Using charts to display data
Charts are useful for displaying data because you can see patterns and trends easily and quickly. You can also compare different sets of data easily. In this section you are going to revise what you already know about how to draw and make sense of pictograms, bar chats and pie charts.
Pictograms
Pictograms are fairly simple charts. Small symbols (pictures) are used to represent quantities. The meaning of the symbol and the amount it represents (a key) must be provided for the graph to make sense.
Example 1: The table shows how many books five different students have finished reading in the past year.
Draw a pictogram to show this data.
Example 2: This pictogram shows the amount of time that five friends spend talking on their phones during one week.
Time spend on the phone
a) Who spend the most time on the phone that week?
Anna- The person with the most clocks.
b) How much time did Isabel spend on the phone that week?
$\displaystyle ~3\frac{3}{4}$
There are three whole clocks, the key shows us each one stands for 1 hour. The fourth clock is only three quarters, so it must be $\displaystyle \frac{3}{4}$of an hour.
c) Who spend $ \displaystyle ~3\frac{1}{2}$hours on the phone this week?
Tara She has three full clocks, each worth 1 hour, and one half clock.
d) Draw the symbols you would use to show $ \displaystyle ~2\frac{1}{4}$hours.
Two full clocks to represent two hours, and a quarter of a clock to represent $\displaystyle \frac{1}{4}$hours.
Bar Charts
Bar charts are normally used to display discrete data. The charts shows information as a series of bars plotted against a scale on the axis. The bar can be horizontal or vertical.
There are different methods of drawing bar charts, but all bar charts should have:
· A title that tells what data is being displayed
· A number scale or axis (so you can work out how many are in each class) and label on the scale that tells you what the numbers stand for.
· A scale or axis that lists the categories displayed
· Bars that are equally wide and equally spaced.
Note! The bars should not touch for qualitative or discrete data.
Example 3: The frequency table shows the number of people who were treated for road accident injuries in the casualty department of a large hospital in the first six months of the year. Draw a bar chart to represent the data. Note that bar charts frequency axes should start from zero.
Patients admitted as a result of road accidents
Note! A bar chart is not the same as a histogram. A histogram is normally used for continuous data.
Compound bar charts
A compound bar chart displays two or more sets of data on the same set of axes to make it easy to compare the data. This chart compares the growth rates of children born to mothers with different educational levels.
You can see that children that born to mothers with secondary education are less likely to experience growth problems because their bars are shorter than the bars for children whose mothers have only primary education. The aim of this graph is to show that countries should pay attention to the education of women if they want children to develop in healthy ways.
Pie charts
A pie chart is a circular chart which uses slices of circle to show the data. The circle in a pie chart represents the “whole“ set of data. For example, if you surveyed the favorite sports played by everyone in a school then the total number of students would be represented by the circle. The sectors would represent groups of students who played each sport.
Like other charts, pie charts should have a heading and a key.
Some examples of pie charts:
Line Graphs
Some data that you collect changes with time. Examples are the average temperature each month of the year, the number of cars each hour in a supermarket car park or the amount of money in your bank account each week.
The following line graph shows how the depth of water in a garden pond varies over a year. The graph shows that the water level is at its lowest between June and August.
When time is one of your variables it is always plotted on the horizontal axis.
Choosing the most appropriate chart
You cannot always say that one type of chart is better than another-it depends very much on the data and what you want to show.However, the following guidelines are useful to remember:
- Use pie charts or bar charts( single bars) if you want to compare different parts of a whole, if there is no time involved, and there are not too many piece of data.
- Use bar charts for discrete data that does not change over time.
- Use compound bar charts if you want to compare two or more sets of discrete data.
- Use line graphs for numerical data when you want to show how something changes over time.
The advantages and disadvantages of each different types of chart/graph. You can use this information to help you decide which type to use.
Pictogram
Data is shown using symbols or pictures to represent quantities. The amount represented by each symbol is shown on a key.
Advantages
- Attractive and appealing, can be tailored to be subject.
- Easy to understand.
- Size of categories can be easily compared.
Disadvantages
- Symbols have to be broken up to represent `in between values` and may not be clear.
- Can be misleading as it does not give detailed information.
Bar chart
Data is shown in columns measured against a scale on the axis. Double bars can be used for two sets of data. Data can be in any order. Bars should be labeled and the measurement axis should have a scale and label.
Advantages
- Clear to look at.
- Easy to compare categories and data sets.
- Scales are given, so you can work out values.
Disadvantages
- Chart categories can be recorded to emphasis certain effects.
- Useful only with clear sets of numerical data.
Pie charts
Data is displayed as a fraction, percentage or decimal fraction of the whole. Each section should be labeled. A key and totals for the data should be given.
Advantages
- Looks nice and is easy to understand.
- Easy to compare categories.
- No scale needed.
- Can shows percentage of total for each category.
Disadvantage
- No exact numerical data.
- Hard to compare two data sets (other category can be a problem).
- Total is unknown unless specified.
- Best for three to seven categories.
Line graph
Values are plotted against `number lines` on the vertical and horizontal axes, which should be clearly marked and labeled.
Advantages
- Shows more detail of information than other graphs.
- Shows patterns and trends clearly.
- Other `in between` information can be read from the graph.
- Has many different formats and can be used in many different ways (for example conversion graphs, curved lines).
Disadvantages
- Useful only with numerical data.
- Scales can be manipulated to make data look more impressive.
Tip! Before you draw a chart decide:
- wow big you want the chart to be
- what scales you will use and how will divide these up
- what title you will give the chart
- whether you need a key or not.