Comparison of two independent groups

SELECT A MEASUREMENT SCALE:

• INTERVAL or RATIO
  An interval scale 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.
   

• ORDINAL
  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
  Scale for grouping into categories with order e.g. mild, moderate or severe. This can be difficult to separate from ordinal.
   

• NOMINAL without order
  Scale for grouping into unique categories e.g. blood group.
   

• DICHOTOMOUS
  As for nominal but two categories only e.g. surgery / no surgery.