Case reports and case series are descriptive studies that provide initial clues about new diseases or exposures. A case report describes the experience of a single patient, while a case series describes the experiences of multiple patients with similar characteristics. These study designs are useful for generating hypotheses, but have limitations due to lack of controls and small sample sizes. Ecological studies examine the relationship between disease rates and other population characteristics using aggregate data. They are useful for initial hypothesis generation but cannot prove causation. Cross-sectional studies measure exposure and outcome simultaneously in a population. They provide a snapshot of disease occurrence and can identify risk factors, but cannot determine temporal relationships.

1. SESSION 4: SOURCES AND USES OF
MORBIDITY AND MORTALITY DATA
BY GESONKO PAUL.
Time: 120 minutes

2. Learning objectives
At the end of this session participants are expected to be able to:
 Define morbidity, mortality, numbers, ratio, proportion and rate
 To outline the sources of morbidity and mortality data
 To explain uses of morbidity and mortality data
 To identify commonly used measures of morbidity
 To identify commonly used measures of mortality
 To identify commonly used measures of fertility.

3. Definition of Terms.
• Morbidity is any departure, subjective or objective, from a state of physiological
or psychological well-being.
• Mortality means death.
• Numbers are used to identify new cases of a disease in a certain population but
the limitation of using numbers is that they do not allow any comparison as the
populations they refer to are unknown.

4. • Ratio is the result of the division of two numbers: it is the basic measure of the
relative magnitude of two numbers.
For example, Sex ratio = Number of males
Number of females
• Proportion: Is the result of the division of a number by the population it refers to.
Unlike a ratio it is a fraction whose numerator is included in the denominator. A
proportion can be expressed as a fraction or as a percentage.

5. • Rate: Is a proportion that includes specification of time period in which there is
density relationship between the numerator and denominator with time being an
intrinsic part of the denominator.
• Rate refers to the instantaneous change (or potential for change) in one quantity
per unit change in another quantity, in a period of time. Rate indicates the
frequency of events (like births, deaths and illness), occurring in a population per
unit time.

6. Sources of Morbidity and Mortality Data.
 Morbidity statistics are available at all levels of health care. These are available
from various documents usually kept at health delivery centers:
o Admission register records
o Clinical records
o Discharge summaries
o Investigation laboratory records

7. o Investigation request forms
o Disease screening programmes
o Diseases and Drugs surveillance systems
 Sources of mortality statistics
o Death certificate
o Burial permits and other mortuary records
o Disease control program
o Census.

8. Uses of Morbidity and Mortality Data.
 Population forecasting
 Health planning
 Evaluation of health services and programs
 Policy development
 Evaluation of medical and nursing procedures.

9.  The following are the common measures of morbidity:
o Incidence rate
 The incidence rate indicates the number of new events in a certain population at
risk of such events during a specified period of time.
 The incidence rate (is also called incidence density) is instantaneous potential
for change in disease status per unit time relative to the size of the disease free
population. In many studies the study members will have different times of
observation.

10.  The denominator in incidence density accounts for the different times of
observation for each individual.
Incidence Rate/Density = Number of new cases
Total person time of observation

11. o Cumulative incidence
 In the case of a fixed cohort with hardly any withdrawals, the calculation of
cumulative incidence will be as follows:
Number of new cases
Number of individuals at risk at the beginning of the study

12. o Attack rate
 Is a form of incidence that measures the proportion of persons in a population
who experience an acute health event during a limited period (e.g., during an
outbreak). Is calculated as the number of new cases of a health problem during
an outbreak divided by the size of the population at the beginning of the
period, usually expressed as a percentage or per 1,000 or 100,000 population

13. oPrevalence rate
 Prevalence, sometimes referred to as prevalence rate, is the proportion of persons
in a population who have a particular disease or attribute at a specified point in
time or over a specified period of time.
= No of cases of a disease present in the population at a specified time x 100
No of persons in the population at that specified time

14. • To avoid dealing with fractions, prevalence rate is often expressed as per 100, per
1000, per 10,000, or per 100,000 depending on the frequency of disease.
• Occasionally reference is made to a term called period prevalence. This refers to
the proportion of individual who ever suffered from the given condition (old cases
+ new cases) during a period of time.

15. • Measures of Mortality.
 The following are the common measures of morbidity:
o Mortality Rate
 Crude Death Rate
• Is the total number of deaths in a defined population in a given time period divided
by the total population.
 Cause Specific Death Rate (CSDR):
= No of deaths from a specific cause during a calendar year x1000
No of persons in the midpoint of that period

16.  Age Specific Death Rate
= No. of deaths of a specific age group
No of persons in the population of that age
• Note: Infant Mortality Rate is a special age-specific death rate

17.  Infant mortality rate
• Number of live born infants under the age of 1 year that died during a
year/number of live births dying during a year, are actually born in preceding
year, the denominator is not exactly the population at risk, except when the
number of births remains constant.
Infant Mortality Rate (IMR) = Number of deaths of infants under 1 year in time period
Number of live births in time period

18.  Maternal Mortality Ratio:- Defined as the number of deaths
assigned to puerperal causes (i.e., childbearing) in a calendar year
divided by the number of live births in that year, the quotient
multiplied by 100,000.
 Case Fatality Rate = No of individuals dying during a specified period
of time after disease onset or diagnosis /No. of individuals with the
specified disease x100%

19.  Proportionate Mortality Rate:
• Defined as the number of deaths assigned to a specific cause in a calendar year,
divided by the total number of deaths in that year (usually expressed as a
percentage)
 Years of Potential Life Lost (YPLL)
• Is an estimate of the average years a person would have lived if he or she had
not died prematurely. It is a measure of premature mortality, or early death.
Example: If an individual dies at the age of 30 years, the person years of life lost
will be difference between the age at death and the life expectancy at that age in that
area.

20. Measures of Fertility.
• Common measures of fertility include:
 Crude Birth Rate (CBR)
 General Fertility Rate (GFR)
 Total Fertility Rate (TFR)
 Gross Reproductive Rate (GRR)

21. Crude Birth Rate
 CBR is called a ‘rate,’ but in practice it is a ratio.
 The rate is ‘crude’ because it does not take into account the risk of giving birth according
to age and sex differences.
Crude Birth Rate = Number of live births in a calendar year
Total population

22. • Fertility Rate/General Fertility Rate (GFR)
 General Fertility Rate (GFR) is more widely accepted than CBR, and is
considered a more conventional and modern measure of fertility. The
denominator is restricted to women at risk of child-bearing rather than the general
population. It is often known simply as ‘fertility rate’.
GFR is = Number of live births in a year
Mid-year population of women aged 15-49

23. • The Total Fertility Rate (TFR)
• TFR is the average number of children a woman would have during her
reproductive lifetime, given that current specific fertility rates would still be
applicable at that time. The total fertility rate is calculated from Age-Specific
Fertility Rates (ASFRs). We get the ASFRs when we divide the number of live
births by the number of women in each age interval. Unlike the CBR and GFR,
the calculation of TFR greatly depends on the age composition although its use is
independent of age distribution.

24. • Gross Reproductive Rate (GRR):- is the average number of daughters a woman
would have if she survived to at least age 50 and experienced the given female
ASFRs. GRR is similar to the TFR only that it considers female live births rather
than all births. This implies that ASFR for GRR is based on females.
• A GRR of 1 means that women are able to replace themselves, while a GRR of 2
means that the population is doubling itself: each woman is on average producing
two daughters.
 Like the TFR, GRR is a hypothetical measure.
 It is a period measure which does not take into account the effect of female
mortality either before age 15 or 15 to 50 years.

25. Key Points.
 Frequency measures include ratios, proportions, and rates
 These measures allow epidemiologists to infer risk among different groups,
detect groups at high risk, and develop hypotheses about causes

26. Session Evaluation.
 What are the measurements of morbidity?
 What are the commonest used measurements of mortality?
 What are sources of data?

27. References
• Bonita, R., Beaglehole, R., & Kjellstrom, T. (2006). Basic epidemiology (2nd
ed.). Geneva, Switzerland: WHO
• Field Epidemiology & Lab Training Program. (2008). Biostatistics workbook.
Atlanta, GA: CDC
• Makwaya et al. (1997). Lecture notes in biostatistics. MUCHS Department of
Epidemiology & Biostatistics.
• McCusker, J. (2001). Epidemiology in community health (Rural Health Series, No.
9).Nairobi, Kenya: AMREF.
• Rosner, B. (2006). Fundamentals of biostatistics (6th ed.). Belmont, CA: Thomson
Brookes/Cole

28. SESSION 05: EPIDEMIOLOGICAL STUDY DESIGNS.
By Gesonko Paul
Time: 60 minutes
Learning Tasks
• At the end of this session participants are expected to be able to:
 Define terms related to epidemiological studies
 Identify types of epidemiological studies
 Differentiate between observational and experimental study designs
 Explain sources of information for epidemiological studies.

29. • Definition of Terms Used in Epidemiological Studies (10 Minutes)
 A study design is a specific plan or protocol for conducting the study, which
allows the investigator to translate the conceptual hypothesis into an operational
one
 Observational study is the that does not involve any intervention (experimental
or otherwise) on the part of the investigator
 Experimental study is the study where by the investigators intervene in the
natural history by actively altering one of the variables and then making
inference on the relationship between the variables based on the outcomes

30.  Randomization is the process by which allocation of subjects to
treatment groups is done by chance, without the ability to predict
who is in what group
 A trial is an experiment
 A clinical trial is a controlled experiment having a clinical event as an
outcome measure, done in a clinical setting, and involving persons
having a specific disease or health condition
 A randomized clinical trial is a clinical trial in which participants are
randomly assigned to separate groups that compare different
treatments

31. Two main Types of Epidemiological Studies.
 Observational studies
o These allow nature to take its course that the investigator measures, but does not
intervene. The aim is to describe the occurrence of disease or disease related
phenomena in population.
o Investigators observe subjects and measure variables of interest without assigning any
intervention other collecting the information/data
o There are two types of observational studies:
 Descriptive Studies
 Analytical Studies

32. o Descriptive Studies
• These studies explain the what, who, when, and where of health events. Data collected
in descriptive studies is oriented to time, place and person. They are useful in hypothesis
generation. Descriptive studies are divided into:
 Cross sectional study
 Case Report and Case Series
 Ecological study

33. o Analytical Studies
• These studies are used to test the hypothesis generated from descriptive studies
• They aim at establishing the cause of a disease by looking for associations between
exposure to a risk factor and its associated occurrence of a disease. In general the
outcome of an analytical study is the conclusion that – a disease and its suspected cause
are or are not associated. Analytical studies are divided into:
Cohort (follow up) studies
Case control studies
• Note: Cross sectional study can also be analytical when the exposure precedes the
outcome.

34. Experimental studies
 These are the types of studies where a researcher has an active role. S/he manipulates
the independent variables and measure their effect on dependent variable.
 Experimental studies are grouped into:

35. o Randomized Controlled Trials
• Randomized Controlled Trials are epidemiological experiments in which
subjects in a population are randomly allocated into groups, usually called study
and control groups, to receive or not to receive an experimental preventive or
therapeutic procedure, maneuver, or intervention. Depending on the purpose or
study area, the trial can be classified as follows:
 Clinical trials
 Community intervention trials
 Field trials

36. o Quasi-experimental studies
• Unlike the true experimental designs (Randomized Controlled Trails), in quasi-
experimental studies, the subjects are not randomly assigned and hence, there is
some loss of control over extraneous variables that may impact the study.

37. Differences between Observational and Experimental Study Designs.
• Observational study designs differ from Experimental study designs in the
following way:
 In an observational study investigators observe subjects and measure variables of
interest without assigning treatments to the subjects. The treatment that each
subject receives is determined beyond the control of the investigator.
 In an experimental study investigators apply treatments to experimental units
(people, animals, plots of land, etc.) and then proceed to observe the effect of the
treatments on the experimental units.

38. Sources of Information/Data for Epidemiological Studies.
• Sources of data can either be primary or secondary.
 Primary data refers to the first hand data gathered by the researcher
herself/himself. This is data that has never been gathered before, whether in a
particular way, or at a certain period of time. Primary data can be collected in a
number of ways. However, the most common techniques are:
o Surveys
o Observations
o Interview
o Experiments

39.  Secondary sources mean data collected by someone else earlier. Secondary data
are the data collected by a party not related to the research study but collected
these data for some other purpose and at different time in the past. If the
researcher uses these data then these become secondary data for the current users.
Sources of secondary data are:
o government publications
o websites
o books
o journal articles
o internal records.

40. Key Points
 Two main types of epidemiological study designs are observation and
experimental study designs
 Descriptive studies generates hypothesis while analytical studies test the
hypothesis
 Sources of epidemiological data can either be primary or secondary

41. Evaluation.
 What are the types of descriptive study designs?
 What are the differences between observational and experimental study designs?

42. References
• Bonita, R., Beaglehole, R., & Kjellstrom, T. (2006). Basic epidemiology (2nd ed.).
Geneva, Switzerland: WHO
• Campbell, D. T., & Stanley, J. C. (2015). Experimental and quasi-experimental
designs for research. Ravenio Books
• Field Epidemiology & Lab Training Program. (2008). Biostatistics workbook.
Atlanta, GA: CDC.
• Gordis, L., 2014. Epidemiology Fifth., Philadelphia: Elsevier Inc.
• Greenberg, R. S., Daniels, S., Flanders, W., & Eley, J. (1993). Medical
epidemiology. East Norwalk, CT: Appleton Lange

43. SESSION 06: DESCRIPTIVE EPIDEMIOLOGICAL STUDIES.
By Gesonko Paul
Time: 120 minutes
Learning objectives
• At the end of this session participants are expected to be able to:
 Explain case report and case series study designs
 Outline the advantages and disadvantages of case report and case series studies
 Explain ecological/correlation study design
 Outline the advantages and disadvantages of ecological study design
 Explain cross section study design
 Outline the advantages and disadvantages of cross section study design.

44. Case Report and Case Series Study Designs.
• Case reports and case series describe the experience of a single patient or a group
of patients with a similar diagnosis. They represent the most basic type of study
design, in which researchers describe the experience of a single patient (case
report), or a group of people (case series).
• They provide first clues in the identification of new disease or adverse effects of
exposure, may lead to formulation of new hypothesis. Unusual cases may prompt
further investigations with rigorously study designs.

45.  Case report
o Description in detail of the experience of a single patient/an individual
o Has an unusual feature of disease, patient exposure history or unusual medical event
or side effect of the drug
o Case series
 Case report
o Collection of individual case reports, or a group of patients with similar characteristics
o Describe some interesting or intriguing observations occurred in small number of
patients

46. Advantages and Disadvantages of Case Report and Case Series Studies.
 Advantages
o Useful in formulating research hypotheses and suggestive of risk factors
o Important step in recognizing new diseases or risk factors
 Disadvantages
o Case report based on one individual, so the risk factor may occur by chance
o Lack the denominator data (the population from which the diseased subjects
arose)
o No comparison group
o Describe highly selected individuals who may not represent the general
population.

47. Ecological Study Design.
• Studies conducted in specific population groups (e.g. Muslims, Catholics, Jews
etc) having specific characteristics in a specified geographical area. Causes or risk
factors are studied with regard to the diseases and deaths occurred in a particular
population. Both are linked together and their co-occurrence (correlation) is
established in these studies for hypothesis formation.
 The variables include measurements taken at the group level e.g. infant mortality
rates of different countries.

48.  The units of analysis are populations or groups of people rather than
individuals. An example is the study of the relationship. Conclusions
of ecological studies may not apply to individuals; thus caution is
needed to avoid the ecological fallacy
 Ecological fallacy is an error in inference that occurs when
association observed between variables of a group level, is assumed
to exist at an individual level

49. Advantages and Disadvantages of Ecological Study.
 Advantages
o The study is conducted at group level, not at individual level, hence relatively
easy to do and quick also inexpensive
o Can be used as the first step in exploring the relationship between an exposure
and a disease
o It generates and support new hypotheses
o Ecological studies conducted over time on a specific geographical area are
more convenient to perform and form hypotheses rather than studying whole

50.  Disadvantages
o Ecologic bias/fallacy (Fail to reflect the effect at the biological/individual
level)
o Potential confounding factors cannot be readily controlled
o The lack of adequate data
o Non-differential misclassification within groups may lead to bias
o Usually rely on data collected for other purpose

51. Cross Section Study Design.
• It is a study design that examines the relationship between diseases (or other health-
related characteristics) and other variables of interest as they exist in a defined population
at one particular time.
• It is so called because a cross section of a community (frequently total population samples)
is studied at a particular point or period of time; and because both exposure and disease
outcome are determined simultaneously for each subject; it is as if we were viewing a
snapshot of the population at a certain point in time. Since it is used in finding the
prevalence of disease/events or conditions, it is sometimes called prevalence study.

52.  It can be of descriptive nature when studying the distribution of the disease/event or condition and cannot establish
cause – effect relationship
 It can be analytical type when sought to provide information about the presence and strength of association (cause –
effect relationship), in this case the exposure must precede the outcome
 It can be done at a single point of calendar time (point prevalence)
 It can be completed in few months or years (period prevalence)
 When the cross-sectional study is repeatedly done, can show a serial trend of disease/event or condition thus serves
the purpose of health and disease surveillance of the population.

53. Advantages and Disadvantages of Cross Sectional Study (05 Minutes)
 Advantages
o Comparatively cheap and quick
o Fairly simple to carry out and analyze
o Good for assessing prevalence and patterns of disease occurrence
o Useful for health care planning
o Investigate trends over time (serial)
o Often provides early clues for hypothesis generation

54.  Disadvantages
o They are not feasible for rare conditions
o Cannot establish temporal relationship (i.e. whether the exposure or presence of a
characteristic preceded the development of the disease or condition).
o They provide no direct estimate of risk or incidence
o They are prone to bias from selective survival
o Not useful for conditions which have a short duration (rare exposures and outcomes)
o Weak in investigating causality
o Potential for bias (non response)

55. Key Points.
• Descriptive studies are concerned with and designed only to describe the existing
distribution of variables without much regard to causal relationships or other
hypotheses.
 Descriptive studies can be used to measure trends in health indicators, generate
hypotheses, monitor public health policies, etc.

56. Evaluation.
 Differentiate between case report and case series study designs
 What is ecological fallacy?
 What are the disadvantages of cross sectional study?

57. References
• Bonita, R., Beaglehole, R., & Kjellstrom, T. (2006). Basic epidemiology (2nd
ed.). Geneva, Switzerland: WHO
• Campbell, D. T., & Stanley, J. C. (2015). Experimental and quasi-experimental
designs for research. Ravenio Books
• Field Epidemiology & Lab Training Program. (2008). Biostatistics workbook.
Atlanta, GA: CDC.
• Gordis, L., 2014. Epidemiology Fifth., Philadelphia: Elsevier Inc.
• Greenberg, R. S., Daniels, S., Flanders, W., & Eley, J. (1993). Medical
epidemiology. East