# Data analysis

1. May 7, 2006

### big man

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.

2. May 7, 2006

### Cyrus

I believe a zero mean data set is just that, a set of data where the mean value is zero. The formula you wrote is the z-score, $$z = \frac{ \bar{x} -\mu }{ \frac{\sigma}{\sqrt{n}}}$$

where n is the sample size. I think it is saying that if you sum the values after finding all the z-scores, the mean should be centered at zero.