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The importance of quality in item generation; a prerequisite for Rasch analysis
Stephen McKenna & James Twiss
Galen Research Ltd, Manchester, UK
Quality in item generation; a prerequisite
for Rasch analysis
Creating a scale
Let’s create a new scale ……..
We need to write a lot of items and then apply Rasch
• But what will it really measure?
• How relevant will the items be to respondents?
The value of Rasch analysis is in perfecting the scale-
not in generating its content
Random item generation
Impact of MS Impact of MS
Targeted item generation based on model of
Impact of MS
• Loss or abnormality of psychological,
physiological or anatomical structure or
• Equates to symptoms
• Disturbances at level of organ
• Fatigue, pain, dizziness, depression, sleep
Disability (activity limitation)
• Any restriction or lack of ability to perform an
activity in the manner or within the range
considered normal for a human being
• Equates to functioning or functional status
• E.g. restricted mobility, problems dressing &
bathing, social restrictions, problems showing
• HRQL measures (such as SF-36) commonly
assess functioning in addition to impairment
What is quality of life?
• Not symptoms, activity limitations, utility or clinical
• Uniquely patient-based
• Holistic – overall impact of disease and its treatment
on the patient
“Human life quality is
dependent upon the
satisfaction of certain
basic needs - lack of
adequate nutrition and
Sir Thomas More (1478-1535)
• Appropriate source of items
• not existing measures
• not the literature
• not test-developers
• not clinicians in most cases
• relevant patients
• Qualitative unstructured interviews
• focus groups do not work well
• do not start with a list of issues – only constructs
• let the interviewee guide the interview as much as
• audio-record and produce transcripts
• invest a great deal of time analysing transcripts
Ensure items assess the required construct. Remove items that:
– are poorly worded
– express more than one idea
– appear difficult to translate
– are personal to one interviewee
– will not apply to all respondents; sex, employment etc
– are likely to produce DIF; interest in shopping, attending gyms
– are specific to one national culture
– are facts or will not change: ‘I worry that my health will get
• Patient Reported Indices of Multiple Sclerosis (PRIMUS)
• consists of 3 scales:
• Symptoms (impairment)
• Activity limitations
• Quality of life
Sample items from the PRIMUS QoL scale
• I feel I am a burden to others
• I have little freedom to do what I want to do
• I have to push myself to do things
• I avoid physical intimacy
• I feel as if I have nothing to offer anyone
Rogues gallery – SF-36
• Does your health now limit you in vigorous activities, such as running, lifting
heavy objects, participating in strenuous sports?
– that rules out 95% of the British! Disliked by older respondents
• General Health - How TRUE or FALSE are these statements for you?
– I seem to get sick a little easier than other people
– I am as healthy as anybody I know
– I expect my health to get worse
– My health is excellent
Are these outcomes?
MCS and PCS
Rogues gallery – EQ-5D
5 items – adequate coverage?
I have no problems in walking about
I have some problems in walking about
I am confined to bed
There seems to be a gap here!
I am not anxious or depressed
I am moderately anxious or depressed
I am extremely anxious or depressed
Are anxiety and depression the same constructs? What if I am anxious and depressed? What if I
am moderately anxious and severely depressed?
Rogues gallery - WHOQOL
Do you get the kind of support from others that you need?
Are you able to accept your bodily appearance?
How available to you is the information that you need?
How satisfied are you with your access to health services?
How satisfied are you with your transport?
How safe do you feel in your daily life?
Not health outcome as intended but static measure of QoL?
The few relevant items are also poor:
How satisfied are you with your sex life?
How often do you have negative feelings, such as blue mood, despair,
Rogues gallery - DLQI
Over the last week, how much has your skin interfered
with you going shopping or looking after your home or
Over the last week, how much has your skin created
problems with your partner or any of your close friends or
8/10 items can be answered not relevant (and scored ‘no
Measure used to decide who should receive biologic
treatment for psoriasis
DLQI: DIF by disease
Prevents working or studying
Person locations (logits)
Quality of patient reported outcome scale depends on:
• Coherent and valid measurement model
• Quality item generation
• Simple response format
Then we can think about fit to Rasch model
Great - now we can make an excellent scale with
the help of Rasch!