How to Display Data- P14

How to Display Data- P14:The best method to convey a message from a piece of research in health isvia a fi gure. The best advice that a statistician can give a researcher is to fi rstplot the data. Despite this, conventional statistics textbooks give only briefdetails on how to draw fi gures and display data.
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How to Display Data- P14 Relationship between two continuous variables 57• If it is necessary to display the correlation between all pairs of a set of three or more variables, this can be done by means of a correlation matrix (Table 5.1) or the preferred graphical equivalent (Figure 5.4).Regression:• The equation of the regression line should be given, together with the r2 value or preferably the residual standard deviation.• The number of observations, n, used to produce the regression equation should be stated.• Wherever possible the regression line should be shown in a plot together with the scatter diagram of the raw data with the predictor (explanatory) variable on the X-axis and the dependent variable on the Y-axis. The line should not extent beyond the range of the predictor variable (x).• The standard error of the slope is useful, as is the P-value from the hypothesis test (for the slope 0).• The accuracy used for the coefficients should be related to the accuracy of the raw data. It makes no sense to give an equation that purports to predict birthweight to the nearest 1/100 g when birthweight was actually measured to the nearest grams.• It is common for the value of the estimate of the intercept to be larger than that of the slope but these are frequently reported to the same number of decimal places. However, when making predictions, it is the slope that is needed with more precision not less, so it should be reported at least as precisely as the intercept.Method agreement data:• Report, n, the number of paired observations, for method 1 and method 2.• A scatter diagram of the measurements of method 1 vs. method 2 with a line of equality (Y X) could be produced.• Preferably a ‘Bland–Altman’ style scatter diagram of the difference between the methods on the Y-axis vs. the average of the two methods on the X-axis should be produced.• The ‘Bland–Altman’ style scatter diagram should show the line of zero dif- ference alongside the mean difference and the 95% limits of agreement.• Size of dots should be relative to the number of observations with that combination of values.ROC curves:• The number of observations, n, used to produce the ROC curve should be stated.58 How to Display Data• The scales for the X (sensitivity) and Y (1 – specificity) axes should range from 0 to 1.• The line of equality of y x should be reported.• The area under the ROC curve should be reported.References1 Sivgaru A, Gaines PA, Walters SJ, Beard J, Venables GS. Neuropsychological out- come after carotid angioplasty: randomised controlled trial. The challenge of stroke. The Lancet conference. Montreal, Canada: Lancet; 1998.2 Campbell MJ, Machin D, Walters SJ. Medical statistics: a textbook for the health sci- ences, 4th ed. Chichester: Wiley; 2007.3 Simpson AG. A comparison of the ability of cranial ultrasound, neonatal neuro- logical assessment and observation of spontaneous movements to predict outcome in preterm infants. Sheffield: University of Sheffield; 2004. PhD thesis.4 Cleveland WS. Robust locally weighted regression and smoothing scatterplots. Journal of the American Statistical Association 1979;74:829–36.5 Hutchinson A, Dean JE, Cooper KL, McIntosh A, Walters SJ, Bath PA, et al. Assessing quality of care from hospital case notes: comparison of two methods. Quality and Safety in Health Care 2007.6 Bland JM, Altman DG. Statistical methods for assessing agreement between two methods of clinical measurement. The Lancet 1986;i:307–10.7 Altman DG. Practical Statistics for Medical Research. London: Chapman & Hall; 1991.8 Johnson CD, Toh SKC, Campbell MJ. Comparison of APACHE II score and obesity score (APACHE-O) for the prediction of severe acute pancreatitis. Pancreatology 2004;4:1–6.9 Machin D, Campbell MJ. Design of studies for medical research. Chichester: Wiley; 2005.Chapter 6 Data in tables6.1 Presenting data and results in tablesData can be presented in a table as well as or instead of a graph. Althoughthere are no hard and fast rules about when to use a graph and when touse a table, when the results of a study are presented in a report or a paperit is often best to use tables so that the reader can scrutinise the numbersdirectly. Tables can be useful for displaying information about many vari-ables at once, while graphs can be useful for showing multiple observationson individuals or groups (such as a dotplot or a histogram). As with graphs, there are a few basic rules of good presentation, includingTufte’s golden rule that the amount of information should be maximised forthe minimum amount of ink.1 Tables should be clearly labelled and a briefsummary of the contents of a table should always be given in words, eitheras part of the title or in the main body of the text. Numerical precision should be consistent throughout and summary sta- ...