Statistics and Probability Discussion

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Questions and Answers

What percentage of participants lost to follow-up is typically ignored in the analysis stage?

10%

In what situations is risk considered more valid as a measure?

When follow-up period is short

How does cumulative risk differ from other measures of incidence?

Looks at new cases of disease

What is a factor that affects the validity of using risk as a measure?

High competing risks Signup and view all the answers

Why is cumulative risk considered to have limitations?

It ignores loss to follow-up Signup and view all the answers

What is the formula to calculate prevalence ratio?

Prevalence in the exposed divided by prevalence in the unexposed Signup and view all the answers

Which measure of association is also known as cumulative risk?

Risk ratio Signup and view all the answers

What is the potential drawback of using prevalence ratio as a measure of association?

It is not commonly used in epidemiology Signup and view all the answers

How does loss to follow up in a research study impact the validity of a risk ratio?

Does not affect the validity of the risk ratio Signup and view all the answers

What factor can affect the validity of a risk measure like odds ratio?

Rate of loss to follow up Signup and view all the answers

What is another term used to refer to the incidence density rate mentioned in the text?

Percent time rate Signup and view all the answers

How is the denominator in the formula for incidence rate calculated?

Total time at risk Signup and view all the answers

In research studies, participants are considered at risk until which of the following occurs?

All of the above Signup and view all the answers

Why might it be challenging to calculate the denominator in the formula for incidence rate?

Varying contributions of person time Signup and view all the answers

What assumption is made regarding participants in relation to their risk for a disease in the text?

They are at risk until they develop the disease or meet specific criteria Signup and view all the answers

What is the main limitation of using risk as a measure in research studies?

It does not account for loss to follow up in the denominator. Signup and view all the answers

How does incidence rate differ from cumulative incidence in research studies?

Incidence rate accounts for loss to follow up while cumulative incidence does not. Signup and view all the answers

What does the numerator represent in both cumulative incidence and incidence rate measures?

The same number of new cases. Signup and view all the answers

How does accounting for total time at risk affect the validity of research study results?

It addresses the issue of loss to follow up in a study. Signup and view all the answers

Why is it important to differentiate between risk and incidence rate in research studies?

To minimize loss to follow up in the analysis. Signup and view all the answers

What is the impact of loss to follow up in a research study?

It decreases the validity of cumulative risk as a measure Signup and view all the answers

In a two by two table, what does 'B' represent?

Number of people without the disease but are still exposed Signup and view all the answers

Which measure ranges from zero to positive infinity and has no units?

Odds ratio Signup and view all the answers

What does the risk in exposed over risk in unexposed represent?

Risk measure affected by loss to follow up Signup and view all the answers

What is the range for incidence rate ratio?

From zero to positive infinity Signup and view all the answers

In the text, why is it important to mention the time period when calculating the one-year incidence of asthma among adults with dementia?

To help differentiate between the incidence in one year and cumulative risk over time Signup and view all the answers

What could be a potential consequence of not adjusting for participants who leave a study during analysis?

Overestimating the actual risk in the population Signup and view all the answers

Why is the idea of cumulative risk not valid when a large number of people leave a study?

Cumulative risk includes individuals who may no longer be at risk Signup and view all the answers

What impact does loss to follow-up have on participant recruitment numbers in a study?

Increases the need for higher initial recruitment numbers Signup and view all the answers

How does excluding individuals who already have asthma at the start of a study impact the estimation of one-year incidence of asthma?

Underestimates the actual one-year incidence rate Signup and view all the answers

What is the main reason for excluding individuals from the denominator in cumulative risk calculation?

They already have the disease Signup and view all the answers

In calculating cumulative risk, what does the numerator represent?

Number of new cases Signup and view all the answers

Why are males often excluded from being considered 'at risk' for certain diseases like ovarian cancer?

They lack the organ associated with the disease Signup and view all the answers

Which measure is also known as incidence proportion in epidemiology?

Cumulative risk Signup and view all the answers

What is the primary difference between cumulative risk and incidence rate?

The time period considered in the calculation Signup and view all the answers

What is the denominator in a cumulative risk calculation?

Number of participants at risk of disease at the beginning of the study period Signup and view all the answers

Why do researchers exclude people who cannot get a disease from the denominator?

To ensure validity of the measure. Signup and view all the answers

What is one reason males may be excluded from being 'at risk' for ovarian cancer?

They lack a certain organ necessary for ovarian cancer. Signup and view all the answers

Study Notes

Research Study Considerations

In research studies, a common goal is to recruit around 400 participants, anticipating a 2-10% loss to follow-up, which will be ignored in the analysis if the pre-decided threshold is not met.
In reality, some participants will always drop out of the study for various reasons.

Cumulative Risk and Incidence Rate

Cumulative risk is a measure of incidence that looks at new cases of disease, but it has limitations because it doesn't account for loss to follow-up in the denominator.
Incidence rate addresses this limitation by accounting for the time each participant is at risk of developing the disease.
The numerator for both measures is the number of new cases, but the denominator for incidence rate is the total time at risk.

Measures of Association

Measures of association quantify the relationship between exposure and disease, and can be reported on either an absolute or relative scale.
The relative scale shows the relative increase or decrease in the effect of exposure on the outcome, while the absolute scale shows the absolute increase or decrease.

Types of Measures of Association

Prevalence ratio: the ratio of prevalence in the exposed group to the prevalence in the unexposed group.
Risk ratio: the ratio of cumulative incidence in the exposed group to cumulative incidence in the unexposed group.
Incidence rate ratio: the ratio of incidence rate in the exposed group to incidence rate in the unexposed group.
Odds ratio: the ratio of odds in the exposed group to odds in the unexposed group.

Calculating Incidence Rate

Incidence rate is calculated by dividing the number of new cases by the total time at risk.
The total time at risk is the sum of the time each participant is at risk of developing the disease.

Assumptions and Considerations

When calculating time at risk, assumptions are made that participants are at risk of developing the disease until they have the disease, die, are lost to follow-up, or the study ends.
The denominator for incidence rate should only include participants who are at risk of developing the disease.

Example: Calculating Incidence

A two-by-two table can be used to calculate measures of association.
The table should have disease status on top (disease vs. no disease) and exposure status on the side (exposed vs. not exposed).
The numbers in the table can be used to calculate incidence, risk ratio, incidence rate ratio, and odds ratio.

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