Hi - I'm programming in PHP. Here is some YTD information:
2011 YTD 14,989.3 2,877,688.8 $73,101.97
2010 YTD 18,409.8 1,911,203.4 $52,875.36
YTD Change Down 1.228% Up 1.506% Up 1.383%

The YTD Change % looks incorrect to me. This is the formula I'm using to calculate the percentage. I know you mathematical people will be horrified but I never did very well in math. :( Help!

The numbers are actually right, just the interpretation is off.
1.228 means you have currently 1.228 times last year, so that's a 22.8% increase.
If you get .9 then that's a 10% decrease. Take your number, subtract 1 and multiply by 100.
So (ThisYear/LastYear - 1)*100 gives the percentage change.

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The number you generate is the ratio of the new number to the old number. To see the change, you subtract the old number. It's the same as (new - old)/old. With a little algebra, you can see that they are identical.
If you lost 10%, this would actually give you a value of -10% gain. But you want to show the number as positive, so we flip it - (new - old)/old = (-new + old)/old = (old - new)/old = 1 - new/old.
I just short cut the algebra.

1.228 means you have currently 1.228 times last year, so that's a 22.8% increase.

If you get .9 then that's a 10% decrease. Take your number, subtract 1 and multiply by 100.

So (ThisYear/LastYear - 1)*100 gives the percentage change.