GCSE Maths Statistics

Histograms

# Histogram

Here we will learn about histograms, including how to draw a histogram and how to interpret them.

There is also a histogram worksheet based on Edexcel, AQA and OCR exam questions, along with further guidance on where to go next if you’re still stuck.

## What is a histogram?

A histogram is similar to a bar chart but is used to display quantitative continuous data (numeric data), whereas a bar chart (or bar graph) is used to display qualitative or quantitative discrete data.

In a bar chart, the heights of the bars represent the frequencies, whereas in a histogram the area of the bars represent the frequencies.

To draw a histogram we need to find the frequency density of each class interval.

The frequency density (D) of a class interval is equal to the frequency (F) divided by the class width (W).

Expressing this as a formula, we have

D=\frac{F}{W} .

These values are then used for the heights of the bars on the vertical axis ( y -axis).

The horizontal axis ( x -axis) is labelled as the data variable with a continuous scale (not grouped).

For example,

Below is a grouped frequency table and the associated histogram.

Notice that the class intervals do not have to be the same size for a histogram. This means that the class width (W) may be different for each bar. Do not assume it is the same value for the data set.

A histogram can be used to show the shape of a frequency distribution of a data set.

Analysing the distribution of data is an important skill and is looked at in more depth in A Level Mathematics.

Step-by-step guide: Frequency density

Step-by-step guide: Frequency density formula

## How to draw a histogram

In order to draw a histogram:

1. Calculate the frequency density for each class interval.
2. Use the frequency density and class intervals to create suitable vertical and horizontal axes.
3. Draw bars for each class interval using the frequency density as the height of the bar.

## Histogram examples

### Example 1: drawing a histogram from grouped data

The table shows information about the ages of people at a cinema.

Use the information in the table to draw a histogram.

1. Calculate the frequency density for each class interval.

First we need to calculate the class width for each row. This is the highest value in the range, subtracting the lowest value in the range.

Here we have

Now we know the class width, we can calculate the frequency density using the formula D=\frac{F}{W} .

For the first row, F=5 and W=10 so D=\frac{5}{10}=0.5.

For the second row, F=16 and W=5 so D=\frac{16}{5}=3.2.

Continuing this for the rest of the table, we have

2Use the frequency density and class intervals to create suitable vertical and horizontal axes.

The maximum frequency density is 3.2 and the horizontal scale needs to go from 0 to 50.

3Draw bars for each class interval using the frequency density as the height of the bar.

Make sure you use a pencil and a ruler to draw each bar with precision.

### Example 2: drawing a histogram from grouped data

The table shows information about the heights of pupils in a mathematics class.

Use the information to construct a histogram.

Calculate the frequency density for each class interval.

Use the frequency density and class intervals to create suitable vertical and horizontal axes.

Draw bars for each class interval using the frequency density as the height of the bar.

### Example 3: drawing a histogram from grouped data

The table shows information about the mass of fish in a lake.

Use the information to construct a histogram.

Calculate the frequency density for each class interval.

Use the frequency density and class intervals to create suitable vertical and horizontal axes.

Draw bars for each class interval using the frequency density as the height of the bar.

## How to calculate frequency from a histogram

In order to calculate frequency from a histogram:

1. Locate the frequency density for the class interval(s).
2. Determine the class width for the class interval(s).
3. Use the frequency density formula to determine the frequency.

## Calculating frequency examples

### Example 4: calculating the frequency of a class interval from a histogram

The histogram below show information about the height h of plants in a garden.

Calculate the frequency of values in the interval 0 \leq h < 10.

Locate the frequency density for the class interval(s).

Determine the class width for the class interval(s).

Use the frequency density formula to determine the frequency.

### Example 5: calculating the frequency of a class interval from a histogram

The histogram below shows information about the time t seconds taken for students to complete a 400m race.

How many students ran 400m in under 90 seconds?

Locate the frequency density for the class interval(s).

Determine the class width for the class interval(s).

Use the frequency density formula to determine the frequency.

### Example 6: calculating the frequency of a random interval from a histogram

The histogram below shows information about the age of children who participated in a research study about mental health.

Estimate the number of students between 12 and 14 years old who participated in the research study.

Locate the frequency density for the class interval(s).

Determine the class width for the class interval(s).

Use the frequency density formula to determine the frequency.

### Common misconceptions

• Frequency vs frequency density

A very common error that occurs in histogram questions is that the frequency is used instead of the frequency density. Frequency density must be found because the groups provided in the frequency table are usually not equal width.

• The height of the bar is the frequency, instead of the area

Similar to the previous misconception, the height of the bar is considered to be the frequency whereas it is the area of the bar that represents the frequency for a histogram.

• The horizontal axis is labelled as discrete groups, rather than a continuous scale

The horizontal axis of a bar chart is divided into discrete categorical variables with gaps between the bars. This knowledge is incorrectly transferred to histograms. A histogram plots values on a continuous scale and so there are no gaps between classes.

### Practice histogram questions

1. What would the vertical axis be labelled as when constructing a histogram to show the height of students in a class?

Cumulative frequency

Frequency

Height

Frequency density

Frequency density is used for the height of the bars of a histogram.

2. Which formula would correctly calculate the frequency density for a class interval?

\text{Frequency density }=\frac{\text{class width}}{\text{frequency}}

\text{Frequency density }=\frac{\text{frequency}}{\text{midpoint}}

\text{Frequency density }=\frac{\text{frequency}}{\text{class width}}

\text{Frequency density }=\frac{\text{cumulative frequency}}{\text{class width}}

The frequency density is the frequency per unit for the data in each class interval.

3. The grouped frequency table shows information about the total number of trees in 1565 areas of woodland.

Calculating the frequency density for each row using the formula D=\frac{F}{W}, we have

The horizontal axis of a histogram should be labelled with the variable (here, the number of trees x ) using a continuous scale. Each bar should be the width of the interval. The scale should range from 0 to 200.

The vertical axis is labelled frequency density. Each bar should be drawn up to the frequency density value for the class. There must not be a break in the vertical axis. The scale should range from 0 to 10 minimum.

This produces the histogram.

4. The table shows information about the mass of artistic structures at a national gallery exhibition.

Calculating the frequency density for each row using the formula D=\frac{F}{W}, we have

The horizontal axis of a histogram should be labelled with the variable (here, the Mass, m \ kg ) using a continuous scale. Each bar should be the width of the interval. The scale should range from 0 to 40.

The vertical axis is labelled frequency density. Each bar should be drawn up to the frequency density value for the class. There must not be a break in the vertical axis. The scale should range from 0 to 5 minimum.

This produces the histogram.

5. The histogram below shows the mass of dogs at a rescue centre.

Select the correct frequency table that represents the data in the histogram.

The frequency table must contain the correct frequencies for each class. The class intervals are determined by the location of the start and end of the bar on the horizontal scale, so the first class would have the interval 0 \leq m <5 as the edges of the bar are at 0kg and 5kg.

The range must allow all values between the limits with no duplicate values for the mass.

The frequencies are calculated by finding the area of each bar (multiplying the height by the width). Specifically, this is found by multiplying the class width by the frequency density for each bar.

6. The histogram below shows the number of emails received over the same 2 hour period every day, recorded over one month.

Estimate how many emails in total were received between 10:00am and 10:20am over the month.

213

1.4

42

28

The frequency is the area of the bar for the given interval. The width of the bar is from 10:00am to 10:20am and so this could be considered as 20 minutes, or W=20. The height of the bar (the frequency density) is D=1.4 for the whole interval.

The frequency is therefore

\begin{aligned} F&=D\times{W}\\\\ F&=1.4\times{20}\\\\ F&=28 \end{aligned}

### Histogram GCSE questions

1. (a) The frequency table shows the ages of guests at a hotel.

Complete the ‘Frequency density’ column of the table.

(b) Use the table to draw a histogram for the data. Use the axes provided below.

(6 marks)

(a)

Attempt to divide frequency by class width seen.

(1)

Minimum of 3 of the frequency density values correct.

(1)

All frequency density values correct.

(1)

(b)

Frequency density used for vertical scale.

(1)

3 bars correct.

(1)

All bars correct.

(1)

2. The histogram shows information about the mass of 20 newborn calves on a farm.

Use the histogram to estimate the number of calves with a mass of more than 31 \ kg.

(3 marks)

Area of the fourth bar ( 32 to 24kg ) =2 \times 1.5=3 .

(1)

Area of bar within the range of 31 to 22 =1 \times 3=3 .

(1)

Frequency =6

(1)

3. The widths of plant stems in a garden were collected. An incomplete histogram and associated table are shown below.

Use the information provided to complete the histogram and table.

(5 marks)

Frequency density of the class 16 \leq x <20 is 3.5 .

(1)

Frequency density of the class 12 \leq x <14 is 2.5

and

Frequency of the class 12 \leq x <14 is 2.5 \times 2 = 5 .

(1)

Frequency density of the class 28 \leq x <32 is 0.25

and

Frequency of the class 28 \leq x <32 is 0.25 \times 4 = 1 .

(1)

Frequency density of the class 14 \leq x <16 is 9 \div 2 = 4.5

and

Drawn correctly on the histogram.

(1)

Frequency density of the class 20 \leq x <28 is 4 \div 8 = 0.5

and

Drawn correctly on the histogram.

(1)

## Learning checklist

You have now learned how to:

• Construct and interpret diagrams for grouped discrete data and continuous data, i.e. histograms with equal and unequal class intervals and cumulative frequency graphs, and know their appropriate use

## Beyond GCSE

Histograms are used extensively beyond GCSE, below are a few examples.

Outliers

Bimodal histograms

Cumulative histograms

The normal distribution

Skewness

## Still stuck?

Prepare your KS4 students for maths GCSEs success with Third Space Learning. Weekly online one to one GCSE maths revision lessons delivered by expert maths tutors.

Find out more about our GCSE maths tuition programme.