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How to calculate SD or SE for results obtained from equation - (Dec/09/2012 )

Dear Guys,
Sorry for this silly question, but I need ur help
I know what is the meaning of Standard error and standard deviation.
and how to calculate them.
My question is, if my results would be expressed as % for example from some kind of equation.
How I can calculate SD or SE.
lets say this, the number obtained from my individual samples would be 5, 6, 5.5, 7.
and I compare it with control gives number of 10.
so if my equation is mean/control X 100
when I put it in my equation, the mean results would be expressed as % so results would be 58%

However, if I calculate SD or SE, from these numbers 5, 6, 5.5, 7. it will be so small (0.85) compared to result and incomprehensible?
How to calculate SD, or SE for results expressed from equations


You can do it either of two ways (don't quote me on this, I could easily be completely wrong here...): Either calculate the SD based on the samples before normalization, so this would give you mean = 5.875+/-0.7395, then normalize it for each of these values (mean: 58.75,upper: 66.14, lower: 51.36%), or you could do the normalization then compute the standard deviation from the % - which gives me the same answer (mean:58.75 +/-7.935%).