lecture 8

Question 1

Measures of prevalence relate to

Answer 1

Measures of prevalence relate to existing cases of disease

Question 2

Measure of incidence refers to

Answer 2

Measure of incidence refer to new cases of disease (how frequently a disease pops up)

Question 3

Prevalence

Answer 3

Prevalence measures the frequency of existing cases of disease. It is useful for measuring the disease burden in a community and is often measured in a cross-sectional survey.
Example: The proportion of students in this class who currently have a cold.
Example: The proportion of Otago students who had flu at 3pm last Tuesday.

Prevalence may also measure the frequency of a risk factor for disease.
Example: The proportion of Otago University students who use e-cigarettes.

prevalence is a cross sectional measure and you measure it at a particular time

Prevalence is a snapshot in time

Question 4

Prevalence features to be aware of

Answer 4

Easy to measure

only a snapshot for the defined time period

example of a disease that can be cured - prevalence is only when people.le have the disease at a give time, so a measurement at a particular time may miss people who develop the disease a short time after/ or were cured a short time before

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. - can refer to a relative time

Example: the proportion of people free from back pain 2 months after back injury. The time point here is relative to an event, rather than an absolute time. - injure at different time, measure prevalence at a specific time point

Question 5

Prevalence vs incidence

Answer 5

Prevalence refers to proportion of persons who have a condition at or during a particular time period, whereas incidence refers to the proportion or rate of persons who develop a condition during a particular time period.

Question 6

Incidence

Answer 6

Incidence measures the frequency of new cases of a disease. As such, it is useful for looking at the causes of disease.

passage of time is important to this measurement, gives you impression that time is flowing forwards

Example
What is the risk of someone in this lecture theatre developing a cold in the coming week?
What is the rate of new cases of influenza among STAT115 students?

Question 7

Cumulative incidence

Answer 7

Cumulative incidence is the proportion of people who become diseased during a specified period of time.

This provides an estimate of the probability, or risk, that an individual will develop the disease during the specified period of time.

The period of time could be one day, one week, one year, five years etc.

CI Is talked about with respect to time moving/interpreted with respect to the particular time period which is how it is different to prevalence

Question 8

Cumulative equation

Answer 8

If follow-up over the time period is complete (so we know what happened to everyone) we can estimate cumulative incidence over the specified time period as:

CI = number of new cases of disease during time period / number of individuals at risk at the start = __/__ over ___ years/hours/seconds etc

for the cumulative incidence to be interpretable, the time period must be specified

Question 9

Cumulative incidence with loss to follow up

Answer 9

The previous cumulative incidence calculations assume that everyone in the sample is followed up for the same duration.Often, in practice, some people are lost to follow up, and people enter the study at different times. We need to allow for this variable follow-up in our measure of cumulative incidence

right censored data

now you specifify total time at risk since it is no longer the same entire observation period, figure it out in time units

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 until the time they are lost to follow-up
Details of this method are not covered in this course, but the following curves (Kaplan-Meier curve and cumulative incidence curve) illustrate the interpretation of the estimate
Bias can still occur if the people who are lost to follow-up are systematically different from those who remain in follow-up (informative censoring)

Question 10

Left vs right censored data

Answer 10

Right censoring occurs when a subject leaves the study before an event occurs, or the study ends before the event has occurred. … Left censoring is when the event of interest has already occurred before enrolment.

Question 11

Kaplan-Meier curve shows …

Answer 11

It estimates the proportion who are still disease free at each time point

Kaplan-Meier curve, and it shows what the probability of an event (for example, survival) is at a certain time interval.

Question 12

Cumulative incidence curve shows…

Answer 12

estimates the proportion who have contracted the disease by each time point

Question 13

Kaplan-Meier curves and loss to follow up

Answer 13

Note that if we calculated the cumulative incidence without allowing for loss to follow-up it would be 2/5=0.4 by the end of Yr 5.

If we allow for the loss to follow-up using the Kaplan-Meier method, the cumulative incidence at the end of Yr 5 is greater than 0.4. - greater than 0.4 because there is less overall study time due to the individuals lost to follow up therefore less years total with no followup, this method takes into account that we haven’t been following every individual for the same amount of time

Question 14

Incidence rate

Answer 14

also allows for differences in follow-up among participants. It is the ratio of the total number of events to the total time on study and at risk over all study participants

instantaneous measure at a given point in time

Scale time with regard to loss to follow up so that we end up with a measure to compare (level the playing field)

Question 15

Incidence rate equation

Answer 15

IR = number of new cases of disease/ total person-time at risk = ______ case per person per _____ (years, hours, minutes, months) etc of observation

note - existing cases are not counted, NEW cases ONLY

Question 16

Person-time at risk is the ….

Answer 16

the total time each person in the population is

• part of the study, and`
• at risk (for the disease being studied).

provides a measure to compare groups when they have different levels of loss to follow up

it is a number/scale giving total time in study AND total time at risk of developing the disease being studied

can be hours/minutes/ years depending on the time unit

Question 17

Notes on the denominator for incidence rate

Answer 17

The denominator for measures of incidence should include only those who are at risk of developing the disease. It 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 true incidence since fewer will develop the condition.

For example when studying the incidence of endometrial cancer we should exclude women who have had a hysterectomy. - because no longer at risk for this disease therefore need to be excluded

Question 18

Prevalence is dependent on

Answer 18

disease duration and incidence rate

Question 19

cumulative incidence takes into account

Answer 19

the duration/passage of time

Question 20

Low incidence and long duration example in terms of prevalence

Answer 20

Adult onset diabetes has a low incidence rate but a long duration, as the disease is neither curable nor fatal. Hence prevalence is high relative to incidence.

Question 21

High incidence and short duration example in terms of prevalence

Answer 21

A cold has a (very) high incidence, but the duration is short, so prevalence is low relative to incidence.