Trimming or decimating to reduce the size of huge data tables.

When a data table is huge (tens of thousands of rows),  data analysis and graphing can be slow.

When you import a large data table, you can choose to Decimate. This is a choice on the Filter tab of Prism's Import dialog. Choose to import a row, skip K rows, import another row...

If you have already imported a huge table, Prism offers two ways to create a smaller table. The Prune analysis averages every K rows to create a new row. The Decimate feature while pasting keeps every K rows and eliminates the rest.

Prune Rows analysis

To prune, click Analyze and choose Built-in analyses. Then choose Prune from the list of data manipulations. Prism will average the data in every K rows, where K is a value you enter. If you enter K=10, Prism averages every set of ten rows to create one row in the results table.  

This analysis takes a large table, and creates as output a smaller table. if you enter K=10, the results table will have 1/10 as many rows as the original data table.  

Since the original data table will still be part of the file,  the project file gets larger when you run the Prune analysis. If you no longer need the full table,  delete it. The results table with the pruned data will now be frozen (since it is no longer linked to the original data) but can still be used for analyzing and graphing.

Decimate while pasting

When you copy data and then use Edit..Paste Special, Prism offers the same options as when it imports data.  Follow these steps to make a smaller table:

1. Create a new empty data table with the same format and subcolumns as your data table.
2. Go to the data table, select all the data, and copy to the clipboard.
3. Go to the new table, and put the insertion point in the upper left corner.
4. Drop the Edit menu and choose Paste Special.
5. Go to the Filter tab of the Paste Special dialog, choose to Decimate, and choose the number of rows to skip between the rows that are used.

If you choose to import a row, skip 9 rows, then import a row... the results table will have the same number of rows as created by the Prune analysis with K=10. But the pruning analysis averages every set of ten rows. Decimating keeps a row, ignores 9 rows (or whatever number you enter), keeps another row..

At this point, your file has the original data table plus the new smaller one, so the file is larger than it was.  Delete the original data table, if you no longer need it, to make your file smaller.