Reduce Data

Hi,

I have a very large data set that I would like to reduce for further processing. I would like to keep only every fifth line from a filtered selection. This means that four fifths of the filtered data should be deleted.

My data set consists of 25 experiments. The first three experiments have 5 times more data than the rest of the experiments. Therefore I only need every 5th measuring point for the first 3 experiments.

* How can I do that?