Epidemiology and clinical evidence
Introduced| English |
|---|
| incidence/ˈɪnsɪdəns/ |
| prevalence/ˈprevələns/ |
A decision before an answer
- A thousand existing cases and a hundred new cases answer different questions; mixing them up misreads every health statistic.
- Your goal: Distinguish incidence from prevalence and compute each.
Separate stock from flow
- Incidence counts NEW cases in a period (number of new diagnoses in a year); prevalence counts ALL existing cases at a time.
- Chronic conditions can have low incidence and high prevalence; short acute outbreaks show the reverse. A count of new cases in the United States in the past year is an incidence figure.
A researcher counts the number of new schizophrenia diagnoses in a country during the past year. This statistic is the:
New cases in a period are incidence; all existing cases would be prevalence.
Give rates denominators
- Rates need denominators: incidence rate is new cases divided by person-time at risk; prevalence is existing cases divided by the population.
- Comparing raw counts across populations of different sizes misleads; compare rates.
Which design follows exposed and unexposed groups forward in time to see who develops the outcome?
Cohort designs move forward from exposure to outcome; case-control designs look backward.
Climb the evidence ladder
- Case reports, case-control studies, cohorts, randomized trials and meta-analyses answer different questions. Random allocation can reduce baseline confounding under appropriate implementation; observational designs need explicit confounder analysis.
- Quality, bias, relevance and uncertainty must be evaluated within each design. A meta-analysis cannot automatically repair weak constituent studies.
Counting this year's new schizophrenia diagnoses measures incidence; counting everyone currently diagnosed measures prevalence. A 50% relative risk reduction from 2% to 1% is 1 point absolute. A rare-condition screen with many false positives follows directly from the low base rate.
All existing cases of a condition in a population at one time give its ____.
Prevalence is the stock; incidence is the flow of new cases.
Read risk against baseline
- Screening and risk run on base rates: with low prevalence, even an accurate test yields mostly false positives among positives.
- Number needed to treat and relative versus absolute risk are read against the baseline, not from the treatment label alone.
Reporting a new-case count as prevalence, or quoting relative risk without the absolute baseline.
Which answer fits this case?
Distinguish incidence from prevalence and compute each
A condition can have low incidence and high prevalence at the same time.
Chronic conditions accumulate: few new cases each year but many existing ones.
Keep the distinctions
- incidence 发病率(新发病例) — The number or rate of NEW cases arising in a period.
- prevalence 患病率 — The proportion of a population with the condition at a given time.
- Distinguish incidence from prevalence and compute each.
- Evaluate clinical evidence by design strength.
- Interpret risk, screening and base rates in populations.
Match each term with its precise meaning in this lesson.
Keep the distinctions stated in the teaching example.
Put this lesson’s reasoning or event sequence in order.
The order follows the stated process; check each stage before the next.