Part 1 Science Notes
MRCOG Part 1
Statistics & Epidemiology
Statistics and Epidemiology for MRCOG
Diagnostic Test Measures
| Measure | Formula | Interpretation |
|---|---|---|
| Sensitivity | TP / (TP + FN) | Ability to detect disease (true positive rate); rules OUT disease when negative (SnNOUT) |
| Specificity | TN / (TN + FP) | Ability to exclude disease (true negative rate); rules IN disease when positive (SpPIN) |
| PPV | TP / (TP + FP) | Probability of disease given positive test; INCREASES with prevalence |
| NPV | TN / (TN + FN) | Probability of no disease given negative test; DECREASES with prevalence |
| LR+ | Sensitivity / (1 - Specificity) | How much a positive result increases the probability of disease |
| LR- | (1 - Sensitivity) / Specificity | How much a negative result decreases probability |
- PPV and NPV are affected by disease prevalence; sensitivity and specificity are NOT
- SnNOUT: high Sensitivity → Negative result rules OUT disease
- SpPIN: high Specificity → Positive result rules IN disease
Study Designs
Hierarchy of Evidence (strongest to weakest)
- Systematic reviews and meta-analyses
- Randomised controlled trials (RCTs)
- Cohort studies (prospective > retrospective)
- Case-control studies
- Cross-sectional studies
- Case series / case reports
- Expert opinion
Key Study Types
- RCT: gold standard for interventions; randomisation reduces confounding; blinding reduces observer bias
- Cohort study: follows exposed/unexposed groups over time; measures relative risk (RR) and incidence
- Case-control study: compares cases (disease) with controls (no disease); looks BACKWARDS at exposure; measures odds ratio (OR); efficient for rare diseases
- Cross-sectional study: snapshot in time; measures prevalence; cannot determine causation
Measures of Association
| Measure | Formula | Used In |
|---|---|---|
| Relative Risk (RR) | Risk in exposed / Risk in unexposed | Cohort studies, RCTs |
| Odds Ratio (OR) | (a×d) / (b×c) from 2×2 table | Case-control studies; approximates RR when disease is rare |
| Absolute Risk Reduction (ARR) | Risk in control - Risk in treatment | Treatment benefit |
| Number Needed to Treat (NNT) | 1 / ARR | Clinical utility of treatment |
| Number Needed to Harm (NNH) | 1 / (Risk in exposed - Risk in unexposed) | Risk assessment |
- RR = 1: no association; RR > 1: increased risk; RR < 1: protective
- NNT of 10 means 10 patients treated for 1 to benefit
Types of Bias
- Selection bias: non-random allocation or loss to follow-up distorts groups
- Recall bias: participants with disease remember exposures differently (common in case-control studies)
- Observer (detection) bias: awareness of group allocation influences assessment (prevented by blinding)
- Publication bias: positive results more likely published; detected by funnel plot
- Lead-time bias: screening appears to prolong survival by detecting disease earlier, not actually improving outcomes
- Length-time bias: screening detects slower-growing, less aggressive disease preferentially
- Confounding: a third variable associated with both exposure and outcome; controlled by randomisation, stratification, multivariate analysis, matching
Statistical Concepts
- P-value: probability of observing results as extreme as obtained if null hypothesis is true; p < 0.05 = statistically significant
- Type I error (alpha): rejecting null hypothesis when it's true (false positive); probability = significance level (usually 0.05)
- Type II error (beta): failing to reject null hypothesis when it's false (false negative)
- Power: 1 - beta; probability of detecting a true effect; usually aim for ≥80%
- Confidence interval (CI): range within which the true value lies with stated probability (usually 95%); if 95% CI for RR crosses 1, result is NOT statistically significant
Screening Criteria (Wilson-Jungner, 1968)
- Important health problem
- Accepted treatment available
- Facilities for diagnosis and treatment available
- Recognisable latent or early symptomatic stage
- Suitable screening test available
- Test acceptable to the population
- Natural history of condition understood
- Agreed policy on whom to treat
- Cost-effective
- Screening should be a continuous process
Clinical Trial Concepts
- Intention-to-treat (ITT): analyses all participants in the group they were randomised to, regardless of compliance; preserves randomisation; reduces bias; may underestimate treatment effect
- Per-protocol analysis: analyses only participants who completed the protocol; may overestimate treatment effect; introduces bias
- Blinding: single-blind (participant), double-blind (participant + investigator), triple-blind (+ analyst)
- Allocation concealment: different from blinding; ensures the person enrolling participants cannot predict group assignment
- Superiority trial: aims to show one treatment is better
- Non-inferiority trial: aims to show new treatment is not worse by a defined margin
- Equivalence trial: aims to show treatments are similar
Important Facts for MRCOG
- Sensitivity and specificity: NOT affected by prevalence; PPV and NPV ARE
- Case-control: measures odds ratio; efficient for rare diseases; prone to recall bias
- NNT = 1/ARR
- Lead-time bias: makes screening look effective when it may not be
- Type I error: false positive (rejecting true null); Type II: false negative
- ITT preserves randomisation; per-protocol may overestimate effect
- If 95% CI for RR crosses 1: not statistically significant
- Wilson-Jungner criteria: 10 criteria for screening programmes
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