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Now let us say that my baseline pain is 2/10, and I come in and I report 4/10 pain, then 6/10, then 8/10, so worsening pain over time. My SPID would be:
[2 hrs x (2-4)] + [2 hrs x (2-6)] + [2 hrs x (2-8)] = [2 x -2] + [2 x -4] + [2 x -6] = -24

Let us say I am going to have my pain monitored every 2 hours for 6 hours. Let us say my baseline pain is 10/10, and I first report 10/10 pain, then 8/10 pain, then 6/10 pain. My SPID would be:
[2 hrs x (10-10)] + [2 hrs x (10-8)] + [2 hrs x (10-6)] = [2 x 0] + [2 x 2] + [2 x 4] = 12

SPID, or sum of pain intensity differences, is a measure of adding the difference in pain weighted with the time difference. The time difference is the time interval from last check, and the pain difference is the difference between current and baseline pain.