Epi - Lecture 2 - Frequency, risks, and associations - 9/1

Prevalence

• includes both new cases and those who contracted the disease in the past and still have the disease → at a specic time → accumulated amount of cases of a disease.

• prevalence often preferred in chronic conditions and important info for pharmacies (how many drugs are needed).

Explain this figure

• left; amount of new cases (incidence) is low/ stable, but the accumulated amount of cases of AIDS is increasing, because people can live longer with AIDS

• usually prevalence and incidence come closer together over time → which means people die close after contracting a disease or they get cured

• right;

  • High prevalence: reflects high incidence or prolonged survival without cure or both.

  • Low prevalence: indicates low incidence, a rapidly fatal process or rapid recovery

studying prevalence

• study designs: cross-sectional is most obvious (because you don’t follow patients over time), but also suitable for cohort and RCTs → definitely NOT in case-control studies (bc cannot match prevalence with controls).

calculations prevalence

incidence

• new cases over a certain period

what is the difference between cumulative incidence and incidence rate

• cumulative incidence → measures the proportion of individuals in a population who develop the disease over a specific period

• incidence rate → measures the rate at which new cases occur in a population over time.

studying cumulative incidence

• always prospective

• cumulative incidence can be calculated → probability of getting disease

incidence rate / incidence density

• new cases relative to the population at risk.

• person time at risk = total amount of time contributed by each individual while he / she remained at risk of developing the disease

what are effect measures

• also called measures of risk and association

• effect measures are used to quantify the association between an exposure and an outcome

• These measures help assess whether and how much an exposure influences the risk of an outcome

risk difference

• the risk of outcome among a group minus the risk of outcome among another group

• risk outcome group 1 - risk outcome group 2

risk ratio

• relative risk

• the probability (risk) of an outcome occurring in the exposed group compared to the unexposed group

• incidence 1 / incidence 2

formula risk ratio

outcomes risk ratio and risk difference

• (close to) 1 → exposure is not associated with the outcome

• >1 → exposure might be a risk factor for the outcome

• <1 → exposure might be a protective factor against the outcome

odds ratio

• compares the odds of an event occurring in the exposed group to the odds of the event occurring in the unexposed group

• for cohort the odds ratio can be calculated but it is not preferred (rr is better)

possible outcomes odds ratio

• (close to) 1 → exposure does not affect the odds of outcome

• >1 → exposure is associated with higher odds of the outcome

• <1 → exposure is associated with lower odds of the outcome

formula odds ratio