Dark data : why what you dont know matters Data describe and represent the world. However no matter how big they may be data sets dont indeed cannot capture everything. Data are measurements and as such they represent only what has been measured. They dont necessarily capture all the information that is relevant to the questions we may want to ask. If we do not take into account what may be missing unknown in the data we have we may find ourselves unwittingly asking questions that our data cannot actually address come to mistaken conclusions and make disastrous decisions. In this book David Hand looks at the ubiquitous phenomenon of missing data. He calls this dark data (making a comparison to dark matter i.e. matter in the universe that we know is there but which is invisible to direct measurement). He reveals how we can detect when data is missing the types of settings in which missing data are likely to be found and what to do about it. It can arise for many reasons which themselves may not be obvious for example asymmetric information in wars; time delays in financial trading; dropouts in clinical trials; deliberate selection to enhance apparent performance in hospitals policing and schools; etc. What becomes clear is that measuring and collecting more and more data (big data) will not necessarily lead us to better understanding or to better decisions. We need to be vigilant to what is missing or unknown in our data so that we can try to control for it. How do we do that We can be alert to the causes of dark data design better data collection strategies that sidestep some of these causes and we can ask better questions of our data which will lead us to deeper insights and better decisions