5 Rules for Plotting the Line of Best Fit in IB Physics Graphs in Class Tests and Exams

Plotting the line of best fit in IB Physics exams is a surprising place for many students to lose easy marks BUT it happens a lot!

If you can follow these very simple rules when plotting your line of best fit in physics graphs - you’ll avoid losing easy marks

Note these rules apply to graphs with no error bars

RULE #1: Use a Clear, Plastic Ruler

This allows you to see the spread of points around your potential line of best fit. The general rule is that there should be an equal number of points above and below the line AND the points should be as close to the line as possible. A clear plastic ruler house makes this process easier.

RULE #2: Don’t Force Your Line of Bets Fit Through the Origin

If your line of best fit does not naturally go through the origin (or the intersection of axes) DO NOT FORCE IT THROUGH! Even if it makes sense that it should go through the origin. 

You can see that the line of best fit below has been forced through the intersection of the axes. The result is that the line of best fit is completely wrong. If the line were twisted slightly and it intersected at around 3.3 on the y-axis, the line of best fit would make more sense.

There’s often a very good reason that your line doesn’t go through the origin. There may be a systematic error in the collection of the data and this should be addressed in the conclusion and evaluation of data.

RULE #3: Use a Thin Pencil

Using a thick pencil means that the examiner cannot see the points clearly enough lying underneath/around the line of best fit.

Often the question will be assessing the plotting of points, as well as the quality of the line of best fit. If you hide the line with a thick pencil line - you’ll lose marks for obscuring the points. 

It also increases the reading error on the value of the y-intercept read off the graph.

Finally on this rule…. you know not to use a pen to draw your line of best fit - right??

RULE #4: No Hairy Lines!

This is not an art class! The line of best fit should be a continuous line through most of the points. Don’t make it all ‘hairy’

RULE #5: NEVER Join the Dots!

I’ve never studied Biology, but I think students are encouraged to join the dots in Biology. That’s a hard NO in Physics!

Some of these rules may be perfectly, frickin’ obvious and some of these rules may come as a big surprise. Either way, take them into account and apply them the next time you tackle a graph question in your IB Physics past papers, exams and class tests.