Objective results require an objective instrument (outcome measure). For measuring the length of a patient, we use a ruler, where each line represents one centimetre. Therefore each line has a fixed value or unit.
However, for many things we want to measure in patients, no ruler is available. Consider for example pain, mood, and sleeping disturbances. Often, questionnaires are used, and each answer is assigned a certain score. However, these scores do not have a fixed unit or value, like ‘centimetres’.
The Rasch model is a mathematical method to resolve this, and to create an outcome measure comparable to a ruler with fixed units. This also creates the possibility to compare different studies. The same method can be used to determine which patient does and which patient does not react to a certain treatment.