Statistics Flashcards
(114 cards)
Distinguish absolute and relative risk.
Absolute:
- Incidence
- Prevalence
- Odds
- Hazard rate/ratio
Relative:
- Risk ratio
- Hazard ratio
- Odds ratio
Define risk.
The likelihood of an event occurring (and, in HACCP, the consequence of that event occurring). It is the number of outcome events / number of all events.
What is relative risk?
Relative risk/risk ratio is the ratio of 2 risks.
RR = risk in group 1 / risk in group 2
How is relative risk calculated?
(A / (A+B)) / (C / (C+D))
Why can numbers be unreliable?
- Small sample size
- Some not sampled at all
- How representative are the different samples
- Where those that were assessed selected at random
- Always think about the potential for bias and how that could be introduced
What does the 95% confidence interval mean?
- 95% of the values lie within the range of 12 and 29
- Prevalence of obesity is 3.6% and 8.7%
- As you increase the number of samples, the values get closer to a normal approximation
If risk is the number of outcome events over the number of all events, what happens if we do not know the total number of events?
Pick random individuals without disease so as not to calculate the whole population. This is when we use odds.
What are odds?
The ratio of positive outcomes to negative outcomes.
= number of outcome events / number of non-outcome events
What is odds ratio?
The ratio of 2 odds is the odds ratio:
OR = odds in group 1 / odds in group 2
How is odds ratio calculated?
(A / B) / (C / D)
Define p value.
The probability that the difference between the value occurred by chance.
What is the significance of low and high p values?
- Low p-value: Unlikely that the difference is due to chance alone
- High p-value: Likely that the difference is due to chance
- P-value for difference between breeds = 0.001 (Chi sq test)
- Convention: a 5% (p=0.05) cut-off value for p-values is used to signify “statistical significance” (below 0.05 = statistically significant [difference])
What does a RR less than 1 mean if statistically significant?
Risk of disease/group is reduced.
What does a RR greater than 1 mean if statistically significant?
Much more likely to be in that particular group, have that disease.
Under what circumstances is odds ratio a good estimate of relative risk?
A good estimate of risk when the prevalence of disease is low or when the disease is rare.
Why use risk?
- More accurate reflection of population prevalence
- Easier to interpret
- Harder to calculate (need to be clear what the denominator us)
- Relative risk is the measure of association calculated from cohort studies
Why use odds?
- Cannot estimate prevalence of disease in the population
- Easier to calculate
- Odds ratio if the measure of association calculated from case-control studies
How can the way we present data affect how we interpret it?
- Always check axes on a graph and what the error bars actually are.
- This is why statistical analysis is used to interpret data.
- Is the right question being asked, for example, in the study above, is brain volume equivalent to intelligence?
What is statistics in veterinary/scientific literature?
Refers to the process establishing the probability of samples coming from the same populations.
What can graphs be used to do?
- Identify the general shape of the data including the centre of its distribution (its location or average) and how variable it is (its spread)
- Identify unusual or outlying points
- Compare the shape of two independent datasets
- Identify relationships between two variables in a dataset
List different ways of displaying data.
Simple scatter/x-y plots
Grouped frequency table
Histogram
Dot display
Bar chart
Box and whisker plot
Describe the usefulness of median and range as measures.
Median does not use all the data and so is not informative.
Useful for summary measures but make limited use of data.
Describe the usefulness of the mean as measures.
Unlike the median, the value of the mean is sensitive to outliers. A ‘truncated’ mean is sometimes used to avoid this, removing a predetermined number of the highest and lowest points before calculating.
What is standard deviation?
- The standard deviation is a measure of variation which gives an indication of the average spread of the data about the mean.
- It is (roughly) the average of the differences between all the points and the mean.
- The standard deviation also provides a useful tool for calculating probabilities from the data.