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Much investigative journalism is qualitative: It looks at why and how things go wrong and who might be responsible. But almost every investigative story has quantitative data as well. How big was the deficit? What are the statistics on illegal fishing in your country? How many patients are turned away from clinics each year?

This means that you need to know how to tell a big number from a small one, and how to make sense of numbers through a few simple calculations like percentages. Most people do not become journalists because they really love numbers. But numbers are not difficult, and in fact, are essential to investigative storytelling.

Many people who believe they have no skill with numbers actually use them in quite sophisticated ways every day: budgeting for living expenses, determining if a season train ticket is a good value or negotiating a pay increase. The way many schools teach numeracy and math has contributed to a fear of 'mathematics' in perfectly numerate people; they have simply not learned to connect the practical applications of numbers with the abstract science of math.

The good news for journalists is that numbers in the media focus mainly on application and have a strong qualitative slant (for example, in understanding who collects statistics, how and why). But you start by understanding the basics. For example, if you want to check out a clinic nurse's official job description and work efficiency, you can employ experts who help you create a timetable of a 'typical day' in this profession. Then, via observation and interviews, you can find out:

>   What tasks occupy most time? Do the nurses use short-cuts? What are they? Are nurses faced with too many tasks to fit into their schedules?

>   How does a nurse’s job description relate to the average number of patients that visit a clinic? How long does the work take per patient?

Similarly, if you need an air sample analysed, you can find out what pollutants are in the air, and ask a medical expert whether these are dangerous and what levels of exposure will damage health. Match the levels against the air purity regulations in your country. You may find that this problem started a long time ago, and the figures have not changed much over time, or that similar 'peaks' seem to occur fairly regularly or even that that figures are lower now than they used to be! The journalist's job is to interpret these numbers and determine whether the problem has become bigger or simply more noticeable. But numbers alone are not enough. The context – why is the problem more noticeable now? – may be where your story lies.

Weather statistics are some of the longest-collected numerical records in most countries. In Africa, for example, they were among the first statistics the colonial authorities recorded, and can be traced back even further by community oral history traditions on floods and drought. In many Asian countries, there is also a dedicated database that documents weather patterns. In Southeast Europe, official weather records can be dated back to the end of the 19th century. So you may want to investigate whether weather conditions such as climate change, floods and drought are really unprecedented in your country. You can compare and analyse data for fluctuations in weather patterns.

The point is, always be on the lookout for how data can contribute to a story. Sometimes people will come to you with tips, but often you must discover the stories yourself. Story ideas can come from what you read, overhear or even from a press release. Press releases are not designed to be placed under statistical scrutiny – or at least, that is what their authors hope. But often press releases provide key information and may lead to bigger stories. Good investigative reporters do not let any possible story clues escape. However, be sceptical about numbers, graphs or and other forms of quantitative data. It may first seem like data present a unique compelling story, but good investigative reporters should question the methods used to generate data, like how a survey was conducted, how a sample selected, who funded and published it and whether it would be in their best interest to leave out any salient details?