Session 2 Lecture 1: Measurement-Scales, Numbers, Rates, Ratios and Risk Flashcards
What are the 3 key features of the sample population taken to make an inference about the entire population?
- Representative
- Unbiased
- Precise
State the two type of error that can occur in a study that may influence the results
- Chance (Random error)
- Bias (Systematic error)
How does Chance (Random error) occur?
- Due to sampling variation
- will reduce as sampling size increases
How does Bias (Systematic error) occur?
- Quantified by the difference between the true value and the expected value
- Does not reduce as sample size increases, bias remains the same
Where does the bias come from that leads to Systematic error?
- Selection biases
- Information biases in the data
Examples of selection biases which are a source of bias that can lead to Systematic error
-Study Sample (External Validity)
not representative of pop of interest
-Group selection within a study (Internal Validity)
groups within a study may not be comparable
-Healthy worker effect
workers usually exhibit lower overall mortality than the general pop
Examples of Information biases in the data which are a source of bias that can lead to Systematic error
-Recall error
differences in recollection from study participants regarding events or experiences from the past
-Observer/interviewer error
study observer/interviewer may have preconceived expectations or knowledge that may influence the result
-Measurement error
Differences in the measurements of participants
-Misclassification
participants put in wrong group
Define the term Prevalence
The proportion of people who have a disease at a given point in time
Calculating prevalence
Number of people with disease / total population (number of people)
Define the term Incidence
The number of new cases of a disease within a given timeframe
Calculating Incidence rate
Number of new cases / sum of the patient time at risk
- patient time at risk = Sum of all the patients times at risk
often reported as a rate: events per person per year (50 per 100000 person-years)
What is the Incidence Rate Ratio used for?
- To compare the incidence rate in one group to another
- relative measure between 2 groups
IRR = incidence rate in group A / incidence rate in group B
Group A= group you are interested in
Group B=comparing group
Calculating Odds
If the probability of an event is p then the odds of that event is given by:
Odds=p/1-p
What is the ratio of odds used for?
To compare two exposure groups
What is the Odds Ratio?
A relative comparison of the odds of an event happening in Group A compared to Group B
“The ratio of ratios”
How is the Odds ratio calculated?
odds of group A = a/b
——————————– = ad/bc
odds of group B= c/d
How is the absolute risk calculated?
Group A = a / a+b
Group B = c / c+d
Calculating relative risk
Absolute risk for group A/Absolute risk for group B
What is the null value for the Odds ratio and relative risk?
1
What is the interpretation if the Odds ratio (OR) and relative risk (RR) are < 1 respectively?
OR < 1 = numerator has a lower odds of the event
RR < 1 = numerator has a lower risk of the event
What is the outcome of the event when the desirable interpretation of odds ratio and relative risk is < 1?
Desired if the event is a bad outcome
What is the interpretation if the Odds ratio (OR) and relative risk (RR) are > 1 respectively?
OR > 1 = numerator has a greater odds of the event
RR > 1 = numerator has a greater risk of the event
What is the outcome of the event when the desirable interpretation of odds ratio and relative risk is >1?
Desired if the event is a good outcome
Calculating the Risk difference
Absolute risk in Group A - Absolute risk in Group B
=Absolute difference
