• Statistics borrow from mathematics an air of precision and certainty but also call on human judgment and so are subject to bias and imprecision
• Knowing what has been counted, and how, tells us whether a study can really answer the question it addresses
• Like words, numbers and statistics mean different things in different contexts
• Just because something is statistically significant it doesn’t mean it is practically significant or of importance to society

It can be too difficult or not practical to make a complete count – it is impossible to know the exact number of illegal immigrants, for example, and we can only estimate the number. The only time when the whole population is asked for information is during the national census, which takes place every ten years and provides figures from a national to neighbourhood level. And even during the census not everyone will be counted.