Correlation describes the pattern
Points trending up to the right = positive correlation; down to the right = negative; no pattern = none. Strong vs weak describes how close the points lie to a line.
The line of best fit balances the points
Draw a single straight line with roughly equal numbers of points above and below; it should follow the trend and pass near the mean point. Use it to estimate one variable from the other.
Correlation is not causation
A correlation between two variables does not prove that one causes the other — there may be a third factor.
Drawn from real examiner reports.
Plotting the points but not drawing the line
Marking all the points correctly yet failing to draw the line of best fit through them.
Reported on the June 2024 Foundation papers' graph questions (e.g. 1F Q12).
A line that does not span the data
Drawing the line too short, or in the wrong place / wrong gradient, so it is not extended across the full range of the data.
Reported on the June 2024 Foundation papers' graph questions.
Trusting predictions far outside the data
Reading a value off the line well beyond the plotted range (extrapolation) is unreliable — the trend may not continue.
Joining the points dot-to-dot
Connecting the plotted points with a zig-zag instead of drawing one straight ruled line of best fit through the trend. A line of best fit is a single straight line, not a series of segments.
Forcing the line through the origin
Making the line of best fit pass through (0, 0) when the data does not support it. The line should follow the points and pass near the mean point, wherever that lies.
Reading off a data point, not the line
Estimating a value from the nearest plotted point instead of from the line of best fit. Predictions must be read off the line, which averages out the scatter of the individual points.
Giving direction but not strength
Writing only "positive" or "negative" when the question wants a full description. State both the strength and the direction, for example "strong positive correlation".
Show construction lines when estimating
To predict, draw a line from the known value up/across to the line of best fit, then read off the other axis; show these construction lines.
State strength and direction together
A description of correlation needs two parts: strength (strong or weak) and direction (positive or negative). Answering with only one, or with just "relationship", drops the mark.
Plot the mean point to place the line
Find the mean of the x-values and the mean of the y-values and plot that point. A good line of best fit passes through it, with roughly equal numbers of points above and below.
A scatter graph plots paired data to reveal a relationship:
"Strong" or "weak" describes how tightly the points cluster around a straight line. Correlation does not prove causation.
Points trending down to the right show what correlation?
A scatter graph of daily ice-cream sales against temperature shows points that rise from the bottom-left to the top-right. Describe the correlation, and say what it suggests.
