Informally Fit a Line to a Scatter Plot

Students draw a line of best fit (trend line) by hand through a scatter plot, attempting to balance the data points above and below the line. They recognize that the line does not need to pass through any particular point and assess the goodness of fit qualitatively.

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Real questions from this skill, graded here. No account, and nothing is saved.

Grade 8 · Statistics and Probability: Bivariate Data · question 1 of 3

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20 rungs. A star at the top.

Use line A to predict the plant height in cm where weeks of growth is 2.

Your answer:

How it is taught

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A fit line follows the trend of the cloud of dots.

Dots should sit close, with some on each side.

Read it: A line of fit

Line A runs through the middle of the dots: 4 dots sit above it and 4 below, and it stays close to them. Line B is flat at 8, with most dots below it. Line A fits.

Look for the line that runs through the middle of the cloud.
Check both sides: about as many dots above the line as below it, all close to it.
Read a prediction off the line: up from the x value to the line, then across to the y axis.

A line of fit sums up the trend, so it runs through the cloud, not beside it.

Dots on both sides mean the line is not pulled toward one part of the data.

The line's slope is how much y tends to change for each 1 of x.

A line of fit runs through the middle of the cloud, with dots on both sides of it.

Picking a flat line, or one that just joins the two end dots.

Line A runs through the middle: 4 dots above it and 4 below.

A line of fit passes through (0, 30) and (5, 45). What is its slope? — 3. The rise is 45 − 30 = 15, the run 5: 15 ÷ 5 = 3.

Hours studied vs. test score

Do students who study more get higher scores? Let's look at the data and find out.

We'll plot hours studied across the bottom and test score going up the side.

Each dot is one student — eight students in all.

Look at the cloud of dots. As hours go up, scores generally go up too.

What if we drew this line? Almost every dot sits below it — that's a poor fit.

Let's try a line that cuts right through the middle of the cloud.

About half the dots land above the line and half land below — that's balanced.

Grade 8 · Drawing a trend line by eye

This is our line of best fit. It won't hit every dot, but it captures the trend.

What you see

While they climb, your page writes one true line to ask about tonight, and a short note each Sunday.

MayaGrade 3 · Mathematics

Did you know?

A chessboard has 64 squares — share them out between the 8 rows and each row holds exactly 8.

Today’s goal: 1 star — one skill mastered, every rung of its ladder climbed
Climbing now: Divide within 100

An example student.

MayaGrade 3 · Mathematics

Maya earned 2 stars this week — Interpret multiplication as equal groups, which took 6 answers over 3 days before its star landed 3 days ago and Multiply by 6, 7, 8 and 9. Next up: Divide within 100.

An example week.

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