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Ann Thorac Surg 2003;75:1066
© 2003 The Society of Thoracic Surgeons

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Correspondence

The use of Z scores in assessing neuropsychological change after cardiac operations

Department of Cardiac Surgery, The Royal Sussex County Hospital, Eastern Rd, Brighton BN2 5BE, UK

e-mail: donaldcwhitaker{at}yahoo.co.uk

To the Editor:

I read with considerable interest the recent review by Collie and associates [1] concerning statistical analysis of cognitive change after coronary artery surgical procedures. It is agreed that the percent change and standard deviation "cutoff" methods of deficit analysis are arbitrary, do not take practice effects into account, and are relatively insensitive, especially as the incidence of cognitive decline has decreased. The main alternative advocated [2], group mean analysis has the flaw of allowing patients who show improvement to mask those whose cognitive status deteriorates even if there is a group to control for regression toward the mean. Further alternatives are clearly required.

It is therefore disappointing that although many potentially superior statistical methods from the neuropsychological and psychiatric literature were illustrated in Table 1 of the review and discussed, little mention was made of the z change score, which was used in a randomized, controlled trial by Arrowsmith and coauthors [3] and an observational study by Steed and colleagues [4] involving patients undergoing coronary artery bypass grafting. Table 1 describes Kneebone and associates [5] as using a "standard deviation index" [(X₂ - X₁)/µSD₁]. However, they actually used the traditional binary standard deviation method, as they converted a score of (X₂ - X₁)/µSD₁ greater than 1 to a deficit and compared this with a reliable change index, which was their main outcome measure.

In contrast, the z score, which is also calculated as (X₂ - X₁)/µSD₁, is not then converted to a deficit or no deficit depending on whether it is greater or less than 1. These scores are left as raw numbers and can then be used to give a mean for each test or the sum of a battery of tests for different groups of patients. This method has the advantage of increased sensitivity because it is a continuous measure without absolute cutoffs, thus giving each individual a score. An additional benefit is that it allows group comparisons, which take into account potential learning effects. Z scores are a recommended way of analyzing cognitive change that should be considered for future studies of cardiac surgical patients.

References

1. Collie A., Darby D.G., Falleti M.G., Silbert B.S., Maruff P. Determining the extent of cognitive change after coronary surgery: a review of statistical procedures. Ann Thorac Surg 2002;73:2005-2011.[Abstract/Free Full Text]
2. Browne S.M., Halligan P.W., Wade D.T., Taggart D.P. Cognitive performance after cardiac operation: implications of regression toward the mean. J Thorac Cardiovasc Surg 1999;117:481-485.[Abstract/Free Full Text]
3. Arrowsmith J.E., Harrison M.J.G., Newman S.P., Stygall J., Timberlake N., Pugsley W.B. Neuroprotection of the brain during cardiopulmonary bypass: a randomized trial of remacemide during coronary artery bypass in 171 patients. Stroke 1998;29:2357-2362.[Abstract/Free Full Text]
4. Steed L., Kong R., Stygall J., et al. The role of apolipoprotein E in cognitive decline after cardiac operation. Ann Thorac Surg 2001;71:823-826.[Abstract/Free Full Text]
5. Kneebone A.C., Andrew M.J., Baker R.A., Knight J.L. Neuropsychologic changes after coronary artery bypass grafting: use of reliable change indices. Ann Thorac Surg 1998;65:1320-1325.[Abstract/Free Full Text]

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This Article Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Alert me to new issues of the journal Add to Personal Folders Download to citation manager Author home page(s): Donald Whitaker Permission Requests Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Whitaker, D. Search for Related Content PubMed Articles by Whitaker, D. Related Collections Coronary disease Chest wall