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. What would you like to do?
What types of bias are there?
- Information bias
- Recall bias
- Selection bias
What is information bias?
- Differences in measurement in the groups
- Correct people wrong info
What is recall bias?
Differences in the groups due to forgetting info
What is selection bias?
Wrong people correct info
What is confounding?
A factor associated with the exposure and disease of interest but not itself on the causal pathway
What is Incidence rate?
Number of new cases of a disease in a defined population in a defined period of time
What is prevalence?
Proportion of people affected in a defined population at a particular point in time
What is a cohort study?
- Identify based on exposure then monitor disease outcome
- Suitable for rare exposures
- Less prone to bias
- Problems with follow up
- Analyse with incidence rate ratio and smr for external comparison
- Can be prospective or retrospective
What is a case control study?
- Identify on disease outcome and compare exposure
- Suitable for rare diseases
- Problems with recall and selection bias
- Analyse using odds ratio then estimate relative risk
What is a randomised control trial?
- Comparison of two treatments interventions or a placebo chance determins who gets what and it is planned
- Can be ethical problems
What is a systematic review?
- Overview of primary studies that used explict and reproducible methods
- Uses bradford hill criteria
What is a meta-analysis?
Mathematical synthesis of two or more primary studies that addressed the same hypothesis in the same way
What are the advantages of a systematic review?
- Limit bias
- Increase prescision of overall result
- Large amounts of info can be understood quickly
- Results of studies can be compared
- Reasons for inconsistencies can be identified
Limitations of systematic reviews?
- Dependant on quality of primary studies
- Susceptible to publication bias
- Reader still needs to consider generalisability of the result
What is the observed incidence rate ratio?
=observed incidence rate in pop 1/ observed incidence rate in pop 2
What is the odds ratio?
- Measure of the risk of disease in one group compared to another
- =exposed disease x unexposed no disease/unexposed disease x exposed no disease
What is the standard mortality ratio?
- = (observed/expected)x100
- Smr >100 means more incidence than expected
What is the error factor?
- Measure of random variation
- Large error factor can be due to small sample size
- = exp(2xsquare root[(1/d1) + (1/d2)])
What is the 95%confidence interval?
= from the obseved value / error factor to the observed value x by the error factor
What is the p-value?
- A measure if how likely the results are due to chance
- If 95% CI excludes the null hypothesis value of 1...
- reject null hypothesis
- There is evidence of a difference
- Result is not due to chance
What is evidence base medicine?
- Use of best current evidence in making descisions about the care of individual patients
- Involves the integration of best clinical expertise best evidence from research and patients unique values and circumstances
What are the levels of evidence?
- Systematic review of RCTs
- Well designed trial
- Well designed non experimental study
- Respected authority
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