Bias and Confounding Flashcards
What is Bias?
Inclination or prejudice for or against one person or group, especially in a way considered to be unfair
What is Error?
Error is the difference between the measurement and the ‘true value’
What are the 2 Types of Error?
Systematic and Random Error
When does Systematic Error Occur?
Systematic error arises when we do not include all groups of people, meaning certain groups are left out. There is a difference between the people in the study and those wuho are not (e.g. only studying rich and not poor, or men and not women). Therefore, the cohort is not representative of the whole population
Systematic Error Occurs where there are limitations in what?
Limitations in the study design, in how patients were recruited, in how the data was collected/analysed
What is a problem with Systematic Error?
Bias will persist even if we increase the sample size. We cannot correct for this type of error using statistical methods or mathematics, making it hard to determine the real effect of this error
Systematic Error is ____ but not _____
Systematic Error is Precise but not Accurate
What is Random Error?
Occurs due to random chance. (e.g. measuring your weight three times, and getting three different values); these differences are due to random error.
Random error changes the variability and distribution of your data, but not the average (i.e. the values will be more all over the place, but the average will come out to be the same).
Random Error is ____ but not _____
Random Error is accurate but not precise
What is an Advantage of Random Error?
The good thing about random error is that we can use statistics to see the effect that it has on our results. We use p-values, hypothesis testing and confidence intervals to determine whether or not any differences in the results are purely due to random error, or are legitimate.
How do we Minimise the Effect of Random Error?
We can evaluate this using statistical methods, and reduce by increasing data/sample size.
What causes Bias?
What does it result in?
How can we minimise this Bias?
Systematic error causes bias towards a particular group.
It results in a lack of accuracy, as all data will be either greater than or less than the true value.
We can try minimise this bias by following the CONSORT principles.
What are the 3 Types of Bias?
Selection Bias
Measurement Bias (or misclassification or information bias)
Confounding Bias
What is Selection Bias?
What is an Example?
Selection bias occurs when there is a different selection of people in the control group vs the test group, hence causing different results in each group
An example would be if in the control group, there was 70% males, but in the test group, there was 30% males. Selection bias has occurred; we would rather there be the same proportion in each. It will arise if you do not properly randomise the cohort, or properly exclude or include certain people
What are Subtypes of Selection Bias?
Attrition bias (e.g. when people drop out due to death or other reasons)
Healthy worker bias (only selecting healthy people)
Nonresponse bias (people not completing a survey)