Question

We're actually learning propigation of error in my chem class, but it seems to be used equally as much in Physics/Stats.

My teacher showed us two methods of doing it:

REAL Method (Addition/Subtraction):
square root[(error absolute 1)^2 + (error absolute 2)^2 +...(error absolute n)^2]

"Our School's" Method:
(Error absolute 1) + (Error absolute 2) +... (error absolute n)

Real Method (Mult/Div)
square root[(error % relative 1)^2 + (error % relative 2)^2 + ... (error % relative n)^2]

"Our" Method:
(Error % rel 1) + (error % rel 2) + ... (error % rel n)

He also has written
Absolute error/magnitude x 100 = Error % relative?

After doing most of a problem out, we have this:

1.0(sub 8) +/- 1(sub 8) %
We divide that by 100 to get rid of the percent and get
1.0(sub 8) +/- .18 REL error

I'm not sure what to do next, but we end up getting
1.0(sub 8) +/- 0.1(sub 9) which gives us an answer of 1.1 +/- .2 g/mL

How do I get from relative error to absolute error? In other words, what happened between the part where I got the relative error, and the part I don't know how to do?