i have a graph of 1/V by 1/U. This should give a straight line where i can read off the intercept and get my focal length.

My problem is doing the errors fromt he graph can someone help me

## errors in graphs how do i do it

### blueheaduk

Posted 21 March 2005 - 04:21 PM

Not entirely sure what your graph is about, but if it crosses through the origin the centroid method may be the way to go. There's information on that in the SQA uncertainties sheets.

### blueheaduk

Posted 28 March 2005 - 08:14 PM

Ya the parallelogram thing is the centroid method lol. I can't remember exactly what to do with it either. I thiiiiiink its soemthing like:

1) Take an average of all the x-axis points you have and the y-axis points you have then plot that as a point. This is the centroid. Draw a straight line from the origin through this point (it should resemble something similar to a best fit line).

2) Look for the point which is furthest above your centroid line (ie most erroraneous) and draw a line parallel to the centroid line through that point.

3) Do the same again but for the point furthest below the centroid line.

4) Draw a straight vertical line through the largest and smallest x co-ordinate points so that you now have the parallelogram.

5) Find the gradients of the diagonals of the parallelogram (ie the bottom left of the parallelogram to the top right, and the top left to bottom right)

http://www.hitsomebody.com/centroid.gif

(The diagonals are the green line and the pink line)

6) Let these two diagonal gradients be m1 and m2

7) The uncertainty in the gradient is given by

Where n is the number of points on the graph (excluding the centroid).

The uncertainty in the itnercept is found by noting where the two 'worst lines' (diagonals) cut the y axis. let these be c1 and c2.

The uncertainty in the intercept is given by:

1) Take an average of all the x-axis points you have and the y-axis points you have then plot that as a point. This is the centroid. Draw a straight line from the origin through this point (it should resemble something similar to a best fit line).

2) Look for the point which is furthest above your centroid line (ie most erroraneous) and draw a line parallel to the centroid line through that point.

3) Do the same again but for the point furthest below the centroid line.

4) Draw a straight vertical line through the largest and smallest x co-ordinate points so that you now have the parallelogram.

5) Find the gradients of the diagonals of the parallelogram (ie the bottom left of the parallelogram to the top right, and the top left to bottom right)

http://www.hitsomebody.com/centroid.gif

(The diagonals are the green line and the pink line)

6) Let these two diagonal gradients be m1 and m2

7) The uncertainty in the gradient is given by

Where n is the number of points on the graph (excluding the centroid).

The uncertainty in the itnercept is found by noting where the two 'worst lines' (diagonals) cut the y axis. let these be c1 and c2.

The uncertainty in the intercept is given by:

### blueheaduk

Posted 01 April 2005 - 03:22 PM

Where the vertical lines drawn on the highest and lowest x-values cut the two "worst lines". You just read it off the graph.