Cramming Sam's top tips from chapter 8
Click on the topic to read Sam's tips from the book
Correlation
- A crude measure of the relationship between variables is the covariance.
- If we standardize this value we get Pearson’s correlation coefficient, r.
- The correlation coefficient has to lie between −1 and +1.
- A coefficient of +1 indicates a perfect positive relationship, a coefficient of −1 indicates a perfect negative relationship, and a coefficient of 0 indicates no linear relationship.
- The correlation coefficient is a commonly used measure of the size of an effect: values of ±0.1 represent a small effect, ±0.3 is a medium effect and ±0.5 is a large effect. However, interpret the size of correlation within the context of the research you’ve done rather than blindly following these benchmarks.
Correlations
- Spearman’s correlation coefficient, rs, is a non-parametric statistic and requires only ordinal data for both variables.
- Kendall’s correlation coefficient, τ, is like Spearman’s rs but probably better for small samples.
- The point-biserial correlation coefficient, rpb, quantifies the relationship between a continuous variable and a variable that is a discrete dichotomy (e.g., there is no continuum underlying the two categories, such as dead or alive).
- The biserial correlation coefficient, rb, quantifies the relationship between a continuous variable and a variable that is a continuous dichotomy (e.g., there is a continuum underlying the two categories, such as passing or failing an exam).
Partial and semi-partial correlations
- A partial correlation quantifies the relationship between two variables while accounting for the effects of a third variable on both variables in the original correlation.
- A semi-partial correlation quantifies the relationship between two variables while accounting for the effects of a third variable on only one of the variables in the original correlation.
