DESCRIPTIVE COMPARISON OF GROUPS - LEARNING OUTCOMES Flashcards
What are the descriptive statistics for categorical data?
Percentages and proportions
What are the descriptive statistics for normally distributed continuous data?
Mean and standard deviation
What are the descriptive statistics for non-normally distributed continuous data?
Median and Interquartile range
What is meant by the ‘mean difference’?
Mean difference = Mean of exposed group - Mean of unexposed group
If the confidence interval for the mean difference spans 0 then we cannot confirm a real difference between the two groups because it means that no difference is a plausible value.
How do we examine the mean difference when we have more than 2 exposure groups?
We first decide which is the unexposed (baseline) group. For an ordered variable this is usually the first or last group. We avoid groups with very small numbers. The mean difference and CI is then calculated as normal, relative to the baseline.
For what kind of data is the mean and mean difference an appropriate statistic?
Continuous data
What are the only kind of studies that risk statistics are applicable for?
Risk statistics are applicable for cross-sectional studies only.
How do we calculate the overall risk of disease?
Overall risk (prevalence) = Number of people with disease / total number of people in the study population
How do we calculate the risk of disease for a particular group, e.g. smokers?
Risk = number of smokers with disease / total number of smokers
How do you calculate the risk ratio?
Risk ratio = Risk of disease in exposed / risk of disease in unexposed
What is the risk difference?
Risk difference = Risk of disease in exposed - Risk of disease in unexposed
For example:
If the risk in diseased is 19% and the risk in unexposed is 4% then we would calculate 19%-4%=15% and this would tell us that the exposed group have 15% more disease than the unexposed group.
What is the benefit of looking at both the risk ratio and the risk difference rather than than the risk ratio alone?
The risk ratio may tell us we have say a 4-fold increase but this may actually only equate to a fraction of a percent difference in reality, which can be seen if we also calculate the risk difference.
What does a risk ratio of 1 mean?
No change in risk
What does a risk ratio less than 1 mean?
Decrease in risk
What does a risk ratio of greater than 1 mean?
Increase in risk
What is the easiest way to carry out an interpretation of risk ratio?
Look at the deviation from 1, which represents no change in risk - e.g. 1.5 - 1 = 0.5 which means 50% more likely
0.4 - 1 = -0.6 which means 60% less likely
What is the Odds ratio?
Odds ratio = odds of disease in exposed / odds of disease in unexposed
Looks at odds rather than absolute risk.
Here we are not using group totals, rather we are comparing disease in exposed to disease in unexposed.
How do we calculate odds?
Odds = number of people with disease / number of people without disease
What measure of risk would we give for a case-control study?
The odds ratio - we can only use risk ratios for cross-sectional studies
For rare diseases the risk ratio and odds ratio should give you ________ values.
For rare diseases the risk ratio and odds ratio should give you similar values.
(rare means
How do we calculate risk ratios or odds ratios if we have more than 2 groups?
We choose a baseline group and calculate statistics relative to that group.
If our 95% CI for risk ratio or odds ratio spans 1 what does this mean?
It means that we can’t say that there is any real difference between the groups. Because our value of 1 is in the plausible range we can’t be confident that there is a real difference there.
Risk ratio is only valid for _________ studies. For case-control studies we must use the __________.
Risk ratio is only valid for cross-sectional studies. For case-control studies we must use the odds ratio.
If we are correcting for other factors is it easier to use the risk ratio or the odds ratio?
If you are correcting for other factors it is easier to use the odds ratio.
What descriptive statistics are appropriate for continuous variables that are normally distributed?
The mean difference an confidence intervals
What descriptive statistics are appropriate for categorical variables?
Risk difference, risk ratio and odds ratio
For continuous variables that are not normally distributed what descriptive statistic / measure of difference would be appropriate?
The median difference, but it is not as easy to calculate confidence intervals for this.
What differences might we see in the risk ratio and odds ratio for more common diseases?
The risk ratio and odds ratio for common diseases will vary more for common diseases than rare diseases and the odds ratio will tend to give us an overestimation of the true risk.
Why is the odds ratio appropriate for case control studies, but the risk ratio is not?
In case-control studies the risk ratio outcome will change greatly depending on the number of cases and controls recruited. Risk ratio depends on the number of cases / controls in the study, which is fixed by the researchers.
The odds ratio depends on proportion of cases / controls with exposure.
The risk ratio is only appropriate in studies where participants are recruited independently of disease status.
The odds ratio is also more commonly used because it is much easier to adjust in a multivariate analysis.
