Percent Change

[h4l3y]

how do I calculate the percent change of several numbers? or is there a way to combine two percent changes or get an avg of the two?
ex. new values: 18 and 25 old values: 37 and 35

I was told the answer is a total reduction of 59.19% but can't seem to get that answer

 

[skeeter]

percent change = [MATH]\dfrac{\text{(new value - old value)}}{\text{old value}} \cdot 100[/MATH]
a negative is a reduction, a positive is an increase

the average of your two examples would be ...

[MATH]\dfrac{100}{2}\left(\dfrac{18-37}{37} + \dfrac{25-35}{35}\right) \approx -40\text{%}[/MATH]
combining the two instead of averaging yields an overall change ...

[MATH]100\left(\dfrac{(18+25) - (37+35)}{37+35}\right) \approx -40.3\text{%}[/MATH]
the whole point here is context, which you haven't provided ... what, exactly, do the numbers represent and what is the time period for each change?

if the two sets of numbers represent a count of the same items that changes over the same period of time, I would probably go with the overall change.

 

[h4l3y]

thanks for the reply.

the numbers are daily avg ozone concentrations. the new values are for march 20th (18) and march 27th (25) 2020 and the old values are for march 22nd (37) and march 29th (35) in 2019 (those dates correspond to the 3rd and 4th Friday for march in both years). Im trying to see the reduction between the two years for those two Fridays as a whole.

I have done the percent changes separately as follows
(18-37)/37 x 100 = -51.35%
(25-35)/35 x 100 = -28.57%

but not sure how to combine the percentages or if the values should be combined before the percent change
I hope that makes sense