PSM/Community Medicine Formulae for FMGE

Essential PSM/Community Medicine Formulae with Calculation Clues

Category	Formula	Clue to Calculate / What it Measures
I. Measures of Disease Frequency
1. Incidence Rate	Incidence Rate = (Number of new cases of disease during a specified period / Population at risk during the same period) * K (e.g., 1000)	New Cases / Population at Risk: Measures how fast new cases are appearing in a healthy population. “Risk” refers to those who *could* get the disease. Think of it as the “speed” of disease development.
2. Point Prevalence	Point Prevalence = (Number of existing cases of disease at a specified point in time / Total population at that specified point in time) * K (e.g., 1000)	Existing Cases / Total Population at a Specific Moment: A “snapshot” of the disease burden at one precise time. Often used for chronic diseases.
3. Period Prevalence	Period Prevalence = (Number of existing cases (new + old) during a specified period / Average population during the same period) * K (e.g., 1000)	Existing Cases (over a period) / Average Population: Measures the total burden over a time interval (e.g., a year). Includes cases existing at the start of the period plus new cases that developed during the period.
4. Attack Rate	Attack Rate = (Number of new cases among exposed individuals during an outbreak / Total number of exposed individuals) * 100	New Cases in an Outbreak / Exposed Population: Specifically for outbreaks. Tells you the proportion of *exposed* people who got sick. Always expressed as a percentage.
II. Mortality Rates
5. Crude Death Rate (CDR)	CDR = (Total number of deaths in a year / Mid-year population) * 1000	Total Deaths / Mid-Year Population: Overall death rate for the entire population, irrespective of age, sex, or cause. Mid-year population is often (Start Pop + End Pop) / 2.
6. Specific Death Rate	Specific Death Rate = (Number of deaths from a specific cause / in a specific group / Mid-year population in that specific cause / group) * 1000	Deaths in a Specific Group / Population in that Specific Group: More detailed than CDR, focusing on specific causes (e.g., cancer), age groups (e.g., age-specific), or sexes.
7. Case Fatality Rate (CFR)	CFR = (Total number of deaths from a specific disease / Total number of cases of that same disease) * 100	Deaths from Disease / Total Cases of Disease: Measures the *severity* or *virulence* of a disease. Proportion of diagnosed cases that die from it. Always a percentage.
8. Infant Mortality Rate (IMR)	IMR = (Number of deaths under 1 year of age in a year / Total live births in the same year) * 1000	Deaths < 1 Year / Live Births: A key indicator of a community’s health and socioeconomic development.
9. Neonatal Mortality Rate (NMR)	NMR = (Number of deaths under 28 days of age in a year / Total live births in the same year) * 1000	Deaths < 28 Days / Live Births: Focuses on mortality in the first month of life, reflecting obstetric care and neonatal health.
10. Under-Five Mortality Rate (U5MR)	U5MR = (Number of deaths under 5 years of age in a year / Total live births in the same year) * 1000	Deaths < 5 Years / Live Births: Broader child health indicator, reflecting nutrition, immunization, and access to basic healthcare beyond the neonatal period.
11. Maternal Mortality Ratio (MMR)	MMR = (Number of maternal deaths in a year / Total live births in the same year) * 100,000	Maternal Deaths / Live Births (per 100,000): Measures risk of death from pregnancy-related causes. Expressed per 100,000 live births.
III. Measures of Association
12. Relative Risk (RR)	RR = Incidence in Exposed Group / Incidence in Unexposed Group	(Ie / Io): Used in Cohort Studies. How many times more (or less) likely are exposed individuals to develop the disease compared to unexposed individuals? RR > 1 means exposure is a risk factor. RR < 1 means exposure is protective. RR = 1 means no association.
13. Odds Ratio (OR)	OR = (Odds of exposure in cases / Odds of exposure in controls) = (a*d) / (b*c) (from 2×2 table)	(ad/bc): Used in Case-Control Studies. Approximates RR for rare diseases. Indicates the strength of association between exposure and disease.
14. Attributable Risk (AR) / Risk Difference (RD)	AR (RD) = Incidence in Exposed Group – Incidence in Unexposed Group	Ie – Io: Used in Cohort Studies. Measures the absolute amount of disease that can be attributed to the exposure, or the extra risk of disease due to exposure. How many fewer cases would there be if the exposure was removed?
IV. Diagnostic Test Evaluation
15. Sensitivity	Sensitivity = (True Positives (TP) / (True Positives (TP) + False Negatives (FN))) * 100	TP / (TP + FN): The ability of a test to correctly identify those *with* the disease. (A positive test means you likely have the disease). “Rule In” test when positive.
16. Specificity	Specificity = (True Negatives (TN) / (True Negatives (TN) + False Positives (FP))) * 100	TN / (TN + FP): The ability of a test to correctly identify those *without* the disease. (A negative test means you likely *don’t* have the disease). “Rule Out” test when negative.
17. Positive Predictive Value (PPV)	PPV = (True Positives (TP) / (True Positives (TP) + False Positives (FP))) * 100	TP / (TP + FP): The probability that a person with a *positive* test result actually has the disease. Influenced by disease prevalence.
18. Negative Predictive Value (NPV)	NPV = (True Negatives (TN) / (True Negatives (TN) + False Negatives (FN))) * 100	TN / (TN + FN): The probability that a person with a *negative* test result actually does not have the disease. Influenced by disease prevalence.
V. Other Key Calculations
19. Vaccine Efficacy (VE)	VE = ((Incidence in Unvaccinated – Incidence in Vaccinated) / Incidence in Unvaccinated) * 100 OR VE = (1 – Relative Risk) * 100	(Iu – Iv) / Iu: Measures the proportional reduction in disease incidence among vaccinated persons. How much does the vaccine *prevent* the disease?
20. Number Needed to Treat (NNT)	NNT = 1 / Absolute Risk Reduction (ARR) Where ARR = Incidence in Control Group – Incidence in Intervention Group.	1 / ARR: The average number of patients who need to be treated to prevent one additional bad outcome (e.g., one case of disease). Lower NNT is better.
21. Dependency Ratio (Total)	Total Dependency Ratio = ((Population 0-14 years) + (Population 65+ years) / Population 15-64 years) * 100	(Young + Old) / Working Age: Measures the burden on the productive (working-age) population. Can be further broken down into Youth and Old-Age dependency ratios.
22. Chlorination Dose	Chlorination Dose = Chlorine Demand + Desired Residual Chlorine	Demand + Residual: The total amount of chlorine needed to be added to water to ensure both disinfection and a protective residual level.