1. Epidemiologic disease measurements

2. Measures of disease occurrence
These are measurements of the disease frequency, magnitude, and amount of disease in populations
Diseases are measured by 4 quantitative descriptors
-Numbers
-Ratios
-Proportions
-Rates

3. Quantitative measurements
Ratio: Is the expression of the relationship between two quantities expressed as X:Y, where X is the count of one item and Y a count of another. Ratio is used to show quantity of disease in a population as: Cases divided by population. It is the basic measure of the relative magnitude of two numbers. E.g., sex ratio = No of males/no of females. Two types of ratios commonly used are; 1.Dimension ratio, example: The # of hospital beds per 1000 population is a ratio possessing dimension. 2.dimensionless ratio, example: Male to female ratio is dimensionless.
Proportion: Is the expression of the relationship of one part to the whole. It is a dimensionless fraction quantity whose numerator is included in the denominator. It can be expressed as a decimal (0.5), or as a percentage (50%).

4. When we call a measure a Ratio, we mean a non-proportional ratio.
When we call a measure a proportion, we mean a proportional ratio that doesn’t measure an event overtime
When we call a rate, we mean a proportional ratio that does measure an event in a population overtime.

5. Quantitative measurements cont.
Since the base always equals 100, a proportion is expressed as a percent. i.e. X\YxK. Where X is a count of one item in the population. K, is the base number. The value of X and Y are determined during the same time interval, e.g. 15 boys and 5 girls, the proportion of female is 5/20x100=25%
The comparisons are achieved by looking at proportions, for instance; The number of new TB cases in Morogoro region is 870, and if the population is 870,000. The proportion of new TB cases is 870/870,000= 0.001, or 0.10% or 0.1/1000. This proportion has to be compared with the # of new TB cases from a different region to decide on which region has more new Tb cases.

6. Quantitative measurements cont
Rate: Is the expression of the probability of occurrence of a particular event in a population during a specified period of time. It indicates the frequency of events(births, illness, deaths) occurring in a population per unit of time. The scale of the measure(percent/1000 or per 100,000) should always be stated, as well as the unit of time (per year, per month, etc). Mathematically X/YxK, where X is the number of events or cases, Y is the total number of population at risk, and K is a round number or base chosen to express the rate as a number greater than one. The denominator must be clearly defined and the numerator completely counted.

7. Quantitative measurements cont
The values of X and Y are determined during the same time interval.
Rate is commonly used in epidemiology because it most clearly expresses probability or risk of disease or other events in a defined population over a specified period of time.

8. Natality Rates Natality rates measures the rate of birth.
Crude birth rate: is expressed as:
Total of live births during given time interval divided by estimated mid-interval population.
It is expressed per 1000 population.
Fertility rate: is expressed as:
Total of live births during given time interval divided by female aged 15-45years at mi-interval.

9. Natality rate cont.
Fertility rate is expresses per 1000 population.

10. Morbidity rates
Morbidity rate measures the rate of illness, it is a rate used to quantify the magnitude of diseases.
(a) Incidence rate: Is expressed as; # of new cases of specific disease during a given time interval over the estimated mid-interval population at risk times K the base.
Note: Incidence rate is a measure of the rate at which healthy people develop disease during a specific time period, it is a statement of probability, since incidence rates are affected by any factor that affects the development of a disease, they can be used to detect etiological factors and monitoring trends, also useful in setting targets for improving health and measuring effectiveness of treatments also used in the study of the efficacy of an intervention. Incidence is a better measure than the prevalence

11. Morbidity rates cont.
Incidence rate, used when examining an outbreak of a health problem
b) Prevalence rate: Is expressed as; #of new and old cases, divided by mid-interval population at risk times K, k= could be per 100, per 1,000, per 100,000 depending on the frequency of the disease.
Number of current cases, i.e., people contracted the disease before time period began and still have the disease plus new cases during specified time period is equal to prevalence.

12. Morbidity rates cont.
Cumulative Incidence, refers to proportion of individuals who become diseased during a specified time period. Time period can be a calendar year, 6 months, 3 years, 5 years, etc.
Prevalence rate contains all known cases in the numerator, it is used primarily to measure the amount of illness in a community and thus, can be used to determine the health care needs of a particular community.
Prevalence rates are influenced by both the incidence of disease and by the duration of illness. Therefore, P= I x D.

13. Morbidity rates cont. Types of prevalence:
Point prevalence, is the # of cases that exist at a given point in time. E.g., 1st June 1990.
Life time prevalence, is the proportion of the population that has a history of a given disorder at some point in time.
Period prevalence, refers to a given time interval.

14. Relationship of incidence& prevalence
Relationship between Incidence and Prevalence
-If Incidence is low, but duration is long(chronic), prevalence will be large in relation to incidence.
-If incidence is low due to short duration caused by number of reasons, recovery, Improved reporting, diagnostic facilities, migration, deaths, etc; prevalence will be small in relation to incidence.
Prevalence provides information for, planning of drug requirement, needed personnel, equipment needed, e.g., beds, laboratory facilities etc. also information for monitoring of disease control programs and for tracking changes in disease pattern.

15. Special types of rates
Attack rate: expressed in percentage as; #of new cases of specific disease at specific time Interval divided by total population at risk during same time interval. Normally expressed as a percentage.
Mortality rate: #of deaths in a population at risk during a specified time period. It is expressed as the total number of deaths during given time interval divided by estimated mid-interval population, multiplied by K. K =1000.

16. Special Rates cont.
Maternal mortality rate: expressed as number of maternal deaths during given time interval divided by #of child-bearing women aged yrs during the same time interval multiplied by K. K=100,000 live births.
Case-fatality rate: expressed as number of deaths assigned to specific disease divided by number of cases of the disease multiplied by K. K=100.

17. Special rates cont.
Crude death rate: expressed as total #of deaths during a given time-interval divided by estimated mid-interval population multiplied by K. k=1000
Specific rates. Are calculated for various segments of the population. The rates are difficult to calculate because more information about the demographic composition of the population must be known than with other rates.

18. Special rates
Cause-specific death rate: expressed as number of deaths assigned to specific cause during given time interval divided by estimated mid-interval population multiplied by K. k=100,000 population.
Adjusted rates: Equalize the differences in the population at risk, so the rates are comparable.

19. Special rates
However, adjusted rates are difficult to calculate because the demographic composition of the population must be known.
Age-adjusted rates are most frequently used to compare mortality in different populations. Are computed either by direct or indirect method.

20. Measures of Association
Are calculations used to measure disease frequency relative to other factors.
Are also indications of how more or less likely one is to develop disease as compared to another.
These are:
Risk ratios, Odds ratios, Rate ratios, Relative risk ,etc.

21. Measures of association cont.
A risk ratio greater than 1, implies that the risk of disease is higher in the exposed group than in unexposed group, while a risk ratio less than 1 occurs when the risk is lower in the exposed group, suggesting that the exposure may be protective.
A risk ratio of 1 occurs when risks are the same in two groups. No association between exposure and the disease.

22. Measures of association cont.
Risk ratio is expressed as risk in exposed group divided by risk in unexposed group
Rate ratio is expressed as rate in exposed group divided by rate in unexposed group
Odds ratio Is a measure of effect size that describes the strength of association or a non- independence between 2 binary data values. In statistics, the odds of an event occurring is the probability of the event divided by the probability of an event not occurring. Think of how likely will the event occur compared to how likely will the event not occur?

23. Measures of association cont.
The risk and odds ratios can both be estimated from longitudinal and from cross-sectional studies with complete follow-up, for example:
A vaccine which is believed that offers full protection to some individuals but none to others, rather than partial protection to all.

24. Measures of association
Odds Ratios
An odds ratio of 1 occurs when the odds, and hence the proportions, are the same in two groups and is equivalent to no association between the exposure and the disease.
The odds ratio is always further away from 1 than the corresponding risk(or prevalence) ratio. Thus:

25. Measurement of association cont.
If Risk Ratio greater than 1 then Odds Ratio is greater than risk ratio.
If Risk Ratio less than 1 then Odds ratio is less than risk ratio
The Odds ratio for exposure; i.e., the odds of disease in the exposed compared to the odds in unexposed group, equals the odds ratio for disease, that is the odds of exposure in the disease compared to the odds in healthy group.

26. Measurement of association cont.
Attributable Risk: The risk ratio that assesses how much more likely, for example, a smoker is likely to develop lung cancer than non-smoker, but it gives no indication of the magnitude of the excess risk in absolute terms. It is measured by Attributable risk.
Attributable risk(AR)= risk among exposed- risk among unexposed=risk difference

27. Measures of association
Relative Risk
Measure of association between incidence of disease and factor being investigated
Ratio of incidence rate for persons exposed to incidence rate for those not exposed.
Estimate of magnitude of association between
exposure and disease.

28. Infant Mortality rates (IMR)
It is defined as the death of babies under 1 year per 1000 live births. The IMR data is acquired through vital statistics, household survey, health service records and census.
Expressed as number of deaths of babies under 1 year divided by total number of live births during the year multiplied by K constant.

29. Maternal mortality ratio
Expressed as X/Y; Number of maternal deaths related to pregnancy divided by the number of live births in a year multiplied by K.
K=100,000 live births.

30. Sources of routine morbidity statistics.
Admission registers, records
Clinical records
Discharge summaries
Investigation laboratory records
Investigation request forms
Burial permits and other mortuary records.
Registers for birth malformations, accidents, toxicity and other infectious diseases can be established if are not currently in use.

31. Records of special health programs
Essential Drug Programs (EDP)
The EDP maintains the records of drug supplied, how long they last, how many patients were treated, and their respective health problems. It is a good indicator of morbidity pattern at the community level.
2. MCH(RCH) . The special cards are available for the mother and the child, the state of health, type of treatment procedure for each mother and child is recorded regularly.

32. records of Special health programs cont.
3. Expanded program on Immunization.
Generate and maintain information on all
vaccinations done, type of vaccination and their batch numbers. Provide information mainly on vaccination coverage.
4. Mental health
Maintains records of people who are mentally ill.

33. Cont. 5.Oral and dental health.
Maintains records of people found with or developing oral or dental health problems.
Other record sources
Health requirements. Recruitment to new employment require someone to undergo medical examination prior to being recruited.
Disease screening programs. Screening of diseases in communities as a basis of evaluation and disease control programs. Census also collects information on people who are sick, disabled or compromised quality of living.

34. Determining Incidence
Consider;
The method of classifying individuals as having disease or not must be clearly stated, i.e. whether it was by detailed examination, screening, or examination of routine sources of health statistics.
The date of onset, thus identifying incidence cases from prevalence
Denominator must be clearly defined and enumerated

35. Period of observation must be long enough for the numerator to stabilize
The denominator of the mid-year population which should be enumerated during the period of the study.

36. Measures of mortality of the fetus or infant in use
Measure time interval Denominator IMR birth-1yr Live births Perinatal MR 28wks gest-7dys Live births +stillbirths Neonatal MR birth-28dys live births PostNMR 28dys-1yr live births ChildhoodMR 1yr-5yrs live births StillbirthRate 28wks gest.-birth live births+stillbirths Early NMR birth-7days live births Late NMR 7dys-28dys live births Fetal deathRate conception-birth live births+stillbirths

37. Other measures of health.
Secondary attack rate, expressed as:
#of exposed people developing the disease of interest within the range of the incubation period divided by the total number of susceptibles exposed to the primary case multiplied by 1000.
Case fatality Rate is expressed as:
# of deaths assigned to a specific disease divided by the number of cases of the disease. It is frequently expressed as a percentage, it predicts the risk of dying if the disease is contracted.