Graphical interpretations of data: An introduction

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This is the first of a series of articles on the design of simple graphs – graphs you could draw with pencil and ruler but are now more likely to be produced using software. You can find examples in the book Plain Figures1 covering the presentation of statistics by graphs and tables. Despite that and other sources of advice, simple two-dimensional graph forms often fail to communicate as their maker intended – or so one assumes.

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How to explain screening test outcomes

Consider the following problem: “The serum test screens pregnant women for babies with Down's syndrome. The test is a very good one, but not perfect. Roughly 1% of babies have Down's syndrome. If the baby has Down's syndrome, there is a 90% chance that the result will be positive. If the baby is unaffected, there is still a 1% chance that the result will be positive. A pregnant woman has been tested and the result is positive. What is the chance that her baby actually has Down's syndrome?”

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