there, then you will look for ways to improve. You might have to make some changes to your clinic and then after some time repeat the survey to see if you are on the right track now.
Even the authorities will evaluate you in a similar manner. They will conduct yet another survey to see the incidence and prevalence and compare the results with your first survey.8
This will enable them to judge how much reduction in the disease load, have you been able to achieve as compared to your friends.
The above is an example of how an epidemiological survey is done to provide data based on which the health service (your clinic) is planned and established.
This data also served as a baseline to compare the results of the next survey. This next survey which tells you about your performance is called as ‘evaluation’ of the health service (which is your clinic in this example). Based on the evaluation, new changes are introduced into the health services followed by re-evaluation later on. This cycle of planning and evaluation is thus continuous, leading to improvement in the health of the community.
Based on this kind of epidemiological information, we know the health problems of our country and therefore launched specific control programs for tackling these.
That's why different countries have national/state programs for different diseases depending on the disease pattern in individual countries. Even different states of a country can have different programs depending on the disease prevalence in each state.
After the above discussion one can understand that the ultimate goal of epidemiology is to: (i) Reduce the health problems, and (ii) Promote well-being and health through a series of continuous planning and evaluation.