Relationship Between Two Variables Measured on the Same Group
Select the appropriate combination of measurement scales (key below):
| Variable X | Variable Y |
| INTERVAL (NORMAL)+ | Interval (normal) |
| Interval (non-normal) | |
| Ordinal | |
| Nominal (ordered) | |
| Nominal (no order) | |
| Dichotomous | |
| INTERVAL (NON-NORMAL)+ | Interval (normal) |
| Interval (non-normal) | |
| Ordinal | |
| Nominal (ordered) | |
| Nominal (no order) | |
| Dichotomous | |
| ORDINAL + | Interval (normal) |
| Interval (non-normal) | |
| Ordinal | |
| Nominal (ordered) | |
| Nominal (no order) | |
| Dichotomous | |
| NOMINAL (ORDERED)+ | Interval (normal) |
| Interval (non-normal) | |
| Ordinal | |
| Nominal (ordered) | |
| Nominal (no order) | |
| Dichotomous | |
| NOMINAL (NO ORDER)+ | Interval (normal) |
| Interval (non-normal) | |
| Ordinal | |
| Nominal (ordered) | |
| Nominal (no order) | |
| Dichotomous | |
| DICHOTOMOUS + | Interval (normal) |
| Interval (non-normal) | |
| Ordinal | |
| Nominal (ordered) | |
| Nominal (no order) | |
| Dichotomous |
Key
INTERVAL is a scale with a fixed and defined interval e.g. temperature in degrees Celsius, so the difference between the 5th and 6th points on the scale is identical to the difference between the 8th and 9th etc. Ratio scales possess all properties of interval scales plus an absolute zero point (complete absence of the property being measured), e.g. temperature in degrees Kelvin. (NORMAL vs. NON-NORMAL: The Shapiro-Wilk W test can be used to investigate a sample for evidence of "non-normality")
ORDINAL is a scale for ordering subjects from low to high with any ties attributed to lack of measurement sensitivity e.g. pain score from a questionnaire.
NOMINAL with order is a scale for grouping into categories with order e.g. mild, moderate or severe. This can be difficult to separate from ordinal.
NOMINAL without order is a scale for grouping into unique categories e.g. blood group.
DICHOTOMOUS is a scale like nominal but two categories only e.g. surgery / no surgery.