Risk

Situation: there is a defined population and each member of the population will be followed for a specific period of time to see if they experience an outcome of interest.

Risk = the proportion of people in the population who experience the outcome within the specified time period

• Because it’s a proportion, risk ranges from zero to 1 (or 0% to 100%).
• The population being followed are disease-free (event-free) at the start of the time period for when they are followed.
• The outcomes counted are those that are new cases (i.e., incident cases).
• Risk is also known as cumulative incidence or incidence proportion (how many new cases “accumulated’ during the year / 5 years / 10 years we followed this population?”)

Example.  Hospital readmissions are an example of a quality of care indicator that is often referred to as a “rate” but is essentially a risk.  There are variations, but many readmission measures start with a group of patients admitted to an acute care hospital (the initial hospitalization is called the “index” admission).  Admissions that involve transfer to another acute care hospital, planned readmissions (e.g., staged procedures) and sometimes admissions that end when the patient leaves against medical advice, are excluded.  The date of discharge starts the follow-up “time clock,” which ends when either the patient is readmitted to an acute care hospital or the chosen number of days to follow up has been reached (often 30, 60 or 90 days).