Determining the number of clusters when performing unsupervised clustering is a tricky problem. Many data sets don't exhibit well separated clusters, and two human beings asked to visually tell the number of clusters by looking at a chart, are likely to provide two different answers. Sometimes clusters overlap with each other, and large clusters contain sub-clusters, making a decision not easy. For instance, how many clusters do you see in the picture below? What is the optimum number of clusters?
World number one Andy Murray has pulled out of next week's Miami Open with a right elbow injury. A lacklustre Murray lost to Canadian world number 129 Vasek Pospisil in the second round at Indian Wells last week. "The focus now is on getting ready for the clay court season. Apologies to the fans, it's one of my favourite tournaments," said the 29-year-old, who won the Florida event in 2009 and 2013. He will be replaced in the main draw by American world number 136 Taylor Fritz.