Day 2 Flashcards
What is the prevalence of a disease?
Prevalence is the proportion of people who have a disease at a given point in time.
- It counts the number of people with existing disease (both old and new)
- Takes a ‘snapshot’ at a given point in time
- Describes the ‘burden of disease’
- Useful to determine resource/service allocation
What is the incidence of a disease?
This is the number of new cases of a disease within a given timeframe.
- It focuses on NEW cases only
- Useful when monitoring epidemics eg the number of new cases of the zika virus in the Americas.
When taking a sample, what three things does it need to be?
Representative
Unbiased
Precise
What are the two types of error they may occur in a study?
Random error (chance)
- This is due to sampling variation
- This will reduce as sample size increases because precision will be increased (reduced uncertainty)
Systematic error (bias)
- Bias is quantified by the difference between the true value and the expected value
- It DOES NOT reduce as sample size increases
What are some sources of bias?
You get selection biases and information biases.
Selection biases incude:
-Study sample (external validity).
For example, this could be where you conduct the study. It is when the study sample is not representative of the entire population of interest.
-Group selection within a study (internal validity).
This means that groups within a study may not be comparable. You have to ensure that other factors are the same in both populations.
-Healthy worker effect
Workers usually exhibit lower overall morbidity then the general population (eg severely ill/disabled are usually excluded form employment.
Information biases include:
-Recall error
Differences in recollection from study participants regarding events or experiences from the past
-Observer or interviewer error
Study observer/interviewer may have preconceived expectations or knowledge that may influence the result
-Measurement error
Differences in the measurement of participants
-Misclassification
Participants are put in the wrong group (eg diseased when they are not). This usually arises form observational or measurement error
How is prevalence often reported?
Prevalence is often reported as a proportion (not a rate!).
Prevalence = No. of people with the disease / Total population.
How is incidence often reported?
Incidence is often reported as a rate (eg 50 per 100,000 person years)
Incidence = Number of new cases / Person time at risk = events per person per year
What is the equation that links incidence and prevalence?
Prevalence = Incidence x Duration
What is the incidence rate ratio?
The incidence rate ratio compares the incidence rate in one group to another.
IRR = indicence rate in group A / incidence rate in group B
The IRR is a RELATIVE measure between two groups. We have use relative measure such as IRR to compare 3 groups for other factors such as mortality rate ratios.
What is the difference between relative rate and relative risk?
Relative risk is the underlying quantity we wish to approximate whereas the relative rate approximates the relative risk if the underlying disease is rare, or if the time period is short.
What is the equation to work out the odds of an event happening?
If the probability of an event is p then the odds of that event is given by p / 1-p
This is the odds of an event in a single group.
We can use odds in a medical setting but it is more common to use the odds ratio to compare two exposure groups.
It is a RELATIVE comparison of the odds of disease.
What are confounding variables? Why are these an issue?
When comparing groups, the association to effect between an exposure sand outcome is distorted by the presence of another variable.
Eg when looking at the association between gender and cancer of the mouth and pharynx, you must account for alcohol consumption because men are more likely to drink more alcohol and alcohol is a risk factor in cancer of the mouth and pharynx.
What do you do about confounding variables?
You can adjust for differences in known confounding factors. A common type of adjustment is standardisation. This form of analysis uses weighted averages to allow us to compare like for like. However, it is difficult to control confounding factors as many of them are unknown!
How do you conduct an ecological study?
- Identify groups to study
- Define the characteristics to be studied: the exposure and the outcome
- Decide whether analysis is to involve categorical or continuous data
- Gather data on the group-level characteristics
What issues are there with ecological studies?
-Definiton of characteristics
-Measurement variation
-Confounding variables -ecological fallacy (falsely inferring individual-level association from group-level association
CHANCE!
