FFT Multiple Response for time T I want to do a fft spectral decomposition of a dataset, but the data has more than one sample for every time t. The data goes from the discrete years 1998 to 2010, each year has 20-30 points. Every example I can find online has a single point for any time t. Does FFT work fine in this situation? If anyone could push me in the right direction, I would be appreciative. The right thing to google is really the only thing I need.
You typically do a spectral analysis on a time series that has a single value per time instance. For example: what's the distance a car has driven after 1 hour. The answer can be 10 km or 50 km but it can't be both at the same time. 

We first we need to understand WHY is there more then one value at the same time

Sometimes data can have multiple dimensions. In our example above you can express the position of the car in x, y, &amp; z co-ordinate in which cases you have 3 values at the same time. In this case you would can analyze each dimension individually or you can derive a single quantity that you are actually interested in (like the total distance).  

It could also be statistic, you have multiple estimates for the same data point. You could reduce this by taking the mean or doing a Monte-Carlo type of analysis. 

Can you elaborate on the type of data and the reason for the multiplicity?