Line Plots of Measurement Data

A line plot is a number line with X marks that shows how many times each measurement appears in a data set.

Reading is good — doing is better. Practice Line Plots of Measurement Data as an interactive lesson.

Definition

A line plot is a simple graph that uses a number line and X marks (or dots) to show data. Each time a measurement appears in your data, you draw one X above that number on the number line. When you are done, you can see which measurement happened the most, which happened the least, and how the data is spread out.

Remember the rule

One measurement = One X. Always count your X marks to make sure you used ALL your data!

Key words

Line Plot: A graph that shows data on a number line by stacking X marks above each value.
Number Line: A straight line with numbers placed in order from least to greatest.
X Mark: A symbol you draw above a number each time that measurement appears in your data.
Data: A collection of information or measurements you have gathered.
Measurement: A number that tells the size or amount of something, like length in inches or weight in pounds.
Most Frequent: The measurement that has the most X marks above it — it showed up the most times.
Least Frequent: The measurement that has the fewest X marks above it — it showed up the fewest times.
Scale: The set of numbers shown on the number line, which should cover all the measurements in your data.

Worked examples

Seven students measured their pencils. The lengths in inches were: 4, 5, 4, 6, 5, 4, 5. Draw a line plot and tell which length was most common.

→ Draw a number line from 4 to 6. Above 4, draw 3 X marks. Above 5, draw 3 X marks. Above 6, draw 1 X mark. The most common lengths are 4 inches and 5 inches — they tied with 3 X marks each. · Always start your number line at the smallest measurement and end at the largest.

Look at this line plot of ribbon lengths (in inches): 2 has 1 X, 3 has 4 X marks, 4 has 2 X marks. How many ribbons were measured in all?

→ Add all the X marks: 1 + 4 + 2 = 7. There were 7 ribbons measured in all. · Adding up all the X marks always gives you the total number of things in your data set.

A line plot shows how many inches of rain fell each day for 5 days: 1, 1, 2, 3, 2. Which amount of rain happened least often?

→ Draw the line plot: above 1 put 2 X marks, above 2 put 2 X marks, above 3 put 1 X mark. The least frequent amount is 3 inches — it only has 1 X mark.

Students measured their hand spans. The data in inches: 5, 6, 5, 7, 6, 5, 6, 7. How many more students had a 6-inch hand span than a 7-inch hand span?

→ Count the X marks: 6 inches has 3 X marks, 7 inches has 2 X marks. 3 minus 2 equals 1. One more student had a 6-inch hand span than a 7-inch hand span. · Line plots make it easy to compare groups just by counting and subtracting X marks.

A teacher collected shoe sizes from 6 students: 3, 4, 3, 5, 4, 3. What numbers go on the number line, and where do the X marks go?

→ The number line goes from 3 to 5. Above 3: draw 3 X marks. Above 4: draw 2 X marks. Above 5: draw 1 X mark. The most common shoe size is 3.

Common mistakes

Forgetting to draw one X for every single measurement — always check that your total X marks match the total number of measurements.
Making the number line skip numbers that appear in the data — every number from the smallest to the largest measurement must be shown, even if no one got that measurement.
Stacking X marks unevenly or different sizes — each X should be the same size and lined up neatly so the plot is easy to read.
Reading the number line wrong by starting at zero when the smallest measurement is larger than zero — start at your actual smallest data value.
Forgetting to give the line plot a title or label — always write what the numbers measure, like 'Length in Inches.'

FAQs

Why do we use a line plot instead of just a list of numbers?

A line plot lets you see the data at a glance. You can quickly spot which measurement happened the most or the least without counting through a long list.

What if no one has a certain measurement — do I still put that number on the number line?

Yes! If a number falls between your smallest and largest measurement, you include it on the number line but leave it blank with zero X marks above it.

Can I use dots instead of X marks?

Yes, some line plots use dots. What matters is that each dot or X stands for exactly one measurement.

How do I know what numbers to put on my number line?

Find the smallest measurement in your data and the largest measurement. Your number line should start at the smallest and end at the largest, counting by ones.

What does it mean when a lot of X marks are bunched together?

It means many things had the same or close measurements. That measurement is very common in your data.

My line plot has 8 X marks but I only measured 7 things. What went wrong?

You drew one too many X marks somewhere. Go back and recount your data one measurement at a time, placing each X as you count, to find the mistake.

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