HEALTH - Detailed Answers (Park's PSM)

Q1. Strategic Action Plan for Malaria Control in India

• 2016 - National Framework for Malaria Elimination in India launched
• 2017 - NSP for Malaria Elimination 2017-2022 launched

1. Case detection - passive and active
2. Early diagnosis and complete treatment (RDT/microscopy; no presumptive treatment since drug policy 2013)
3. Sentinel surveillance

1. Indoor Residual Spray (IRS) - with DDT, synthetic pyrethroids
2. Insecticide Treated Bed Nets (ITNs) and Long Lasting Insecticidal Nets (LLINs)
3. Anti-larval measures including source reduction (larvicides, bioenvironmental methods)

1. Capacity building
2. Behavioural Change Communication (BCC)
3. Intersectoral collaboration
4. Monitoring and Evaluation (M&E)
5. Operational research and applied field research

• Complete cure
• Prevention of progression to severe malaria
• Prevention of deaths
• Interruption of transmission
• Minimizing risk of drug resistance

Q2. Telemedicine

1. Video conferencing - real-time audio-video consultation
2. Store and forward - asynchronous transmission of data, images (radiology, dermatology, pathology)
3. m-Health (Mobile health) - using mobile phones for health delivery
4. Remote patient monitoring - chronic disease management at home

• RMP to Patient
• RMP to Caregiver
• RMP to RMP (specialist referral)
• RMP to Health Worker

• Teleconsultation for rural/remote areas
• Teleradiology, Tele-dermatology, Tele-ophthalmology
• Continuing medical education
• Disaster management
• Monitoring of chronic diseases

• Started as a pilot by ISRO in 2001 - linking Apollo Hospital, Chennai to a rural hospital in Andhra Pradesh
• Telemedicine Practice Guidelines (TPG) issued by Government of India to regulate telemedicine for Registered Medical Practitioners (RMPs)
• Key players: Narayana Hrudayalaya, Apollo Telemedicine Enterprises, Aravind Eye Care, ISRO-supported networks
• Accelerated adoption post-COVID-19 pandemic

• Expands access to specialty care in remote areas
• Reduces cost and travel time for patients
• Supports timely diagnosis and management
• Promotes doctor-to-doctor knowledge sharing

Q3. Integrated Vector Control (IVC)

• Source reduction - drainage, filling, covering water collections
• Larviciding - temephos, BTi (Bacillus thuringiensis israelensis)
• Biological control - larvivorous fish (Gambusia, Poecilia), cyclopoid copepods

• Indoor Residual Spray (IRS) - DDT, malathion, synthetic pyrethroids
• LLINs (Long Lasting Insecticidal Nets) - permethrin/deltamethrin impregnated
• Space sprays and fogging during epidemics

• Intermittent irrigation
• Water level management
• Deepening/flushing water bodies

• Repellents (DEET), protective clothing, mosquito nets
• House screening

• Sterile insect technique
• Genetically modified organisms

• Insecticide resistance developing in vectors
• Reduces dependence on a single insecticide
• More cost-effective use of resources
• Less environmental impact
• Sustainable long-term control

Q4. NIKSHAY

1. Registration of TB cases (both public and private sector)
2. Ordering diagnostic tests
3. Recording treatment details
4. Monitoring treatment adherence
5. Transferring cases between facilities
6. Real-time data reporting to government
7. Acts as India's National TB Surveillance System

• Available to health workers in both public and private sector
• Mandatory notification of all TB cases through Ni-Kshay
• End-to-end patient record keeping: diagnosis, follow-up tests, treatment, prescription, adherence, outcome
• Patient data maintained confidentially, accessible only to concerned provider and District TB Office

• Nutritional incentive scheme linked to Ni-Kshay
• Rs. 500/month credited directly to patient's bank account via Direct Benefit Transfer (DBT)
• Over Rs. 3,202 crores disbursed to 1.13 crore beneficiaries (as of 2025)

• Private providers who notify TB cases through Nikshay receive financial incentives
• Bank details entered into Nikshay for DBT

Q5. Vatsayan Kendra / ICTC (Integrated Counselling and Testing Centre)

1. Early detection of HIV
2. Provision of basic information on modes of transmission and prevention of HIV/AIDS
3. Promoting behavioural change and reducing vulnerability
4. Linking PLHIV (People Living with HIV) with HIV prevention, care and treatment services

1. Standalone ICTC (SA-ICTC):
   • High client load
   • Full-time counsellor and laboratory technician
   • Located in medical colleges, district hospitals, sub-district hospitals, CHCs
2. Facility-Integrated ICTC (F-ICTC):
   • Set up below block levels at 24x7 PHCs
   • Existing health facility staff trained in HIV counselling and testing
   • Logistic support from DAC (Department of AIDS Control)
   • Public-Private Partnership (PPP)-ICTCs also established in private facilities

• A van equipped to conduct general examination, counselling and blood sample collection/processing
• Team: health educator/ANM, counsellor, laboratory technician
• Serves hard-to-reach areas with flexible working hours
• Provides: HIV counselling and testing, syndromic management of STI/RTI, antenatal and immunization services

• Conducted by frontline health workers (ANMs) at sub-centre level
• Aims to test every pregnant woman to detect HIV and prevent MTCT (Mother-To-Child Transmission)

Q6. PPTCT - Prevention of Parent-to-Child Transmission of HIV

• 2002: Started with single-dose Nevirapine (SD-NVP) prophylaxis during labour for HIV+ mothers and newborn
• 2012: Transitioned to multi-drug ARV prophylaxis (Option B, WHO 2010 guidelines) in 3 southern states
• 2013: National Strategic Plan for PPTCT developed for nationwide roll-out
• 2013 (Dec): India adopted Lifelong ART (Triple drug: TDF+3TC+EFV) for all pregnant and breastfeeding women with HIV regardless of CD4 count or clinical stage (Option B+)

1. Routine offer of HIV counselling and testing to all pregnant women with 'opt-out' option
2. Involvement of spouse and family members - shift from ANC-centric to Family-Centric approach
3. Lifelong ART (TDF+3TC+EFV) regardless of CD4 count
4. Promotion of institutional deliveries for HIV+ pregnant women
5. Care for associated conditions (STI/RTI, TB, opportunistic infections)
6. Nutrition counselling and psychosocial support
7. Exclusive breastfeeding counselling within 1 hour of delivery for 6 months
8. ARV prophylaxis to infants from birth to minimum 6 months
9. Integrating follow-up of HIV-exposed infants into routine healthcare (immunization)
10. Co-trimoxazole Prophylactic Therapy (CPT) and Early Infant Diagnosis (EID) using HIV-DNA PCR at 6 weeks
11. Community follow-up and outreach through local networks

Q7. IPV (Inactivated Polio Vaccine)

• Trivalent: contains all three poliovirus serotypes (types 1, 2, 3)
• At least 40 units type-1, 8 units type-2, 32 units type-3 D-antigen
• Contains enhanced potency (eIPV)
• May contain trace amounts of formaldehyde, streptomycin, neomycin, or polymyxin B
• Preservative: phenoxyethanol (0.5%) - neither thiomersal nor adjuvants used

• Route: Intramuscular injection OR fractional dose intradermal injection
• Storage: Refrigerated (2-8°C); freezing diminishes potency
• Available as standalone or in combination vaccines (DTPw, DTPa, HepB, Hib)

• Primary course: 4 doses
• 1st, 2nd, 3rd dose at 1-2 month intervals (starting at 6 weeks of age)
• 4th dose: 6-12 months after 3rd dose
• Booster before school entry, then every 5 years until age 18

• Induces humoral antibodies: IgM, IgG, IgA serum antibodies
• Does NOT induce intestinal/local immunity
• Protects individual against paralytic polio
• Does NOT prevent gut reinfection by wild viruses

1. Safe for immunocompromised persons
2. Safe for patients on corticosteroids/radiation therapy
3. Safe for adults over 50 receiving vaccine for the first time
4. Safe during pregnancy

1. Does not induce intestinal immunity - wild virus can still multiply in gut
2. Multiple doses needed before immunity develops
3. Not suitable for epidemic control (OPV preferred for epidemics)
4. Injections during epidemic times may precipitate paralysis

• Minor local reactions: erythema (0.5-1%), induration (3-11%), tenderness (14-29%)
• No serious adverse reactions reported

Q8. NPCDCS - National Programme for Prevention and Control of Cancer, Diabetes, Cardiovascular Diseases and Stroke

• 11th Five Year Plan: 100 identified districts in 21 states
• 12th Five Year Plan: Expanded to all districts of India

1. Prevent and control common NCDs through behaviour and lifestyle changes
2. Provide early diagnosis and management of common NCDs
3. Build capacity at various levels for prevention, diagnosis, and treatment
4. Train doctors, paramedics, and nurses to cope with increasing NCD burden
5. Establish capacity for palliative and rehabilitative care

• Implemented in 20,000 sub-centres and 700 CHCs in 100 districts
• Strategies:
  • Health promotion through mass media and health education
  • Opportunistic screening of persons above 30 years (BP measurement, blood glucose by strip)
  • NCD Clinics at CHC and District levels
  • Trained manpower development
  • Strengthening tertiary health facilities

• IEC services for prevention and early detection
• Early warning signals: non-healing ulcers, unexplained lumps, change in bowel/bladder habits, blood in urine, abnormal bleeding, nagging cough, difficulty swallowing
• PAP smear screening (desirable)
• Referral of suspected cancer cases

• Increased intake of healthy foods
• Increased physical activity
• Avoidance of tobacco and alcohol
• Stress management

• Opportunistic screening: BP measurement, glucometer-based blood sugar testing (strip method)
• Referral of suspected diabetic/hypertensive cases to CHC

Q9. National Tobacco Control Programme (NTCP)

• COTPA 2003 - Cigarettes and Other Tobacco Products (Prohibition of Advertisement and Regulation of Trade, Commerce, Production, Supply, and Distribution) Act, 2003

1. (a) Health education and IEC (Information, Education, Communication) activities regarding harmful effects of tobacco use and secondhand smoke
2. (b) Promoting quitting of tobacco in the community
3. (c) Making PHC tobacco-free

• Watching for implementation of ban on smoking in public places
• Monitoring ban on sale of tobacco products to minors
• Monitoring ban on sale of tobacco products within 100 meters of educational institutions

1. Reduce prevalence of tobacco use in the country
2. Establish tobacco control cells at state and district levels
3. Implement provisions of COTPA 2003 effectively
4. Build capacity for tobacco cessation services
5. Establish tobacco-free educational institutions and workplaces
6. Conduct IEC campaigns against tobacco

• Tobacco Cessation Centres (TCCs) in major government hospitals
• Tobacco-free educational institutions (TFEI) initiative
• Training of healthcare workers in brief advice for tobacco cessation
• School health programmes targeting youth
• Quitline services

Q10. IDSP - Integrated Disease Surveillance Programme

• Central Surveillance Unit (CSU) - at National Centre for Disease Control (NCDC), New Delhi
• State Surveillance Units (SSU) - at all State/UT headquarters
• District Surveillance Units (DSU) - at all districts (741 districts, 94% reporting weekly data)

1. Strengthen/maintain a decentralized, laboratory-based, IT-enabled disease surveillance system for epidemic-prone diseases
2. Monitor disease trends and detect/respond to outbreaks in early rising phase through trained Rapid Response Teams (RRTs)
3. Establish functional mechanism for inter-sectoral coordination for zoonotic diseases
4. Strengthen data quality, analysis, and links to action

• Data is collected on weekly basis (Monday-Sunday)
• Weekly data provides information on disease trends and seasonality

• Whenever a rising trend of illness is detected, it is investigated by Rapid Response Teams (RRT)
• RRTs diagnose and control outbreaks at field level
• Data analysis and action by State/District Surveillance Units

1. Integration and decentralization of surveillance activities
2. Human Resource Development (training of SSOs, DSOs, RRTs)
3. Use of ICT for data collection, collation, analysis, and dissemination
4. Strengthening of public health laboratories
5. Inter-sectoral coordination for zoonotic diseases

• Launched in November 2019 to improve digital surveillance
• Captures individualized data disaggregated by age, gender, locality
• Links S, P, L formats with Early Warning Signals
• Monitors more than 33 health conditions
• Near real-time or daily surveillance data

Q11. End TB Strategy (National Strategic Plan for TB Elimination in India)

• India bears the highest global burden of TB: ~2.82 million new TB cases/year (199/lakh population, as of 2022)
• Estimated TB mortality: 3,31,000 deaths/year
• India's goal: Eliminate TB by 2025 (5 years ahead of global SDG target of 2030)
• Prime Minister pledged TB-free India by 2025 at the End TB Summit (2018)

• WHO's End TB Strategy
• SDG Goal 3 (Good Health and Well-Being)
• National Health Policy

1. Nikshay Poshan Yojana - Rs. 500/month nutritional support to TB patients via DBT
2. PM TB Mukt Bharat Abhiyaan (PMTBMBA) - community and private sector mobilisation
3. Mandatory notification of all TB cases (public + private)
4. Universal Drug Susceptibility Testing (UDST) - using CBNAAT/TrueNat
5. Single-window service for TB-HIV co-infected patients
6. Programmatic Management of Drug-resistant TB (PMDT)
7. 100-Day Campaign for active TB case finding

• National: Central TB Division (CTD), DDG-TB, supported by NTI Bengaluru, 6 National Reference Laboratories
• State: State Tuberculosis Officer (STO) at State TB Cell (STC), supported by STDC and IRL
• District: District Tuberculosis Officer (DTO) at District Tuberculosis Centre (DTC)
• Sub-district: Tuberculosis Unit (TU) with MO-TC, STS, STLS

• TB incidence declined by 16%
• TB deaths declined by 18%

BIOSTATISTICS & DEMOGRAPHY - Detailed Answers (Park's PSM)

Q1. Census

• Conducted at regular intervals of 10 years in most countries
• Covers the entire population (universal, not a sample)
• Provides a snapshot at a specific point in time

• First regular census: 1881
• Conducted at 10-year intervals since 1881
• Last census: March 2011 (2021 census delayed due to COVID-19)
• Legal basis: Census Act of 1948
• Supreme officer: Census Commissioner of India (under Ministry of Home Affairs)
• Conducted at the end of the first quarter (January-March) of the first year in each decade - because most people are resident in their own homes during that period

1. Total population count
2. Age and sex distribution
3. Literacy rate
4. Marital status
5. Religion and caste
6. Occupational structure
7. Housing and household amenities
8. Migration data
9. Economic characteristics (income, employment)

1. Provides basic data (population by age and sex) needed to compute vital statistical rates
2. Provides denominator for calculating health, demographic and socio-economic indicators
3. Baseline for planning, action and research in medicine, social sciences
4. Without census data, quantified health and demographic indicators cannot be obtained
5. Provides frame of reference for the entire governmental planning system

• Full results are usually not available quickly (takes several years to analyze)
• Conducted only once in 10 years - so data becomes outdated

Q2. Birth & Death Registration Act of India

• 1873: Government of India passed the Births, Deaths and Marriages Registration Act - but only provided for voluntary registration
• Individual states (Tamil Nadu, Karnataka, Assam) later passed their own Acts
• However, the registration system was unreliable with grossly deficient data

• Enacted by Parliament of India
• Came into force from 1st April 1970
• Provides for compulsory registration of births and deaths throughout India
• Uniform legislation across the country

1. Compulsory registration of all births and deaths
2. Responsibility of registration lies with the Registrar General of India at the central level
3. At state level: Chief Registrar oversees the system
4. At district level: District Registrar
5. At local level: Local Registrar (village officer/health worker)

• Head of the household must report birth/death within 21 days
• In hospitals/institutions: the medical officer-in-charge
• In case of death: a doctor must certify the cause of death

• Name, date, place of birth
• Sex of child
• Name of parents, their age, address, occupation
• Order of birth

• Name, age, sex of deceased
• Date, place, cause of death
• Occupation and address

• Introduced in 1964-65 as a large-scale demographic survey
• Provides annual estimates of birth rate, death rate, and infant mortality rate at state and national levels
• Operates on a continuous basis - fills the gap between census years

• Under RBD Act, doctors are required to certify cause of death on the prescribed form
• Provides data on cause-specific mortality

• Foundation of vital statistics
• Provides continuous check on demographic changes
• Basis for computing birth rate, death rate, infant mortality rate
• Essential for health planning and policy

Q3. Sources of Health-Related Data

PRIMARY SOURCES (Routine Data Systems)

• Conducted every 10 years
• Provides demographic data: population size, age-sex distribution, literacy
• Provides denominator for rate calculations
• Drawback: data becomes outdated quickly

• Births, deaths, marriages, divorces
• Governed by RBD Act 1969 in India
• Continuous system, unlike census
• Source of birth rates, death rates, cause-specific mortality
• Drawback: incomplete registration, especially in rural areas

• Large-scale continuous demographic survey
• Dual record system: independent concurrent and retrospective recording
• Provides annual state-wise and national estimates of birth rate, death rate, IMR
• More current than census

• Compulsory notification of specified diseases (cholera, plague, smallpox, etc.)
• Health workers/doctors must report to health authorities
• Provides morbidity data on epidemic-prone diseases
• Drawback: underreporting is common

• Data on admissions, diagnoses, procedures, length of stay, outcomes
• Useful for studying disease patterns and healthcare utilization
• Drawbacks: only "tip of the iceberg" - mild/subclinical cases missed; admission policy varies; population at risk undefined

• Permanent records for specific diseases: cancer, TB, leprosy, blindness, stroke, MI
• Follow-up of patients; provides data on duration of illness, case fatality, survival
• Example: Cancer Registry, National Leprosy Register

• Bringing together records of one individual from different times/places
• Birth, marriage, hospital admission, death records linked
• Useful for studying disease associations, chronic disease epidemiology, family studies

SECONDARY/SPECIAL SOURCES

• IDSP (Integrated Disease Surveillance Programme) - S/P/L reporting formats
• Continuous monitoring of epidemic-prone diseases

• NFHS (National Family Health Survey) - reproductive health, child health, nutrition
• DLHS (District Level Household Survey)
• AHS (Annual Health Survey)
• Cross-sectional surveys providing detailed morbidity and social data

• Data from PHCs, CHCs, district hospitals
• Covers immunization, ANC, delivery, family planning services

• Case-control studies, cohort studies, RCTs
• Provide etiological and interventional evidence

• WHO World Health Statistics
• UNDP Human Development Reports
• World Bank Health Data

Q4. Difference Between Standard Deviation & Mean Deviation; Uses of Standard Deviation

(A) MEAN DEVIATION (M.D.)

M.D. = Σ|x - x̄| / n

1. Calculate the arithmetic mean (x̄)
2. Find deviation of each value from the mean (x - x̄)
3. Take absolute values (ignore + and - signs)
4. Sum all absolute deviations
5. Divide by n

• Mean = 810/10 = 81
• Sum of |deviations| = 56
• M.D. = 56/10 = 5.6

(B) STANDARD DEVIATION (S.D.)

S.D. = √[Σ(x - x̄)² / n]

S.D. = √[Σ(x - x̄)² / (n-1)]

1. Calculate arithmetic mean (x̄)
2. Find deviation of each value from mean (x - x̄)
3. Square each deviation: (x - x̄)²
4. Sum all squared deviations: Σ(x - x̄)²
5. Divide by n (or n-1 for small samples)
6. Take square root

DIFFERENCES BETWEEN SD AND MD

USES OF STANDARD DEVIATION

1. Measures dispersion/variability of data around the mean - the higher the SD, the more spread out the data
2. Basis for Standard Error (SE): SE = SD/√n - used to estimate how representative the sample mean is of the population mean
3. Setting Confidence Intervals:
   • Mean ± 1 SD covers 68.27% of observations (in normal distribution)
   • Mean ± 2 SD covers 95.45% of observations
   • Mean ± 3 SD covers 99.73% of observations
4. Hypothesis testing - used in t-test, z-test, ANOVA
5. Comparing variability between two groups/datasets (Coefficient of Variation = SD/mean × 100)
6. Defining normal ranges in clinical medicine (e.g., reference ranges for lab values)
7. Quality control in laboratory and epidemiological studies
8. Sample size calculation for research studies

Q5. Normal Distribution (Curve & P value)

NORMAL DISTRIBUTION

1. Bell-shaped and symmetrical about the mean
2. Mean = Median = Mode (all three coincide)
3. The curve is continuous and extends from -∞ to +∞
4. Total area under the curve = 1 (or 100%)
5. The curve never touches the x-axis (asymptotic)
6. Determined by just two parameters: mean (μ) and standard deviation (σ)
7. Unimodal (one peak)

P(x) = (1/σ√2π) × e^[-(x-μ)²/2σ²]

THE NORMAL CURVE

1. Setting reference ranges/normal values in clinical medicine
2. Basis for hypothesis testing (z-test, t-test)
3. Central Limit Theorem - even non-normal populations, sample means tend to be normally distributed for large n
4. Basis for confidence interval calculation

P VALUE

• P < 0.05 (1 in 20): Result is considered statistically significant - i.e., unlikely to have occurred by chance
• P < 0.01 (1 in 100): Result is considered highly significant
• P < 0.001: Very highly significant
• P > 0.05: Result is not significant - the difference could be due to chance

• At the 5% significance level, the critical z-value is 1.96 (i.e., 2 SDs from mean)
• If the test statistic exceeds this value, p < 0.05 and we reject the null hypothesis

• P-value does NOT tell you the magnitude or clinical importance of a difference - only whether it is likely due to chance
• Statistical significance ≠ Clinical significance

• 95% CI: If 95% CI does not include zero (for differences) or 1 (for ratios), the result is statistically significant (p < 0.05)
• Provides range within which the true population value is likely to lie with 95% probability

Q6. Sampling Technique (Types & Methods)

• Cannot study the entire population (practical, economic reasons)
• Saves time, money, and manpower
• More detailed information can be collected
• Feasible for destructive testing or rare conditions

• Universe/Population: The entire group to be studied
• Sample: The selected subset
• Sampling Frame: List from which samples are drawn (e.g., electoral rolls, household lists)
• Sampling Unit: The individual element selected (person, household, village)

TYPES OF SAMPLING

A. PROBABILITY SAMPLING (Random Sampling)

• Every individual has an equal chance of selection
• Methods: Lottery method or Random Number Table
• Requires complete sampling frame
• Best for homogeneous populations
• Limitation: Impractical for large dispersed populations

• Every k^th individual selected (k = N/n = sampling interval)
• Example: If population N=1000, sample n=100, select every 10th person
• Easy to execute; requires complete list
• Risk: periodic bias if list has cyclical pattern

• Population divided into homogeneous subgroups (strata) based on relevant characteristic (age, sex, socioeconomic status)
• Random sample drawn from each stratum
• Proportionate stratified sampling: Sample from each stratum proportional to its size
• Disproportionate stratified sampling: Larger samples from smaller/more variable strata
• Ensures representation of all subgroups; more precise than SRS

• Population divided into naturally occurring clusters (villages, wards, schools)
• Clusters (not individuals) are randomly selected
• All individuals in selected clusters are studied
• Two-stage cluster sampling: Clusters selected first, then individuals within clusters
• Practical and economical for large, geographically dispersed populations
• Used in national health surveys (NFHS, EPI cluster surveys)
• Limitation: Less precise; sampling error higher (Design Effect)

• Sampling done in multiple stages at different administrative levels
• Stage 1: Select districts, Stage 2: Select blocks, Stage 3: Select villages, Stage 4: Select households
• Used in large national surveys
• Combines various methods at different stages

B. NON-PROBABILITY SAMPLING

• Select individuals who are readily available/accessible
• Quick and easy; but biased results
• Example: studying patients attending OPD

• Researcher deliberately selects individuals based on judgement
• Used in qualitative research; expert opinion studies

• Set quotas for different subgroups; fill them by convenience
• Resembles stratified sampling but without randomization
• Used in market research; opinion polls

• Initial participants refer others with similar characteristics
• Used for hard-to-reach populations (IDUs, sex workers)

SAMPLING ERROR vs NON-SAMPLING ERROR

Q7. Bias in Statistics

MAIN TYPES OF BIAS

1. SELECTION BIAS

• Admission (Berkson's) Bias: Hospital-based studies - hospital patients differ from community population in disease and exposure prevalence
• Non-response bias: Those who respond differ systematically from non-responders
• Volunteer bias: Volunteers are healthier/more health-conscious than non-volunteers (Healthy Worker Effect)
• Survivor bias: Only survivors are studied; those who died/dropped out are missed
• Loss to follow-up bias: In cohort studies, if those lost differ from those retained
• Referral/Detection bias: More severe or unusual cases referred to specialist centres

2. INFORMATION (MEASUREMENT/OBSERVATION) BIAS

• Recall bias: Cases (e.g., mothers of malformed babies) remember exposures more carefully than controls - common in case-control studies
• Observer bias / Interviewer bias: Observer's prior knowledge or expectations influence how they record data; reduced by blinding
• Reporting bias: Participants under-report socially undesirable behaviours (smoking, alcohol, sexual behaviour) or over-report desirable ones
• Diagnostic/Classification bias: Different diagnostic criteria applied to cases and controls
• Misclassification bias:
  • Non-differential: Misclassification equally distributed across groups - biases result toward null (underestimates true effect)
  • Differential: Misclassification unequal between groups - can bias in either direction
• Lead time bias: In screening studies, earlier detection appears to prolong survival even if treatment does not work
• Length bias: Screening detects slow-growing (less severe) disease more often; overestimates survival benefit

3. CONFOUNDING BIAS

• Example: Smoking confounds the relationship between alcohol and lung cancer
• Not truly a bias in the strict sense but causes systematic error in effect estimates
• Control methods: Restriction, matching, stratification, multivariable analysis

HOW TO CONTROL BIAS

BIAS vs RANDOM ERROR