Time Series Analysis
The best way to do this is to use a dBase Table (DBF file). You can create this with Excel to CSV to dBase in MICRODEM. You must insure that your readings are equally spaced in time, and it is best if you have a column with sequential numbers (set the value first row to 1, the second row to A1+1 [or A2+1 if you have labels]), and then copy the formula down the spreadsheet.
File, Open database to open a database.
DB for the time series. Note:

To see a graph of the data, try the STATS button and the 2D graph option.
Pick two fields for the x and y axes. For the time series, the x axis should be be the time axis (DAYS or SEQUENCE in the example above) and the yaxis the one you want to investigate for periodicity. 
You will not want another data filter. That would let you put a second series on the graph, but is not allowed for time series. 
The graph will appear. In this case there appears to
be some bad data around day 19. You can remove it with a Filter. In this case a range of
2037 looks like it would work well. After you filter, you must create a another graph, so you can close this graph. 
You can access the time series options from the Stats button, and the Time series options on the pop up menu.
Time series analysis options also appear on the Analyze Graph menu.
Last revision 2/27/2006