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Annexes 194Psychometric Tests’ Sensitivity to Cognitive Changes 3TABLE 1. Demographic and health characteristics in thesample (n ¼ 2,252), the PAQUID Study, France, 1989–2001No. %Male 974 43.3No diploma 608 27.0Age (years) at visit at 1 year70 1,100 48.9>70–75 368 16.3>75–80 412 18.3>80 372 16.6Prevalent dementia at visit at 1 year 14 0.6Incident dementia between visitsat 3 and 13 years 368 16.3test. The estimated beta cumulative distribution functionsrepresent the shapes of the transformations between the latentcommon factor and each of the four tests and provideinteresting metrologic properties for each test. The completemethodology was previously detailed in the report by Proustet al. (4) and is described in the Appendix.SampleMeasurements at the initial visit were excluded fromthe analysis because of a first passing effect previously described(17). Indeed a test-retest improvement was observedbetween the initial visit and the visit at 1 year, possibly due tothe stress induced by the first evaluation at the initial visit ora practice effect observed after the first evaluation. Of the3,777 subjects in PAQUID, we retained 3,043 subjects whowere followed up after the initial visit, and we studied theevolution of the common factor between the visits at 1 and13 years. The Digit Symbol Substitution Test (DSST) was notcompleted at the visit at 3 years. Moreover, a subsample ofPAQUID completed a nutritional questionnaire at the visitat 3 years that could impact on the Isaacs Set Test shortenedat 15 seconds (IST15) score for the fruit and animal categories.So, we excluded the measurements of IST15 at the visitat 3 years. From the sample of 3,043 subjects, we retaineda sample of 2,252 subjects with at least one measure at eachneuropsychological test between visits at 1 and 13 years toensure that every subject brought information about eachneuropsychological test. The median number of measureswas four for the MMSE (interquartile range (IQR): 2–5) andthe Benton Visual Retention Test (BVRT) (IQR: 2–5) andthree and two for the IST15 (IQR: 1–4) and the DSST (IQR:1–4), respectively.RESULTSDescription of the sampleCharacteristics of the subjects in the sample are describedin table 1: 56.7 percent of the subjects were women, and 73percent had at least graduated from primary school. Themean age at the beginning of the follow-up (visit at 1 year)was 75.16 (standard error: 6.24) years. Among the 2,252subjects, 14 had a positive diagnosis of dementia at the visitat 1 year, and 368 subjects were diagnosed as demented duringthe follow-up. At the 1-year visit, 1,623 subjects completedthe MMSE with a median score of 28 (IQR: 26–29),1,622 subjects completed the IST15 with a median score of28 (IQR: 24–33), 1,619 subjects completed the BVRT witha median score of 11 (IQR: 9–13), and 1,614 completed theDSST with a median score of 28 (IQR: 20–37) (table 2).Cognitive evolution with ageThe latent common cognitive factor ranges from 0 to 1,with 0 corresponding to a minimal cognitive performanceand 1 corresponding to a maximal cognitive performance.The predicted mean evolution of the latent common cognitivefactor and its 95 percent confidence interval are displayedin figure 1, part A. It shows a decline of cognitive performancesin the population according to age, with an accelerationof the decline at older ages. This evolution can be interpretedas the mean cognitive decline with age in a population ofsubjects aged 65 years or older, including both dementedand nondemented subjects.The predicted mean evolutions of the four tests and their95 percent confidence intervals are displayed in figure 1, partB. The decline of the DSST is quite linear, whereas the declineobserved in the other tests is nonlinear, with an accelerationat older ages.Comparison of the sensitivity of the four testsThe estimated transformation shapes between each testand the common factor are displayed in figure 2. They arevery different for the four tests: The transformation is concavefor the MMSE and the BVRT, convex for the DSST, and closeto linear for the IST15. Moreover, the MMSE and the BVRTcover, respectively, only 78 percent and 85 percent of thecommon factor range, while the DSST and the IST15 coveralmost all of the entire range with, respectively, 94 percentand 95 percent of the common factor range.Ceiling effect. For the MMSE and the BVRT, the maximumvalue of the test is reached for a value of the commonfactor lower than its maximum (respectively, 0.78 and 0.87),which shows that these tests are not appropriate to assesscognitive changes in subjects with a very high general cognitivelevel of functioning. This underlines the ceiling effectof these two tests. In contrast, the maximum values of theIST15 and the DSST are reached for a value of the commonfactor close to 1, underlining that these tests do not sufferfrom a ceiling effect and can identify changes at the highestlevels of cognition.The MMSE transformation. The nonlinear shape of theMMSE transformation shows that a decline of the MMSEdoes not correspond to the same intensity of decline of thecommon factor in all the range of the test. Indeed, a loss of0.1 in the common factor scale between 0.7 and 0.6 representsa loss of 1 point for the MMSE, whereas the same lossbetween 0.3 and 0.2 represents a loss of more than 4 pointsfor the MMSE. This shows that a decline in the MMSEcannot be interpreted without accounting for the initial valueand that the MMSE is more appropriate to detect a decline atlow cognitive levels than at high cognitive levels.