Mini Mental Status test

Data came from the Medical Research Council Cognitive Function and Ageing Study (MRC CFAS) . Briefly, MRC CFAS is a multi-centre study on over 18, 000 persons from across six centres in England and Wales; five of the centres have the same standardised design. These centres used a two-phase sampling design with a screening interview followed by an assessment interview. Participants were selected from Family Health Service Authority lists and were stratified by age to include persons aged 65 years and over at the index date for each centre and living within a specified geographical area. The study began in the late 1980s; baseline interviews took place between 1989 and 1993.

In this study data were used from the five centres with a standardised design: Cambridgeshire, Gwynedd, Newcastle, Nottingham, and Oxford (total n = 13, 004). The population under investigation contained individuals who were cognitively assessed at the baseline screening interview or the assessment interview around two months later (n = 2, 640, both tests were completed by 2, 275 participants). The population invited to the assessment interview was weighted towards those in a potentially frail cognitive state (identified using details from the screen interview, including MMSE scores) although all levels of ability were represented. For full details of the questionnaires used at the screen and assessment waves please see .

Cognitive Assessment

The Mini-Mental State Examination (MMSE) was administered to participants at both the screen and assessment interviews. The version of the MMSE used in this study included serial sevens, but not spelling 'world' backwards . The words to repeat and recall were 'apple, penny, table' at screen, and 'tree, clock, boat' at assessment. Items that could not be answered due to sensory or mobility problems were considered failed, all other items that were not answered were kept as missing data . Incomplete MMSE scores tend to come from individuals who are severely cognitively impaired.

MMSE scores range from 0-30 and there have been several definitions proposed to categorise these scores into cognitive states. The three definitions used in this paper were suggested by MRC CFAS, Tombaugh and McIntyre and Folstein et al. . The MRC CFAS categorisation was based on the ROC curve findings from Figure One of Stephan et al. 2010 , which showed the MMSE to be as accurate as other diagnostic definitions of Mild Cognitive Impairment in predicting future risk of dementia. The graph indicated MMSE groupings as follows: < 18 (severe impairment), 18-22 (moderate impairment), 23-26 (slight impairment), 27-30 (no impairment). . who devised the MMSE also recommended splitting the MMSE scores into four groups (< 11 severe impairment, 11-20 moderate impairment, 21-26 mild impairment, 27-30 no impairment) while Tombaugh and McIntyre's seminal review reported a trend towards a three group categorisation (< 18 severe impairment, 18-23 mild impairment, 24-30 no impairment).

Interview Administration

Interviewers at both screen and assessment had a range of backgrounds, mainly professions allied to medicine. These included psychologists, psychiatrists, registered nurses and others with similar backgrounds. All interviewers received identical training from the CFAS study co-ordinators. Wording, prompting and feedback were all strictly controlled by a combination of training and computer assisted interviewing. Monitoring of the quality and consistency of interviews was carried out to ensure comparability both within and between centres through observation, role play, and analysis of audiotapes of interviews in the field. Interviews took place in the respondents' homes.

Statistical Methods

MMSE scores were categorised into groups, which were relabelled in ascending order from 1 (low cognition) to 4 (high cognition) (or 1 to 3). Cognitive change was measured by subtracting the assessment group number from the screen group number. This created a scoring range of -3 to 3 (or -2 to 2) where 0 represented no change in group. Descriptive statistics were used to compare the classification performance of each categorisation method.

To determine whether baseline cognitive score had an effect on cognitive change, weighted logistic regression was used to test for differences between those who changed group compared to those who did not. Age, sex, and study centre were entered as covariates along with the MMSE score from the screen interview and the duration in months between screen and assessment interviews. Inverse probability weights were calculated using logistic regression-study participation was regressed on age, sex, screening MMSE score, and GMS-AGECAT (Geriatric Mental State-Automated Geriatric Examination for Computer Assited Taxonomy), which is a computerised diagnostic system that can be used to diagnose dementia. This enabled the cohort under investigation to be back-weighted to the original population-based cohort of 13, 004 individuals. All analyses were conducted in R version 2.10.1 .