nominal data provides the weakest level of measurement and is often known as categorised or frequency count data (can be allocated into categories by counting frequency or occurrence within particular categories)
ordinal data is capable of being placed in rank order (e.g. on a scale)
interval data is measured in fixed and precisely defined units with equal distances between all the points on the scale concerned
difference or correlation
difference generally involves experiments and determines if there is a statistically significant difference between the means of two variables
correlations show relationships or associations between two variables
related or unrelated test of difference
a related design is when a matched pairs or repeated measures experimental design has been used
an unrelated design is when an independent groups experimental design has been used
