I'm doing a course in data analysis using IDL and we're doing trend removal. This is the heading of the section I'm on: Trend Removal/Zero Mean Data Sets The exercises are easy to program, but I just need to find something that explains data analysis. I was hoping to find an explanation of zero-mean data sets because I'm a bit unclear on that. I did find a formula somewhere, but the site didn't look too reliable. The formula was: zero-mean=(x-mean)/(standard deviation) Also the question says to eliminate the trend, that is, "find the data set". Now the data that we're analysing is the carbon dioxide readings. I eliminated the sinusoidal trend (period of 12 months), but then it says to: "sum this set to test whether it is approximately zero. Graph your detrended series" When I sum the set without the sinusoidal trend it obviously doesn't equal zero since the graph will still be plotted around the region of 350 ppm. I don't particularly get this and I really need help. If anyone can make sense out of my explanation I'd appreciate any help or direction to some useful resources that could help me with data analysis.