In the year 2000 300 out of a possible 600 completed a phone survey for a total of 50% completion rate. In the year 2001 20 out of 100 possible people completed the survey when it was re-done. This is a completion rate of 20%

To figure out the percentage change between the years can just use the difference between the percentage figures I already have? Or can I calculate it the same as you would for non-percentage numbers. For example you gave an example here of a salary increase from $20 to $95 being an increase of 375%.

Would I solve my problem the same way expect substituting my percentages (.5 and .2) in place of $20 and $95?