Subject: % Change
Name: Michael Weinberg
Who is asking: Other
Level: All

Question:
I am the Laboratory information system supervisor in a hospital. Our computer system calculates the change of specific results to monitor a patient's status. But there is a problem with the calculation in the base code of the program.
   If one day the patient has a Blood count of 100 (P for Previous) then the next day the Blood count is 75 (C for current), the computer should calculate the change as -25% using the calculation |P-C|/P= D (D for Delta) and then check for positive or negative by the equation If P>C then D*-1 or If P    The program is calculating using the equation: ((P/C)-1)*100 in the above situation this would calculate the Delta as 33.33333% Referenced in a Laboratory manual published before computers were used to calculate the delta changes.
   The computer's calculation would mark a change from 100 to 50 as a 100% change! If I had a 100% loss I should have nothing remaining. So I am trying to find any information or Documentation to send to the company that created the program code. Is there a one-step calculation? Or is the 2 step needed? Is there any general Delta check Formula documentation? Thank you for any help in this matter.

Sincerely
Michael P. Weinberg

Hi Michael

Percentage change can be confusing. Let's look at your first example with P = 100 and C = 75.

  • You calculated a percentage of -25% using (C - P)/P * 100. What you have is the change, C - P, as a percentage of P, the PREVIOUS value.

  • The computer used ((P/C)-1)*100 which is equal to (P - C)/C * 100. This is the change, P - C, as a percentage of C, the CURRENT value.

Both can be called the percentage change. One is the change as a percentage of where you started and the other is the change as a percentage of where you ended up.
   To me it seems more natural to consider percentage change as you have done, as the change as a percentage of where you started. You can convert the computer calculation by multiplying by -C/P. In the first example with P = 100 and C = 75,
33.33333*(-C/P) = 33.33333*(-75/100) = 25%.

Cheers,
Penny

Go to Math Central