# Absoloute Error

1. Apr 7, 2006

### khizman

Anyone can explain to me what Absoloute Error is and what this means:

Worst case % error = Absolote error/ Smallest reading * 100

For Voltmetre ______ %
For Ammetere ______ %
For Length _______ %
For Diametre _______%
_________________
Overall % error __________%

this if for a project on resistance of wires. 3 parts length, thickness, type of material.

heres a screenie of a sample of my reuslts
http://img439.imageshack.us/img439/9977/screenei4xw.jpg

Thanks For any help in advance

Last edited: Apr 7, 2006
2. Apr 7, 2006

### Chi Meson

I have never heard of "worst case error." But anyway...

Error is stated in two ways: relative error and absolute error.

The relative error is the percentage form such as " 20 meters plus or minus 5%"

The absolute error is in the unit of measurement, indicating the uncertanty of the final digit, such as "20 meters plus or minus 1 meter"

Notice in both cases the error is the same. When adding or subtracting quantities, you must add the absolute error. When multiplying or dividing quantites, you add the releative error.

3. Apr 7, 2006

### khizman

this plus or minus, in my case i dont have any plus or minus i dont think?

I know that my crocodile clip was 2 mm so that must affect my length measurement in some way/

4. Apr 7, 2006

### Chi Meson

the plus or minus is the uncertainly of the measurement. If your alligator clip made your measurement uncertain by 2 mm, so that your measurement is "between 23 and 25 mm," then the measurement is :
24 mm plus or minus 1 mm. (relative error) or
24 mm plus or minus 4% .

5. Apr 7, 2006

### khizman

hmm so that mean when i meaure my length of 5 cm (50mm) it could be plus or minus 4% is that corect?

6. Apr 8, 2006

### Hootenanny

Staff Emeritus
No because your croc clip was always 2mm its length is not affected by an increased length of wire, you need to use the absolute error. Therefore, your measurements will always be $X mm \;\; \pm 2mm$. Do you follow?

-Hoot

7. Apr 8, 2006

### khizman

but say i measure 5 cm isnt my crocodile clip affecting that length by 2mm
plus or minus?

8. Apr 8, 2006

### Chi Meson

2 mm is 4% of 50 mm, so it would be
50 mm +/- 4% [latex isn't working for me: "+/-" means "plus or minus"]
2mm is 3% of 66mm, so that would be
66 mm +/-3%

both have the same absolute error, but different relative errors.

Last edited: Apr 8, 2006
9. Apr 16, 2006

### khizman

so to work this formula out of,

worst case % error = absoloute error (is that 4% or +/- 2mm?) / smallest reading ( 5cm? ) *100

10. Apr 16, 2006

### Hootenanny

Staff Emeritus
Worst case;

$$=\pm \frac{2\times 10^{-3}}{0.05} \times 100$$

Regards,
~Hoot