Demographic characteristics and mean DRS total scores for all 1,175 participants in the updated sample are presented in Table . The three groups differed on age, F(2, 1172) = 9.46, p < .001, and education, F(2, 1172) = 4.83, p < .01. Specifically, robust normal participants were marginally younger than single encounter (p = .05) and significantly younger than preclinical dementia (p < .001) participants. Robust normal participants had a significantly higher level of education than single encounter participants (p < .05), but not when compared with preclinical dementia participants. There was no significant difference between the groups on sex distribution, χ2(2) = .57, p = .75. Robust normals had the lowest proportion of APOE ε4 allele carriers (23.6%); however, this was not significantly different when compared with the proportion of APOE ε4 carriers among single encounter, χ2(1) = 1.85, p = .17, or preclinical dementia, χ2(1) = 3.63, p = .06 groups. When considering only the robust normal sample, 11 adults (1.2%) were APOE ε4/4 homozygotes.
Characteristics of “robust normal” (n = 894), “single encounter” (n = 101), and “preclinical dementia” (n = 180) participants
As expected, mean total DRS scores differed among the three groups, F(2, 1172) = 16.30, p < .001, with robust normals obtaining a significantly higher mean DRS score compared with single encounter (p < .001) and preclinical dementia (p < .001) participants. Consistent with prior studies on the effects of robust norming, the conventional (full) sample of 1,175 participants (M = 135.8, SD = 5.8) underestimated the DRS mean and overestimated the variance when compared with the robust subsample. Moreover, this effect was also evident when comparing the current robust subsample with the mean DRS score (M = 134.7, SD = 6.8) calculated from the original subsample of 623 participants.
Frequency distributions of demographic data and study encounters for the robust normative sample are presented in Table . Approximately 89% of the sample had at least two follow-up visits and almost half (45.3%) had a minimum of five follow-up visits during which a consensus diagnosis of normal was retained. Among preclinical dementia cases, the median number of follow-up visits was four and the mode was six (range = 1–8). Single encounter participants were lost to follow-up for a variety reasons, but no formal attempt has been made to ascertain the specific factors contributing to their study withdrawal.
Frequency distribution of robust normative sample (n = 894)
Table shows the association between demographic variables and DRS scores in robust normal participants. As expected, age and education were significantly associated with the majority of DRS subtest and total scores. Sex was associated only with the Construction subtest, accounting for less than 1% of variance (r2 = .008). To determine if this association was sufficiently relevant to stratify the Construction subtest in our normative tables, we then analyzed the mean Construction scores between men and women across 5-year age bands. Mean Construction scores were significantly different for men and women only between the ages of 80 and 84, with women obtaining on average 0.12 points more than men. Across all ages, the mean Construction subtest score for men was 5.73 and women 5.81. Given the rather trivial relationship between sex and Construction subtest scores in our sample, and the lack of association to any other DRS subtest, it was not considered for the normative analyses.
Correlation coefficients and shared variances of DRS subtest and total scores with demographic variables (n = 894)
Normative data for DRS subtest and total scores are presented in Tables –. Robust age-corrected scaled scores are presented in the leftmost column of each table, with corresponding percentile ranks in the rightmost column. To use these norms, first select the table corresponding to the patient's age at the time of test administration. Then search for the patient's raw score and refer across to the corresponding scaled score (M = 10, SD = 3) and percentile rank. The age range and sample size used to create norms for each group are provided beneath each table.
Robust norms for persons under age 67
Robust norms for persons of age 67–69 years
Robust norms for persons of age 70–72 years
Robust norms for persons of age 73–75 years
Robust norms for persons of age 76–78 years
Robust norms for persons of age 79–81 years
Robust norms for persons of age 82–84 years
Robust norms for persons of age 85–87 years
Robust norms for persons of age 88–90 years
Robust norms for persons of age over 90 years
Steps to obtain robust age- and education-corrected scaled scores for total DRS raw scores
Results from the hierarchical linear regression model showed education to contribute unique variance to DRS total scores, F(2, 891) = 108.9, p < .001, with an increment from r2 = .12 in the age-only model to r2 = .20 in the age and education model. To aid clinicians and investigators using the DRS, robust age- and education-corrected scaled scores can be derived for the total raw score using the equation presented in Table . Scaling constraints due to marked deviation from normality precluded education adjustments for each DRS subtest.
Finally, a preliminary post hoc analysis was performed of the incremental diagnostic validity and clinical utility of these robust norms in the detection of “baseline” cognitive impairment in preclinical dementia cases. Scaled scores were obtained for baseline DRS total scores from 49 preclinical dementia cases (39 AD, 10 with other dementia diagnoses) using robust and original norms. These scores were compared with the baseline scaled scores from 49 cognitively normal adults selected from the robust sample. To minimize the likelihood of spurious findings, 3 separate random samples (with replacement) of 49 cognitively normal adults were drawn. Cognitive impairment was defined as a scaled score lower than 7. Across the three comparisons, use of the original norms resulted in sensitivity of 14.3%, whereas the new robust norms resulted in sensitivity of 24.5%. Specificity using original norms ranged from 91.8% to 95.9%, and using robust norms ranged from 89.8% to 93.9%. Positive predictive values ranged from 63.6% to 77.8% using the original norms and 70.6% to 80.0% using robust norms, an average increase of 3.2%. Negative predictive values ranged from 51.7% to 52.8% using original norms and 54.3% to 55.4% using robust norms, an average increase of 2.4%.