In addition, the total correct score typically combines performance scores for FS and BS. For example, the average Wechsler total correct score for patients with mild Alzheimer’s disease shows mean z-score differences of only −0.22 for FS and −0.44 for BS ( Wilde et al., 2004), indicating that the typical patient with mild Alzheimer’s disease is poorly discriminated from age-matched control subjects. The high variance reduces sensitivity to clinical abnormalities. This inflates standard deviations, as reflected in high coefficients of variation (COV, the standard deviation divided by the mean) that is reportedly 23.4% for FS and 36.4% for BS ( Wilde, Strauss, & Tulsky, 2004). Second, because different subjects receive different numbers of trials, the variance of the total correct metric is high relative to its mean, and is highly skewed.
Digit span test average score trial#
For example, a subject with variable performance who misses one trial in FS testing at lengths of 3, 4, 5, 6, 7 and 8 will have the same total correct score as another subject who accurately reports all trials at lengths 3, 4, and 5, but who fails twice at list length 6. First, it conflates inconsistent performance with limits in maximal DS. In addition, the standard Wechsler total correct metric of DS performance is problematic for two reasons. However, it may seriously underestimate the ML of subjects who are distracted or who encounter idiosyncratically difficult digit strings (e.g., permutations of their telephone area code) at a particular length. This procedure assumes that the subject’s “true” maximum length (ML) span can be assessed by only four list presentations: two at the ML and two above. A second, more serious problem relates to the suboptimal sampling of list lengths that bound the subjects’ maximal DS: testing ceases as soon as the subject misses two lists of the same length. This procedure is relatively inefficient for subjects with normal memory spans (e.g., 7 digits in FS) because eight trials are delivered before the subject reaches list lengths that challenge memory capacity. First, WMS-III testing involves the presentation of two digit lists at each span length, beginning with three digits in FS testing and two digits in BS testing. There are also two problems in the methods used by the Wechsler Memory Scale-III (WMS-III) ( Wechsler, 1997b) to sample digit list lengths.
Digit span test average score code#
Certain digit sequences (e.g., the local telephone area code) may inflate digit span in geographic regions where particular digit sequences are overlearned and underestimate span where digit sequences conflict with previously overlearned strings (e.g., the digit sequence “415” in a region where the local telephone area code is “451”). In addition, the digits in each list are not selected randomly. First, because each list is read aloud by an examiner there are variations in the rate, intensity, emphasis and clarity of digit enunciations on each presentation as well as variations in clarity between different examiners ( Reeves, Schmauder, & Morris, 2000 Silverman, 2007). In traditional DS testing digit salience is influenced by two factors. The total number of lists reported correctly is combined across forward span (FS) and backward span (BS) to produce a Wechsler total correct score. Testing ceases when the subject fails to accurately report either trial at one sequence length or when the maximal list length is reached (9 digits forward, 8 backward). Digit sequences are presented beginning with a length of two digits and two trials are presented at each increasing list length. In each case, digit span is measured for forward and reverse-order (backward) recall of digit sequences. For decades they have been a component of the widely used Wechsler memory scales (WMS) and Wechsler intelligence scales for adults and children ( Wechsler, 1997a, 1997b). Measures of forward and backward digit span (DS) are among the oldest and most widely used neuropsychological tests of short-term verbal memory ( Richardson, 2007).
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