How to Display Data- P7

How to Display Data- P7: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- P722 How to Display Data 2500 2000Frequency 1500 1000 500 0 Normal Emergency Planned Ventouse Forceps Vaginal vaginal caesarean caesarean delivery breech delivery section section delivery(b) Type of deliveryFigure 3.4 (Continued.)individuals in this category compared to 2221 in the normal delivery categoryand so vaginal breech births comprise 1% of births. However this is notthe impression given in Figure 3.4. Above all else, a graph should be simpleand accurately reflect the data so that the reader can easily understand theinformation being conveyed. Neither Figure 3.4 nor b do this and should notbe used. A final point about three-dimensional bar charts is that it can behard to read the scale, particularly for those bars furthest away from the scalemarkers, as it is not clear whether the scale should be read from the left orfrom the back. While Figures 3.1 and 3.3 are less visually exciting than Figure 3.4aand b they are much clearer and less ambiguous and more accurately reflectthe data.3.5 Clustered bar chartsThe data in Table 3.1 can be further classified into whether or not the babyis the first (primiparous) or subsequent child (multiparous) (Table 3.3).It now becomes impossible to present the data as a single pie chart or bar Displaying univariate categorical data 23Table 3.3 Self-reported type of delivery for new mothers (n 3221)1What kind of delivery? Primiparous (%) Multiparous (%)Normal vaginal delivery 857 (58.1) 1364 (78.2)Emergency caesarean section 302 (20.5) 132 (7.6) (once labour had started)Planned caesarean section 72 (4.9) 179 (10.3)Ventouse (vacuum extractor) 162 (11.0) 48 (2.8)Forceps delivery 76 (5.1) 13 (0.7)Vaginal breech delivery 7 (0.5) 9 (0.5)Total 1476 (100.0) 1745 (100.0)chart. These data are categorised in two ways, by type of delivery and par-ity, enabling the distribution of delivery type to be compared between thosewomen who had no previous children and those who had at least one. Table3.3 is an example of a 6 2 contingency table with 6 rows (representing typeof delivery) and 2 columns (representing parity) and it is said to have 12cells (6 2). More generally, a contingency table with r rows and c columnsis known as an r by c contingency table and has r c cells. Type of deliveryis said to have been cross-tabulated with parity. The data could be presented as two separate pie charts or bar charts sideby side but it is preferable to present the data in one graph with the samescales and axes to make the visual comparisons easier. In this case theycould be presented as a clustered bar chart (Figure 3.5). When presentingdata in this way (as percentages), you should include the denominator foreach group (total sample size), as giving percentages alone can be mislead-ing if the groups contained very different numbers of subjects. It is possible to use different colours to distinguish between the differentgroups, but as with pie charts, it is best to use different shades of the samecolour to represent different groups. This has been done in Figure 3.5. Note that the bars and vertical scale now represent the percentage of casesrather than the actual number (i.e. the relative frequency). The relative fre-quency scale has been used rather than the count scale as this enables com-parisons to be made between the groups when the numbers in each groupdiffer, as in this example with parity. If the relative frequency scale is used, itis recommended good practice to report the total sample size for each groupin the legend. In this way, given the total sample size and relative frequency(from the height of the bars) it is possible to work out the actual numbersof mothers with the different types of delivery. An alternative method would24 How to Display Data Parity 80% Primiparous (n 1476) Multiparous (n 1745) 60%Percent 40% 20% 0% Normal vaginal Emergency Planned Ventouse Forceps Vaginal breech delivery caesarean caesarean delivery delivery section section Type of deliveryFigure 3.5 Self-reported type of delivery by parity for mothers at 8 weekspostnatally.1be to display the data for primiparous and multiparous women separatelyas in Figure 3.6. However, this would be a poor method of display since thepurpose in plotting the data together is to compare the primiparous andmultiparous women. This comparison is much less easy with Figure 3.6 andso the data should be plotted together as in Figure 3.5. The clustered bar chart in Figure 3.5 clearly shows that there is a differ-ence in the self-reported type of delivery experienced by first time motherscompared to mothers who already have a child. Primiparous mothers areless likely to report a normal vaginal delivery and more likely to report hav-ing an emergency caesarean section than multiparous women. If the actualcounts had been used on the vertical axis, then t ...