Determining the correlation of a variable with itself over time I have a mixture of organic compounds that have been stored for a period of 6 months.  I want to determine the effects of storage on the compounds (i.e. whether it has caused their amounts to decrease).  I was specifically asked to use spearman's rank to do this, but am confused about how it would work.  Would I check the correlation between the compounds at month 0 and month 1, and then at month 1 and month 2 and so on?

You need two variables, a value variable - telling you the amount of the compound at a specific time, and a time variable, matching the values. Then you would estimate a correlation coefficient between the values and times. How you code the time variable really depends on how you collected the data, on what frequency.
As an example, consider the following data, which was measured each day for 6 months, so we simply start the sequence from 1 and continue on forever.




Time
Amount




1
0.99


2
0.98


3
0.965


4
0.94


5
0.9


...
...


180
0.65




Now you can estimate a cor. coef. between these two variables.
