Lecure 8

Question 1

Measures of prevalence relate to

Answer 1

Existing cases of disease

Question 2

Measure of incidence refer to

Answer 2

New cases of disease

Question 3

Prevalence is used up for measuring

Answer 3

• the disease in a community and is often measures in a discription study / survey

E.g: the proportion of students in this class who currently have a cold, the proportion of Otago students in this class who currently have a cold

• the frequency of risk factor for disease

E.g: the proportion of Otago uni students who use e-cigarettes

Question 4

Prevalence calculation

Answer 4

Question 5

Prevalence time line

Answer 5

Question 6

Prevalence definition

Answer 6

Prevalence is the proportion of people in a population who have the disease at a given point in time

• the time point may refer to calendar time, or to a fixed point in the course of events (relative to an event rather then an absolute time)

Question 7

What is incidence and what does is measure

Answer 7

Incidence measures the frequency of new cases of a disease
- useful for looking at the cause of disease

Question 8

What is cumulative incidence

Answer 8

• the proportion of people who become diseased during a specific period of time
• if follow-up over the time period is complete we can estimate cumulative incidence.

Question 9

(If follow up over time periods is complete) we can estimate cumulative incidence over the specified time period as:

Answer 9

Question 10

What does cumulative evidence provide?

Answer 10

• an estimate of the probability, or risk, the an individual will develop this disease during the specific period of time

Answer 11

Question 12

For culumulative incidence to be interpretable, what must be specified?

Answer 12

Time period

Question 13

Cumulative incidence with loss to follow up

Answer 13

Time to event analyses are able to take account of the censoring of follow-up. We use methods for censored data.
- people are included in the analysis up into the time they are lost to follow up
- bias can still occur if the people who are lost to follow-up are systematically different from those who remain in follow up

Question 14

What is the difference between these curves

Answer 14

• the curve on the left estimate the proportion who are still disease free at each time point
• the curve on the right estimates to proportion who have contracted the disease by each time point

If we calculated the cumulative incidence without allowing for loss to follow-up it would be 2/5 = 0.4 by the end of year 5
If we allow for the loss to follow-up using the Kaplan-Meier method, the cumulative incidence at the end of year 5 is greater then 0.4

Question 15

What also allows for differences in follow-up among participants

Answer 15

• we can compute an incidence rate
• it is the ratio of the total number of events to the total time on study and at risk over all study participants
  
  • person-time is the total time each person in the population is part of the study and at risk for the disease
• it is the same if we follow 16 people for one year or four people for four years, or one person for 16 years

Question 16

How to calculate the incidence rate

Answer 16

• the denominator for measures of incidence should only include those who are at risk of developing the disease and should exclude those who:
  
  • already have the disease
  • those who cannot develop the disease
    Failure to do this will lead to an underestimate of the tru incidence since fewer will develop the condition 3

Question 17

Incidence vs prevalence

Answer 17

Question 18

Prevalalnce depends on the:

Answer 18

• incidence rate, ASWELL as the duration of the disease