Best fit line
Also called Line of best fit, Trend line
A best fit line is a single straight line drawn to follow the overall trend of plotted data rather than to pass through any particular points. AP scores it as a line that approximates the trend, and its slope is what the analysis then uses.
Drawn once, through the scatter, with roughly as many points above it as below. It does not have to touch a single one.
The AP Physics 1 CED names two habits it wants broken: students often think that to make a graph they need to connect the data points, or that the best fit function is always linear. Dot to dot is not a fit, and curved data call for a best fit curve.
How the exam pays for it. In the published scoring guidelines for the Experimental Design and Analysis question, one point goes simply to a line that approximates the trend of the data, and a separate point to using two points from the best fit line to calculate the slope with appropriate units.
Read that second wording twice. Take the two points off your line, not off your data. Picking two measured points throws away every other trial and hands you the scatter of two readings instead of the trend of all of them.
Drawing it well:
- Extend the line across the plotted range rather than stopping at the outermost points.
- Keep it straight, ruled, and single. A wobbling line through every point is not a fit.
- Do not force it through the origin unless the physics says the y-intercept is zero.
- Read the slope with units attached, since those units are what identify the physical quantity.
Choosing what to plot so that a straight line is the right shape in the first place is linearization.