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GENERAL OBJECTIVE:
To know the basic concepts and strategies of Epidemiology
SPECIFIC OBJECTIVES:At the end of the lecture, the student
must know the:
1. Definition of Epidemiology2. Concepts of Epidemiology
SPECIFIC OBJECTIVES:
3. Principles of Epidemiology4. Aims and purposes of Epidemiology5. Relationship of the other branches of Science
to Epidemiology6. Ecologic Concepts of diseases7. Types of Disease Agents8. Attributes of the Human host9. Attributes of the Environment10. Relationship among Agent, Host and
Environment
SPECIFIC OBJECTIVES:
11. General and specific defense mechanisms of the body
12. Mode of Disease Transmission13. Nature of communicable diseases14. Classification of disease in human15. The stages of the natural history of
disease16. Levels of disease prevention17. Steps in scientific inquiry
SPECIFIC OBJECTIVES:
18. Strategies of Epidemiology19. Methods of hypothesis formulation20. Description of the different study designs21. Concept of Descriptive Epidemiology in
relation to Person, Place and time22. Sources of Epidemiological data23. Description of disease frequency as to
counts, ratios, proportions and rates
DEFINITION OF EPIDEMIOLOGY
- The study of the distribution and determinants of disease frequency in man
- From the Greek word epi meaning ON or UPON and demos which means POPULATION
MODERN DEFINITION OF EPIDEMIOLOGY
- A branch of medical science concerned with the relationships of the various factors and conditions which influence the frequencies and distribute health, infectious disease process, defect, disability or death as it occurs in aggregations or groups of individuals in a population
CONCEPT OF EPIDEMIOLOGY
1. Concept of the “shrinking world“- The world is becoming smaller not
because of physical size but due to:
a). Marked improvement in transportationb). Better communication and
transportation makes easier exchange of information about diseases
CONCEPT OF EPIDEMIOLOGY
2. Changes in nature of disease problema). Socio-economic conditionsb). Improvement and progress of medical
science 3. Much have been done to fight
communicable diseasesa). Better facilities for early Dx & Txb). Better control measures- vaccines
CONCEPT OF EPIDEMIOLOGY
4. Epidemics are now a rarity - study more the endemic behavior of
disease rather than epidemic occurrences
5. Realization of the different applications of methods in Public Health both for infectious and non-infectious diseases
PRINCIPLES OF EPIDEMILOGY
1. Exact observation (strict, accurate, precise)
2. Correct interpretation ( free from error)
3. Rational instruction (by expert knowledge and technical skills)
AIMS & PURPOSES OF EPIDEMIOLOGY1. To analyze carefully the roles and
interactions of agents, host and environmental factors in the natural history of disease to discover gaps in the knowledge and contribute to preventive medicine
AIMS & PURPOSES OF EPIDEMIOLOGY 2. To describe and analyze disease
occurrence and distribution according to such variables as age, sex, occupation, temporal frequency, periodic fluctuation, long term trends and geographic distribution to make community diagnosis making an estimate of morbidity and mortality risks
AIMS & PURPOSES OF EPIDEMIOLOGY3. To aid in filling gaps in knowledge
about the causes of disease processes by observing the range, amplitude and group behavior of clinical syndromes in populations
AIMS & PURPOSES OF EPIDEMIOLOGY4. To study immediate and special
problems in the field of health. This would include the study of new diseases, endemic disease problems, epidemics and administrative problems
5. To measure the effectiveness of preventive and control programs in health
RELATIONSHIP OF OTHER BRANCHES OF SCIENCE TO EPIDEMIOLOGY
1. Clinical Medicine to obtain details of clinical diagnosis for epidemiological descriptions of the distribution of diseases or disorders
2. From Bacteriology, Entomology, Parasitology, Zoology for information about nature and characteristics, reservoirs, sources and modes of transmission of living agents of disease
RELATIONSHIP OF OTHER BRANCHES OF SCIENCE TO EPIDEMIOLOGY
3. From Demography on the composition and characteristics of the population
4. From Chemistry, Physics, Nutrition and Industrial Medicine for information about health hazards
5. From Anthropology and Sociology for information about habits, customs, cultural and social characteristics of the population
RELATIONSHIP OF OTHER BRANCHES OF SCIENCE TO EPIDEMIOLOGY
6. Genetics and Psychology for characteristics of persons which may influence occurrence of diseases
7. Meteorology for data on weather and climate in relation to occurrence of diseases
8. Biostatistics – analytic and descriptive9. Vital statistics containing data on
natality, morbidity and mortality
ECOLOGIC CONCEPT OF DISEASEThe ecologic concept of disease is based
on the 3 premises of the biologic laws:1. The disease results from an imbalance
between disease agents and man2. That the nature and extent of
imbalance depend on the nature and characteristics of the host and the agent
ECOLOGIC CONCEPT OF DISEASE3. That the characteristics of agent
and host and their interaction are directly related to and depend largely on the nature of physical, social, economic and biologic environment
DISEASE AGENT
- May be defined as an element, a substance or a force, either animate or inanimate, the presence or absence of which, may following effective contact with susceptible human host under proper environmental conditions, serve as stimulus to initiate or perpetuate a disease process
TYPES OF DISEASE AGENTS
1. Biologic2. Nutrients3. Chemical4. Physical5. Mechanical6. Psychological
ETIOLOGIC FACTORS OF AGENTS OF DISEASE1. Biologic – living plants and animals, parasites2. Nutritive agents – cholesterol, vitamins,
proteins, fats3. Chemical – poisons, allergens, metals, drugs4. Physical – climate, season, weather,
radiation5. Mechanical – machines, cars, engines6. Psychological/ social – mental stress
CHARACTERISTICS OF THE LIVING AND NON-LIVING AGENTS
1. Inherent nature and characteristics in morphologic agents
2. Viability and resistance3. Infectivity and pathogenicity to man4. Reservoirs and sources of infection5. Vehicles and condition of
dissemination
CHARACTERISTICS OF LIVING AND NON-LIVING AGENTS
LIVING AGENTS
Morphology
Mortality
Physiology
Reproduction
Metabolism
Nutrition
Toxic products
NON-LIVING AGENTSDustCrystalsSolutionsInsoluble substancesCorrosive & non-
corrosive substances
THE INCUBATION OF NON-INFECTIOUS AGENTS DEPENDS ON:
1. Strength of stimulus2. Repeated increments of the agent3. Period of exposure4. Tissue involved
DISEASE AGENTS MAY:
1. Fail to lodge in the body, being expelled or dislodged by the non-specific outer defenses of the host
2. In-apparent infection – lodge, multiply and yet produce no discernible action on the host
3. Sub-clinical cases- reactions may be so mild to escape detection
Whatever the original reaction is, the end result may be:
1. Elimination of the agent leading to complete recovery
2. Clinical recovery without elimination of the agent (carrier state)
3. Death, defect or disability of the host
ATTRIBUTES OF THE HUMAN HOST1. IMMUNE RESPONSE2. HABITS & CUSTOMS3. AGE, SEX & RACE4. MARITAL FACTORS5. OCCUPATIONAL FACTORS6. OTHERS: Constitution, heredity,
psychological factors
GENERAL & SPECIFIC DEFENSE MECHANISMS
Resistance: the sum total of the defense mechanism of the host
1. Non-specific resistancea). Skin & mucous membranesb). Phagocytosisc). Reticuloendothelial systemd). Hormonese). Different reflexes or physiologic
mechanisms
Different reflexes or physiologic mechanisms:
Winking reflex Tears Sneezing Coughing vomiting
Diarrhea Urination Genito-urinary
discharge Sweating
Specific Resistance or immunity of the human host- Possession of antibodies for a specific
disease1. Passive immunity – either attained
by maternal transfer or by inoculation of specific antibodies; brief duration of immunity
2. Active immunity – natural immunity and artificial immunity; vaccines
3. Latent immunity – developed due to the giving of small doses for a long period of time
The human host may be :
1. Susceptible – a person not possessing resistance against a particular pathogenic agent
2. Immune – a person who possesses antibodies that are specific and protective
CHARACTERISTICS OF AGENTS IN RELATION TO HOST:
1. INFECTIVITY –the ability of an agent to invade and adapt itself to the human host
2. PATHOGENICITY – the measure of the ability of an agent when lodged in the body to set up either a local or general tissue reaction
3. VIRULENCE – the measure of the severity of the reaction produced
4. ANTIGENIC PROPERTIES – the activity to stimulate the host to produce agglutinins, opsonins, presipitins, antitoxins, complement fixing, neutralizing and sensitizing antibodies
ATTRIBUTES OF THE ENVIRONMENT:
- Influence existence of the agent- Influence exposure or susceptibility to
agenta). Physical environment –geology,
climateb). Biologic environment – human
population density, crowding index - flora and fauna – sources of food,
vertebrate hosts, arthropod vectorsc). Socio-economic – occupation,
urbanization, pollution, disruption (wars, floods, earthquakes)
RELATIONSHIP AMONG AGENT, HOST & ENVIRONMENT
A H
E
MODE OF DISEASE TRANSMISSION
Infectious diseases can spread through human population by:
1. Common vehicle epidemics - the etiologic agent is transmitted by water, food, air or inoculation
a). Vehicle – water, food, soilb). Vector – snails, mosquitoes2. Epidemics propagated by serial transfer
from host to hosta). Airborne droplets, anal, oral, genital
routeb). Infected blood or sera
NATURE OF COMMUNICABLE DISEASESCOMMUNICABLE DISEASES - an illness
due to specific infectious agents or its toxic products
CONTAGIOUS DISEASES – implies transmission of the disease through direct contact
Periodicity of Communicable Disease Occurrence:
1. EPIDEMIC2. Endemic – a disease constantly
occurring in a geographical area3. Pandemic – epidemic occurring within
more than one country or territory4. Sporadic – occasional or infrequent
occurrence of a disease5. Epizootic or enzootic – diseases
occurring in animals6. Zoonotic – disease of animals
transmissible to man
Biologically, disease agents are classified according to decreasing sizes into the following seven categories:1. Arthropods – scabies, lice, ticks
2. Helminths – schistosoma, ascaris3. Protozoa – plasmodium4. Fungi – yeast and molds5. Bacteria – spirochetes, streptococci,
TB bacilli6. Ricketssiae – typhus7. Viruses – flu, measles, Hepa B, HIV
Host factors in the occurrence and distribution of communicable diseases
1. Reservoir of agents – man, animals, plants, soils, inanimate organic matter
2. Portals of entry and exit – interrupt the natural history before man is infected
- by detecting agents, abolishing reservoir, interfering with transmission, barriers against infection
CLASSIFICATION OF DISEASES
Communicable or Infectious
Bacteria
Virus
Ricketsiae
Arthropods
Helminths
Protozoa
Fungi
Non-communicable or non-infectious
OccupationalEnvironmentalLife-style diseasesFamilial or
hereditary
THE NATURAL HISTORY OF DISEASE
1. Stage of susceptibility a). Portal of entry2. Stage of pre -symptomatic disease a). Incubation period3. Stage of Clinical Disease a). Clinical horizon4. Stage of disease outcome a). Complete recovery b). Disability or defect c). Carrier state d). death
STAGE OF SUSCEPTIBILITY
- The disease has not developed but the groundwork has been laid by the presence of risk factors which favors its occurrence
- Portal of entry is a pre-requisite for successful infection
STAGE OF PRESYMPTOMATIC DISEASE
- There is no manifest disease but usually through the interaction of factors, pathogenic changes have started to occur
- INCUBATION PERIOD – the interval between the time of entry of agent into the host and the onset of signs and symptoms
STAGE OF CLINICAL DISEASE
- Sufficient end-organ changes have occurred so that signs and symptoms of the disease can be recognized
- Classification and Staging of Disease
STAGE OF DISEASE OUTCOME
- Some diseases run their course and then resolve completely either spontaneously or under the influence of therapy
- Some diseases will give rise to residual defect of short or long duration with disability
NATURAL HISTORY OF DISEASE DISEASE
INCUBATION PERIOD
CLINICAL CASE
RECOVERY DEATH
COMPLETE CARRIER
DISABILITY OR DEFECT
LEVELS OF DISEASE PREVENTION
1. PRIMARY LEVEL- Pre-pathogenesis- Isolation, quarantine- Health education, food sanitation,
proper waste disposal, eradication of animal reservoir, specific protection
- Active and passive immunization
LEVELS OF DISEASE PREVENTION2. SECONDARY LEVEL- PATHOGENESIS period- Early diagnosis and prompt
treatment of the disease to prevent its spread
- Example: killing of rabid dogs, cows with MCD, pigs with FMD
- Screening tests for diseases , periodic examinations, case-finding, adequate treatment and follow-up
LEVELS OF DISEASE PREVENTION3. TERTIARY LEVEL- Stage of Disease outcome- Disability limitation- Rehabilitation- Intensive, periodic follow-up and treatment
Health Screening
For CVS diseases: Chest x-ray & ECG Blood sugar for diabetes Pap’s smear for cervical cancer Digital rectal examination Newborn Screening Hearing Screening Mantoux Test
STEPS OF SCIENTIFIC INQUIRY 1. Examining existing facts and
hypotheses and identifying gaps in knowledge
2. Formulating a new and a more specific hypothesis
3. Obtaining additional information to test the acceptability of the new hypothesis
4. Evaluating the new evidence and deriving appropriate conclusions
The Epidemiologic Approach or Strategy 1. identify the problem 2. review of related literature 3. identify critical knowledge gaps
about the problem 4. special data collection activities 5. hypothesis formulation 6. hypothesis testing
Features of a good hypothesis: Formulating the Hypothesis The population to whom the
hypothesis will apply (the target population)
The cause being investigated is usually a particular environmental exposure. It could be a physical, chemical, biologic or psychological factor
The outcome is usually the disease or condition of interest
Features of a good hypothesis: Formulating the hypothesis The dose response relationship is the
amount of exposure necessary for the disease or condition to develop
The time-response relationship is the time period between the exposure and the development of the outcome. This concept is synonymous to the incubation period for infectious diseases and to the latency period of non-infectious diseases
Some considerations in formulating hypothesisa). New hypotheses are commonly
formed by relating observations from several different fields
b). The stronger the statistical association between the exposure and the disease, the more likely it is to suggest a causal hypothesis
Some consideration in formulating hypothesisc). Observed changes in the frequency of
disease can lead to very productive hypothesis
d). An isolated or an unusual case should receive particular attention in forming hypothesis
e). Observations that appear in conflict or those which present a paradox should be considered
Study Designs
Descriptive studies – case studies, case series and cross-sectional studies
Observational studies – cohort studies, case-control studies
Experimental studies – clinical trials, RCT’s, community trials
Sources of Data
Two Types of Data according to Source:1). Primary Data – obtained by the
investigatorExample: interviews
physical examinationlaboratory examinations
- More accurate and up-to-date but more expensive and difficult to obtain
Sources of Data
2). Secondary Data – data actually gathered by other individuals or agencies
Example: published reportsclinical/hospital
recordscensus
- More readily available but incomplete
- Confidentiality of information
Surveillance
In most health departments, routinely collected statistics provide the key data for monitoring morbidity and mortality trends
Surveillance System – includes a functional capacity for data collection, analysis and dissemination linked to public health programs
Epidemiologic Surveillance Has been defined by the Centers for
Disease Control (CDC) as the ongoing systematic collection, analysis and interpretation of health data essential to the planning, implementation and evaluation of public health practice, closely integrated with the timely dissemination of these data to those who need to know
Specific Data that are Useful in Epidemiologic Studies
1. Data on vital events – birth, death, marriages, divorces, adoptions, total births/deaths, deaths by specific causes, mortality rate, case fatality rate, etc.
2. Disease statistics – prevalence and incidence of specific diseases
Specific Data that are Useful in Epidemiologic Studies
3. Data on physiologic or pathologic conditions – prenatal Hgb levels, blood sugar levels among diabetics, BP readings
4. Statistics on Health Resources and Services – number of hospital beds, vaccine vials consumed, number of health center staff
Specific Data that are Useful in Epidemiologic Studies
5. Statistics pertaining to the environment – number of households with sanitary water source, number of snail breeding places, amount of pollution in the air, level of noise in the factory, workers’ protective gears
6. Demographic data – total number of population, age groups, gender, rural-urban residence, occupation, income
7. Socio-cultural data – knowledge, attitude, practices of people regarding health
Sources of Data
1. Registers of births and deaths2. Certificates of death3. Disease registers4. Disease notifications5. Census6. Clinic/hospital records7. Surveys
10 Key Sources of Data for Surveillance Systems Designated by the WHO:1. Mortality registration2. Morbidity reporting3. Epidemic reporting4. Laboratory investigation5. Individual case investigations6. Epidemic field investigations7. Surveys8. Animal-reservoir and vector
distribution studies9. Biologic and drug utilizations10. Knowledge of the population and the
environment
Other Sources of Surveillance Data1. Hospital and medical care statistics2. Panels of cooperating physicians3. Public health laboratory reports4. Absenteeism from work or school5. Telephone and household surveys6. Newspaper and news broadcasting
reports
Data can readily be available from1. National Center for Health Statistics2. Centers for Disease Control &
Prevention3. Birth Defects Monitoring Program4. Metropolitan Atlanta Congenital Defects5. Cancer Surveillance, Epidemiology and
End Results (SEER) Program6. Morbidity and Mortality Weekly Report
from the CDC7. Medical Specialty Societies8. World Health Organization
Locally, data may be available from:1. Department of Health2. National Institutes of Health3. Medical Specialty Societies4. National Congenital Defects
Registry5. Newborn Screening Program6. Specialty Hospitals7. National Census and Statistics
Office
Descriptive Epidemiology: Person, Place and TimeDescriptive Epidemiology – the study
of the amount and distribution of disease within a population by person, place and time.
WHO (person) is affected?WHERE (place) do the cases occur?WHEN (time) do the cases occur?
PERSON
Characteristics of PERSON1. age 7. family variables2. sex 8. other personal3. ethnic group variables:4. social class Blood type5. occupation Environmental exposures6. marital status Personality traits
AGE
- The most important determinant among the personal variables
- Death rate is fairly high in infancy- Lowest point is between 5-14 years
old- Doubling in rate from 40 and every
decade of life
AGE
- Chronic conditions tend to increase with age whereas the relation of age
to acute infectious diseases is less consistent
- Age is related to the frequency and severity of infectious diseases
- High rate of injury in particular age group
SEX
- Death rates are higher for males than females, but morbidity rates are higher for females
- In utero and neonatal death rates are also higher for males
- The higher death rates for males throughout life maybe due to sex-linked inheritance, differences in hormonal balance, environment or habit patterns
SEX
- The higher mortality rate for men are not paralleled by higher rates of illness
- Women have more episodes of illness and more physician contacts than men have
- Rate of attempted suicide is higher in women but completed suicides are more common in men
- Toxic shock syndrome
SEX
Possible explanations for the relatively high morbidity and low mortality in women:
1). That women seek medical care more freely and perhaps at an earlier age of disease
2). That the same disease will tend to have less lethal course in women than in men
ETHNIC GROUP & RACE
Blacks have higher rates of deaths caused by CHVD, CVA, TB, SY
Whites have higher rates of death from suicide, leukemia and atherosclerotic disease
Many differences in rates of diseases and death reflect at least in part, differences in various environmental exposures, in lifestyle and in the extent and quality of medical care
SOCIAL CLASS
Difference in wealth, power, prestige: difference in access to medical care and facilities
Poverty affects utilization of medical services
More common cases of mental illness in lower strata
OCCUPATION
This influence may occur thru a variety of exposures – unfavorable physical conditions (heat, cold, changes in atmosphere), chemicals, noise, stress in work
Silica (pulmonary fibrosis), asbestos (lung cancer), aniline dyes (bladder cancer)
Injury, trauma, social and psychological climate of the job or workplace
MARITAL STATUS
Marital status is associated with level of mortality for both sexes
Psychological and physical support from the spouse
For women, marital status may also be related to health through differences in sexual exposure, pregnancy, childbearing and lactation
FAMILY VARIABLES
Family size: larger families – especially if they are poor, children may be in a disadvantage: higher rates of fetal, neonatal and infant deaths, higher childhood mortality, and a tendency to poorer intellectual performance
Birth order: first borns tend to be more healthy and better educated
Personal Variables
Maternal age: etiologic importance in congenital malformations
Parental deprivation – psychiatric, psychosomatic disorders, TB incidence, attempted suicides and accident repeaters
Blood type A- gastric CA, Type O –doudenal ulcer
Environmental exposurePersonality traits – medical advice,
compliance
PLACE
Frequency of disease can be related to place of occurrence in terms of areas set off either by natural barriers or by political boundaries
Frequency of disease may be related to temperature, humidity, rainfall, altitude, mineral content of soil or water supply
Lack of iodine, mottled dental enamel Rural-urban differences Migrants in national and international
borders
TIME
Disease occurrence is usually expressed on a monthly or annual basis
Secular trends – refers to changes over a long period of time, years, decades
Cyclic change – refers to recurrent alterations in the frequency of disease
If there are enough cases of a particular disease annually for stable rates, cases for several years around a census may be combined
Measures of Disease Frequency Count – the basic measure of disease
frequency Ratio – a measure that shows the
relationship between quantities Proportion – the numerator in the
proportion is part of the denominator Rate – the most common measure
Measures of Morbidity
Prevalence – the proportion of individuals with the disease during a a given point in time
- the probability that a person randomly chosen from the population will have a disease at the time he was examined.
Prevalence = no. of existing cases of a disease
-----------------------------------------
Total population examined
Measures of Morbidity
Incidence – refers to the proportion of the population who developed the disease in a given interval time
- the numerator is the count of new cases of disease in the population
- a measurement of the risk of developing the disease in the population at risk of the disease
Cumulative incidence – no. of new cases of disease----------------------------------population at risk
Relationship of Incidence and Prevalence The higher the number of new
(incident) cases, then there would be a greater number of existing (prevalent) cases
Study of Epidemics
Two Types of Epidemics:1. Common Vehicle Epidemic2. Propagated or Progressive
Type
Common Vehicle Type of Epidemic Single exposure, single source or
point epidemic It indicates simultaneous exposure of
the population to a common source Example: food or chemical
poisoning- Multiple exposure- Water-supply epidemic (Dysentery)
Propagated or Progressive Type of Epidemic Also called contact epidemic,
propagated by:a). Person to person spreadb). Arthropodc). Vector reservoir
Characteristics of Epidemic1). Type of onset or manner of onset
a). Sudden, abrupt or explosive – time factor is the period of incubation
b). Insidious or gradual – most cases start after the incubation period
2). Types of infectiona). Mass infection – more of
primary casesb). Progressive infection – more
of secondary cases
Types of Epidemic Curves
1). Classical Epidemic Curve –short ascending and descending limbs- picture of common source- rapid transmission due to big dose of organism- longer descending limb is due to the development of secondary cases- more deaths on the ascending limb because of heavier dose of the organism and less resistance
Classical Epidemic Curve
Inverted Epidemic Curve
- Long ascending and short descending limb
- Indicates that the transmission is more complex and the disease has a longer incubation period
- Person to person spread, insect-borne diseases (malaria, DHF)
Inverted Epidemic Curve
Bell-shaped Epidemic Curve- Rapid ascending and rapid
descending limb- The spread is rapid and the
transmission is simple so there is rapid elimination of susceptible
- Measles and poliomyelitis
Bell-shaped Curve
Investigation of Epidemics1. DEFINITION OF THE PROBLEM
- define clearly the nature and extent of the problem
a). Verify the diagnosis through clinical signs and symptoms of the disease aided by laboratory examinations
b). Verify whether epidemic exists. Compare the incidence of the disease with its usual incidence in the community
Investigation of Epidemics2. APPRAISAL OF EXISTING
INFORMATIONa). Orient as to time, the chronologic
order of the disease (epidemic curve)b). Orient as to place, determine
geographic distribution of casesc). Orient as to the person’s
characteristics (age, sex, occupation, socioeconomic conditions, etc.)
Investigation of Epidemics
3. FORMULATE TENTATIVE HYPOTHESIS
What could be the cause/ causes?Classify according to mode of transmission:
3.1. Common Vehicle – single exposuremultiple or continuous exposure
3.2. Propagated – person to person arthropod vector reservoir
Investigation of Epidemics4. TESTING THE HYPOTHESIS
a). Epidemiological investigation of all cases or representative sampleb). Search for additional informationc). Analyze detailed data – attack rates, common source or vehicled). test various hypotheses
e). Formulate conclusions as to source, mode of transmission and all features of the epidemic which require explanations
Investigation of Epidemics
5. CONCLUSIONS & PRACTICAL APPLICATIONS- evaluate the results in terms of local situations
- conclusions are based on pertinent evidence to control present outbreak and prevent future similar epidemics
- reports should be simple, clear and honest
- collection of data must be made in a scientific manner which requires exact observation, correct interpretation and rational explanation