Outcome Frequency Measures

Question 1

Define Frequency

Answer 1

The number of occurrences in a defined population over a defined period

Question 2

What are the Steps to determine frequency?

Answer 2

1. Define a case:
   - individuals with the outcomes of interest
   - case definition: the criteria that need to be met to be counted as a case (does not have to be clinically defined)
2. Define a Population
   - Who is the target pop? - who’s health do you want to investigate
   - Who is the study pop? - the people with specific and relevant characteristics e.g.Aged 18-20 living in london

• Sample pop? : sample of people from the study population

3. Define the population at risk (not always known)
   - This is only the individs who could develop the outcome.

E.g if the outcome is pregnancy, only women of childbearing age could be counted.

Question 3

Define Prevelence?
Define Formula and unit of measurement?
Whats it used for?

Answer 3

Number of existing cases in a defined population at a defined point in time in % (usually)

Formula: Total Cases / Total pop at risk (or pop)

Not useful for quick outcomes
Useful for chronic diseases etc.

PRESENT, POINT

Question 4

Define Incidence
Define formula and unit of M?
Whats it used for?

Answer 4

Number of new cases in a defined population over a defined period of time. in % (usually)

Formula: New cases / total pop at risk (or pop)

• Better measure for studying causation
• used to calculate risk, odd and incidence rate

INCOMING, PERIOD

Question 5

Define Risk (cumulative incidence)
Formula and unit of measurement?
When would you use Risk? (what type of pops?)

Answer 5

The prob that a person will develop an outcome during a specified time period (NB time period). Valued between 0 & 1.

Formula: No of new cases in a specified time period
/ Total pop at risk at the beginning of the time period

Note: the pop at risk excludes existing cases at the start of the time period. IE ONLY USE THE POP WHO DO NOT HAVE THE OUTCOME

Used in closed pops ie. when they are followed over a period of time and those included at the beginning are also included at the end.

Question 6

Define Secondary Attack Rate

Formula ?

Answer 6

The prob that a contact of a case will develop the outcome over a period of time.

Formula: No. of new cases among contacts / total number of at risk contacts in a time period

Question 7

Define Odds
Formula and measurement?
When would you use odds (what type of pop?)

Answer 7

The probability of becoming a case vs. the prob of not becoming a case in a ratio

Formula: New cases / Non cases

Notes: not used very often in Epi

Used in closed pops ie. when they are followed over a period of time and those included at the beginning are also included at the end.

Question 8

What is an open population?

Answer 8

Where people enter and exit at different points

Question 9

What is an incidence RATE reported in?

Answer 9

No. of cases per person-time at risk

Total new cases / total pop at risk during the specified time period

Use a midpoint to estimate population at risk

EXAMPLE if time period is 5 years, use the population at 2.5 years.

Question 10

What are measures of frequency good for?

Answer 10

Assessing PH importance of an outcome

Predict burden on HC services and plan in response