Stepped sampling

STEPPED SAMPLING... This routine computes, plots, and saves sequentially sampled data at user-set intervals; among other uses, this helps determine the degree to which data sampled at one time are correlated to data sampled at different times (i.e., autocorrelation). For example, if you wanted to compute a relationship between (say) voluntary running speed and metabolism from a file containing many different running speeds, you would need to obtain your measurements at large enough intervals so that there was no autocorrelation between measurements. The intuitive expectation is that successive measurements obtained at very short intervals will closely resemble each other, but eventually become independent as the interval between measurements increases. The goal would be to use an inter-measurement interval large enough to be sure that autocorrelation ('pseudoreplication') is minimal.

The iterative sampling process starts at an initial point (a particular sample) in the file, then steps forwards and backwards by user-defined intervals (the 'skip' interval) and takes the mean, range, and SD of blocks of data of user-set duration, symbolized as:

-------||||||||-------|||||||--------|||||||-------||||||||-------

where ----- = skipped samples, ||||| = blocks, and | is the initial point

This process is repeated until user-defined limits are reached, or the start of end of the data are reached. The initial point can be a user-set sample number or the central point in a data block (for example, you could search for the highest point in a data file, use the 'set block' option, and then use the resulting block as the initial point). If a block is selected, the start and end points are set to the beginning and ending points of the block. The default value is 1/2 of the total number of samples. The control window looks like this:

When ready, click either the "delta-t step plot" button or select a new channel to display results, as in the following example:

Results are plotted on-screen (blue for measurements later than the initial point, red for measurements earlier than the initial point, and gray for the mean value for both + and - values at a given interval).

The 'scatterplot' button shows an X-Y scatterplot of step-sampled data from any two of the available channels (it's only available if there are 2 or more channels in the file).

You can use the channel buttons to select up to 10 channels that can be analyzed and stored in a tab-delineated (Excel compatable) or .cvs file with the "save as ASCII" button. Note that you can select more than 10 channels, but only results from the first 10 will be stored. If the file contains interpolated data as indicated with the standard interpolation markers "»" and "«", any stored values that were computed from interpolated data will be marked in the Excel file.