Finding percentage uncertaintys

[greener1993]

I need help on how to work out uncertianity is there a set equation for it?

i found this on the web
Step 1:
Add up all your values and divide by the number of values

Step 2:
Subtract each value from this average value and take the absolute value from this (positive numbers)

Step 3:
Add up all these absolute values and divide by the number of values

Step 4:
Take this value and divide by the square root of the number of values you have

This 4 step process gives you the uncertainty of the data you have collected

is this correct?

1) when measuring wire i got the resualts of
0.28mm
0.28mm
0.28mm
0.29mm
0.29mm
0.29mm
0.28mm
0.29mm

The actual width of the wire said on the roll was 0.28mm and theses were taken with a electronic vernier scale which measures to 2dp of a mm. (if this is of any use)

i also read that if the uncertainity is below what the equipment can measure it is neglecable. And i also need to know how to turn it into a percentage

thanks in advanced

 

[jbsteele]

Yes, the 4-step process you found is correct. For your measurements of 0.28 mm to 0.29 mm, you can calculate the uncertainty as follows:Step 1: Add up all your values and divide by the number of values: (0.28 + 0.28 + 0.28 + 0.29 + 0.29 + 0.29 + 0.28 + 0.29) / 8 = 0.285Step 2: Subtract each value from this average value and take the absolute value from this (positive numbers): |0.28 - 0.285| + |0.28 - 0.285| + |0.28 - 0.285| + |0.29 - 0.285| + |0.29 - 0.285| + |0.29 - 0.285| + |0.28 - 0.285| + |0.29 - 0.285| = 0.005Step 3: Add up all these absolute values and divide by the number of values: 0.005 / 8 = 0.000625Step 4: Take this value and divide by the square root of the number of values you have: 0.000625 / sqrt(8) = 0.00254The uncertainty in your measurements is 0.00254 mm. To convert this to a percentage, simply multiply the uncertainty by 100: 0.00254 * 100 = 0.254%.