The association can be studied via a tabular or graphical display, or via sample statistics which might be used for inference.
[2] The method used to investigate the association would depend on the level of measurement of the variable.
For two quantitative variables (interval or ratio in level of measurement), a scatterplot can be used and a correlation coefficient or regression model can be used to quantify the association.
In a bivariate correlation, outliers can be incredibly problematic when they involve both extreme scores on both variables.
If each variable can only take one of a small number of values, such as only "male" or "female", or only "left-handed" or "right-handed", then the joint frequency distribution can be displayed in a contingency table, which can be analyzed for the strength of the relationship between the two variables.