Epidemiology 2.6 Flashcards
How can sample size be determined?
several mathematical equations – depending on the type of study you’re running and the variables involved - while you don’t need to know how to calculate the sample size at this stage of training by hand, you should be aware of the factors that influence sample size
What are the three main statical factors that will push you sample size up or down?
- Difference between groups the you would be looking at
- The study power
- The study alpha
How does the difference between the groups you are looking at affect sample size?
- For example, if investigating blood pressure, we might want to look for perhaps 5 mmHg or 10 mmHg difference
- A larger difference will require a smaller sample size – just like photographing a larger object doesn’t require as good a camera (as photographing a smaller object).
How does the study power affect sample size?
- aim for 80%
2. as you increase your power, your sample size will increase.
How does the study alpha affect sample size?
- how much you want to rule out chance causing a positive finding
- it’s the equivalent of specifying the p-value and it’s also connected with one- and two-tailed testing
- Decreasing alpha from 0.05 to 0.01 will increase your sample size.
What else effects sample size?
- consider loss to follow-up
- as a rule of thumb, it’s not unusual to lose 15% of your participants – and of course longer studies will have higher loss
What is a type I error?
‘false positive’ finding
What is the p value?
the probability of obtaining a result as extreme as the observed results of a statistical test, assuming the null hypothesis is correct
What does. the p value mean?
- probability of getting your results if actually there’s no real difference
- that means it’s the probability of a false positive
When is there statical significance?
<0.05
Why are p values not so helpful?
- If you run multiple analyses, likely that one of your analyses will come back around the p = 0.05 mark - running multiple analyses on your data will eventually come back with something <0.05 when there’s no real underlying difference between your groups.
- The p-value is often mistaken as implying clinical significance. It doesn’t.
- All a p-value tells you is how likely your findings are due to chance alone
- It’s perfectly possible to have highly ‘statistically significant’ findings of a clinical difference which is entirely clinically meaningless.
When are p values helpful?
consistent findings over multiple tests <0.05 or single tests <0.01 then it gives some reassurance that chance alone is less likely to be playing a part
What is a type II error?
‘false negative’ finding
What does going form 80% to 90% poor do to the risk of deriving false negative?
Halve, but need to increase sample size which increase cost and complexity of trial
What is narrative review?
- Brings together published literature into a single article enabling the reader to rapidly understand issues
- Sometimes referred to as a literature review, scoping review or non-systematic review
What is a systematic review?
- Sets out highly structured approach to searching, sifting, including and summarising the literature
- Often presented as the basis for meta-analysis, but also exists separately
What are the strengths of a narrative review?
- Agile, normally easier and faster to write; may often be more up to date than latest systematic review
- Particularly useful when looking at areas with limited research or in higher levels or variation in research approaches
- Can be useful where work from different discipline is being brought together with a less easily-answerable research question
What are the limitations of a narrative review?
- Subject to potential bias as
1. Author free to select works (sometimes that may support their opinion): although ethical authors and robust peer review should prevent overly-speculative or unbalanced views
2. With no search being specified it is possible that important evidence is omitted by chance (rather than. intent)
What are the strengths of a systematic review?
- Aims to collate all available evidence that relates to a highly focused research question
- Implements a highly specified protocol that enables reproducibility - this will usually involved a structured search
- ‘Includes’ evidence based on pre-specified criteria: inclusion criteria
- Can take many months to design the search, review all sources (often thousands) and then synthesise the findings
What are the steps for a systematic review?
- Research questions
- Structured search
- Indices (categorise indices)
- Screening/inclusion
- Reporting
- Writing
- Submitting/revising/publishing
- Can take 18-24 months
What is a structured search?
- approach enables transparency and future researcher to reproduce the approach
- Words and heading and apply to index e.g. med and give reproducible approach to finding literature
- Develop an approach and series of searches that combine later on and justify why used some articles and why omitted some