Reading tables and graphs
Getting the number you need out of a table, a bar graph or a chart, and saying what it does and does not show.
A worked example
A table lists Monday 12 mm, Tuesday 0 mm, Wednesday 8 mm, Thursday 19 mm, Friday 7 mm of rain.\n\nTotal: add every row, including the dry one — 12 + 0 + 8 + 19 + 7 = 46 mm.\nMean: divide by the number of days, not the number of rainy days — 46 / 5 = 9.2 mm.\nRange: largest minus smallest, and zero is a value — 19 - 0 = 19 mm.
The mistake people actually make
Three that catch almost everybody. A day with a zero is still a day, so it counts in the mean. On a bar graph the heights are the data and the labels underneath are the categories — adding the labels answers a different question. And on a rate like litres per 100 km, the smaller number is the better one.