Community Med

What is epidemiology?

The study of the distribution (who, where, when) and determinants (causes, risk/protective factors) of health-related events in populations, and applying this knowledge to prevent, control, and treat health problems.

What is the difference between prevalence and incidence?

• Prevalence: Total number of cases in a population at a given time. * Incidence: Number of new cases developing in a population during a specific time period.

Example of calculating incidence in influenza among students?

• 10,000 medical students baseline. * 1,000 already had influenza → prevalence = 10%. * Remaining 9,000 at risk → 500 new cases in 6 months. * Incidence = 500 \div 9,000 = 5.6\% over 6 months.

What is a study design?

A specific plan or protocol for conducting a study that translates a conceptual hypothesis into an operational one.

What are descriptive vs. analytical studies?

• Descriptive: Describe occurrence of disease (patterns of person, place, time). * Analytical: Test hypotheses, study cause–effect relationships (exposure vs. outcome).

What is a cross-sectional study and its limitation?

Observational study where data is collected at one point in time to measure prevalence. Limitation: Not good for rare or progressive diseases (e.g., Parkinson’s, ALS).

What is a cohort study?

Observational, longitudinal study following a group over time to measure incidence; good for common diseases.

Case-control vs. cohort study?

• Case-control: Start with disease → look back at past exposures (retrospective). * Cohort: Start with exposure → follow forward to see disease development (prospective).

What is relative risk (RR)?

Ratio of disease risk in exposed group vs. non-exposed group. * RR > 1 → increased risk. * RR < 1 → decreased risk (protective factor).

What is a clinical trial?

Experimental study with participants randomly assigned to treatment groups to test safety and effectiveness.

When is it unethical to use placebo in clinical trials?

When there is already a proven, effective treatment for the condition.

Clinical trial vs. community trial?

• Clinical trial: Individual patients, randomized, tests drugs/treatments. * Community trial: Whole communities randomized, tests public health interventions; individuals don’t have equal chance of group assignment.

What is an “arm” in trials?

The treatment group to which a participant or community is assigned.

Point prevalence vs. period prevalence?

• Point: Proportion with disease at a single time point. * Period: Proportion with disease during a specific time interval.

What is a case report?

A detailed report on a single patient’s unusual condition, new disease, or adverse drug effect.

What is a case series?

A collection of case reports describing multiple patients with the same disease. Usually cannot calculate incidence/prevalence (except if all cases in a defined population over time are included).

What is an ecological study?

Observes exposures and outcomes at the population level, not individual level (e.g., countries with high coffee use having high cancer rates).

What are surveys in epidemiology?

Tools used to gather information about exposures, behaviors, or health outcomes. A cross-sectional study is a type of survey where the data itself forms a study design.

Prospective vs. retrospective studies?

• Prospective: Follow subjects forward in time, outcomes not yet occurred. Fewer biases. * Retrospective: Look back at existing data on exposures and outcomes. Cheaper, quicker, but more prone to bias.

What are patient registries?

Organized systems that collect uniform data on patients with a specific disease, condition, or exposure for clinical, scientific, or policy purposes. Special form of case series.

Types of registries?

• Mortality registry: Records deaths. * Research patient registry: For clinical trials/research. * Disease/condition registries: For specific diseases (e.g., cancer registry). * Service/device registries: For specific treatments/devices (e.g., bone marrow transplant registry).

What is Real World Evidence (RWE)?

Analysis of treatment results in routine practice, using patient cohorts with comorbidities and adverse events, giving a fuller picture than clinical trials.

What is epidemiology?

The study of the distribution (who, where, when) and determinants (causes, risk/protective factors) of health events in populations.

What is the main goal of epidemiology?

To prevent, control, and treat health problems at the population level.

What are descriptive studies used for?

To describe the occurrence and distribution of health-related events, without testing hypotheses.

Define a case report.

A detailed report of a single patient with an unusual condition; provides early warning signals but has no comparison group.

Define a case series.

A description of multiple patients with the same disease or exposure; shows patterns but usually cannot measure incidence/prevalence. Exception: If it includes all cases in a defined population over time, incidence can be calculated.

What is an ecological study?

A study where the unit of analysis is groups or populations, not individuals. Uses existing statistics to compare exposures with outcomes (e.g., red meat consumption vs colorectal cancer rates).

What is the main limitation of ecological studies?

The ecological fallacy
—
group-level associations may not apply at the individual level.

Define a survey.

A structured method of data collection (e.g., questionnaire); when done at a single point in time, it becomes a cross-sectional study.

What is a cross-sectional study?

An observational study measuring exposure and outcome simultaneously in a population at a single point in time to estimate prevalence.

When are cross-sectional studies useful?

For describing burden of disease, healthcare planning, and investigating chronic/common conditions.

What are the disadvantages of cross-sectional studies?

Cannot establish temporality (cause-effect), not suitable for rare/short-duration diseases, cannot measure incidence.

What is the exception where cross-sectional studies can be strong?

When exposures are fixed and pre-disease (e.g., genetic factors such as HLA or blood groups).

Define point prevalence vs period prevalence.

Point prevalence = cases at a single time point. Period prevalence = cases during a defined period.

What defines analytical studies?

They test hypotheses about exposure
→
outcome associations.

What is a case-control study?

Retrospective design starting with diseased individuals and comparing past exposures with controls.

When are case-control studies most useful?

For rare diseases and diseases with long latency periods.

What measure of association is used in case-control studies?

Odds ratio (OR).

What is a cohort study?

A study where disease-free individuals are classified by exposure status and followed over time to measure incidence of disease.

What measure of association is used in cohort studies?

Relative risk (RR).

What is the main advantage of cohort studies over case-control studies?

Can directly measure incidence and establish temporal sequence (cause precedes effect).

What are limitations of cohort studies?

Inefficient for rare diseases, expensive, long duration, large samples required, drop-out bias.

Define prospective cohort.

Groups are followed into the future from exposure to outcome.

Define retrospective cohort.

Uses existing records to look back at exposures and outcomes; quicker and cheaper but limited by data quality.

Define ambidirectional cohort.

Combines retrospective and prospective follow-up on the same cohort.

Define a birth cohort vs reception cohort.

Birth cohort = individuals born in the same year followed over time. Reception cohort = individuals followed from first exposure.

Define incidence.

New cases of a disease in a population at risk over a period of time.

Define prevalence.

All cases (old + new) in a population at a point or over a period.

Formula for relative risk (RR).

RR = [a/(a+b)] \div [c/(c+d)] where a = exposed cases, b = exposed non-cases, c = unexposed cases, d = unexposed non-cases.

Interpret relative risk values.

RR = 1
→
no association; RR > 1
→
exposure increases risk; RR < 1
→
exposure decreases risk.

Define prevalence rate ratio (PRR).

Ratio of prevalence in exposed
÷
prevalence in non-exposed (used in cross-sectional studies).

What is a clinical trial?

Experimental study where individuals are randomized to intervention vs control.

When is it unethical to use placebo in a clinical trial?

When an effective treatment already exists.

What is a community trial?

Experimental study where entire communities, not individuals, are randomized.

What is a disease registry?

An organized system collecting uniform data on patients with a disease/condition for tracking and research.

Types of registries?

Mortality, disease/condition, service/device, research.

What is Real World Evidence (RWE)?

Analysis of outcomes in routine practice (outside clinical trials), including comorbidities and adverse events.

What are the four quantitative descriptors used to measure disease occurrence?

Numbers, Ratios, Proportions, Rates

Give an example of using Numbers to describe disease.

100 cases of TB in community A

Define Ratio in epidemiology and give an example.

A ratio quantifies the magnitude of one event (X) relative to another (Y) as X/Y. Example: ratio of TB cases in community A to B is 1:10

Define Proportion and give an example.

A ratio where the numerator is included in the denominator. Example: Proportion of TB cases in community A is 10%

Define Rate in epidemiology.

A proportion with a time element; measures occurrence of an event over time. Example: US measles cases in 2000
÷
US population in 2000

What do morbidity measures quantify?

The magnitude/frequency of diseases in a population

What are the two common morbidity measures?

Incidence (cumulative incidence & incidence density) and Prevalence

Define Incidence rate.

The proportion of a population that develops a disease over a period of time; measures risk/probability and rapidity of new cases

What are the two types of incidence?

Cumulative incidence and Incidence density (rate)

Define Cumulative incidence.

The proportion of initially disease-free individuals who develop the disease during a period

Define Incidence density.

Number of new cases per unit of person-time (accounts for varying observation times)

What does incidence measure in terms of disease risk?

The risk/probability of developing the disease

What is the formula for cumulative incidence?

( ext{Number of new cases during period} ext{
÷
} ext{Population at risk at baseline})

What does “population at risk at baseline” mean?

The group of disease-free individuals at the start of the study who are capable of developing the disease

Define Case Fatality Rate (CFR).

Proportion of cases with a specified disease who die within a specified time; measures severity; expressed as %

Define Attack Rate (AR).

Number of people affected
÷
number exposed; used in outbreaks in narrowly defined populations (e.g., food poisoning at a party)

How do CFR and AR differ?

CFR = measures deadliness* (severity). AR = measures infectivity/spread* in outbreaks

What are the practical challenges in measuring incidence rate?

1. Identifying the population at risk (disease-free, susceptible). 2. Populations fluctuate due to births, deaths, migration. 3. People are at risk only until they develop the disease

Define Prevalence.

The proportion of a population with a disease at a point in time

What does prevalence describe in public health?

The current burden of disease in a population (important for planning & resource allocation)

Give examples of prevalence questions.

Prevalence of cognitive disorder among school children in Jordan; prevalence of anxiety disorder among JU medical students

What is the formula for prevalence?

( ext{All persons with disease} ext{
÷
} ext{Total population})

Is prevalence a rate or proportion?

It is a proportion, not a rate

What is the relationship between prevalence and incidence?

( ext{Prevalence} = ext{Incidence Rate} imes ext{Average Duration})

What happens if incidence increases (tap analogy)?

Prevalence rises

What happens if recovery or mortality increases (drain analogy)?

Prevalence falls

How does disease rarity affect prevalence/incidence relationship?

If disease is rare (<10% population affected), preventive measures that reduce incidence
→
lower prevalence; developing a cure that shortens duration
→
lower prevalence

Give an example of how treatment affects prevalence.

In late 1990s, antiretroviral therapy increased HIV survival (longer duration without cure)
→
prevalence of HIV increased

Is prevalence always a proportion or a rate?

Always a proportion

Is incidence always a rate?

Incidence can be a proportion (cumulative incidence) or a rate (incidence density/person-time)

What is the definition of epidemiology according to J.M. Last (1988)?

Epidemiology is the study of the distribution and determinants of health-related states or events in specified populations, and the application of this study to control health problems.

What is the etymology of the term 'Epidemiology'?

The term comes from Greek: * Epi = upon or among * Demos = people * Logos = study → Literally: “The study of what is upon or among the people.”

What was Frost's (1927) early definition of epidemiology?

The science of the mass phenomena of infectious diseases.

What was Stallbrass's (1931) early definition of epidemiology?

The science of infective diseases, their prime causes, propagation, and prevention.

What is epidemiology's role in public health?

Epidemiology is a core science of public health.

How did Winslow (1920) define public health?

“The science & art of preventing disease, prolonging life, and promoting health through organized community effort.”

Define Health (WHO, 1948).

Complete physical, mental, and social well-being.

Define Disease.

Physiological or psychological dysfunction.

Define Illness.

A subjective feeling of not being well.

Define Sickness.

A state of social dysfunction.

What are the objectives of epidemiology? (List 5)

1. Investigate etiology and modes of transmission. 2. Determine extent of disease problems in a community. 3. Study natural history and prognosis of disease. 4. Evaluate preventive and therapeutic measures. 5. Provide a foundation for public health policy.

How does epidemiology function as a science?

It uses systematic collection, analysis, and interpretation of health-related data to identify patterns and relationships.