There’s an order to things: Item response theory as a way to make sense of functional decline in dementia
Sarah McGrory is a PhD Student at the Alzheimer Scotland Dementia Research Centre at the University of Edinburgh. She Tweets at @ alzscotdrc
Are some activities harder than others for people with dementia? In our research, recently reported in Age and Ageing, we looked at how people with dementia differed in their day to day activities. 202 people with mostly mild to moderate dementia in Scotland were asked about their activities
Activities (using the telephone, shopping, food preparation, housekeeping, laundry, travelling, taking medications, handling finances) can be measured using a questionnaire called the Lawton Instrumental Activities of Daily Living (IADL) scale. Usually scores on the individual tasks are added together to give a total score ranging from 0 to 8. This number can hide a lot of information about a patient’s functional ability. It assumes every functional activity is equally difficult, which is rarely the case. For example, being able to manage your finances is likely to be harder for most people than being able to eat.
Our research used a statistical technique called item response theory (IRT) to get more information from a questionnaire about function. IRT allows the different activities to be ranked according to their difficulty. This can be especially useful for assessing progression of cognitive impairment. Knowing the expected order of decline can help to monitor progression, so any changes from the usual course, or changes in rates of decline, can be identified and studied. IRT can help to identify key tasks in a scale, and can also show in what order tasks might become more difficult as dementia progresses. To do this, though, clinicians and researchers have to record answers to every task on a scale, not just to the total number.
We found that the tasks included in the scale could be ordered by increasing difficulty from being able to use the telephone (easiest) to the ability to shop (most difficult). This means a person may be having problems shopping independently may showing the first sign of functional difficulties related to cognitive impairment. Problems with this task should alert doctors as a possible early symptom of cognitive decline. Recognising these early stages is very important and can help people live independently for as long as possible with the help of medications, family education and counselling. Looking at the individual tasks within a scale instead of relying on the total score can help us to understand more about dementia progression and help us to identify care requirements for patients.