Graphing Data

Navigate the knowledge tree: 🌿 Biology ➡ NCEA Level 2 Biology ➡ 2.1 Investigation ➡ Lesson 8: Grahing Data

Learning Intentions

By the end of this topic, you should be able to:

Choose the right graph for my data.
Plot data clearly and accurately with correct labels, scales and units.
Spot patterns or trends in my graph.
Describe what a graph shows using scientific language.

Glossary

Key terms and definitions.

anomaly: A result that does not fit the pattern of the other data.

axis: Lines on a graph that show variables. The horizontal line shows the independent variable values, the vertical line shows the dependent variable values.

categorical data: Data that fits into named groups, not numbers on a scale.

continuous data: Measured numerical data that can take any value within a range.

correlation: The relationship between two variables.

dependent variable: The variable that is measured in the investigation.

discrete data: Counted data that can only be whole numbers.

graph title: A sentence that explains what the graph shows. It includes both variables.

independent variable: The variable that is changed on purpose in an investigation.

label: Words written beside each axis that name the variable and include units.

line graph: A graph used when the independent variable is continuous numerical data. Points are connected with a line of best fit.

line of best fit: A smooth line drawn to show the overall trend of the data, not connect-the-dots.

mean: The total of all results divided by the number of results. Its use in data analysis helps reduce the effect of random error.

pattern / trend: The overall direction of change shown in the data.

scale: The numbers used on an axis.

units: The measurement used for a variable. Always written in brackets.

How to choose which type of graph is suitable

Choosing what charts or graphs to draw is influenced by the nature of the data. There are many different types of graphs, and different graphs are appropriate for different kinds of data. Before representing data graphically, it is important to identify the kind of data you have.

For continuous data:
- - scatter plots (visualise relationship between variables)
  - line graphs (visualise patterns / trends over time)
  - histogram (visualise distribution)

For continuous data with calculated means, points can be connected. On scatter plots, a line of best fit is often drawn.

For categorical data:
- - box plot (visualise distribution)
  - bar graphs (visualise trends over time, better than pie chart)
  - pie chart (only use for few categories, great for communicating with the public)

This is an excellent video that clearly distinguishes between the different types of data and which graphs to choose to represent different types of data.

Guided practice

For each investigation, choose the correct graph type.

Sugar concentration (M) vs average % change in mass of kumara
Type of vegetable vs average mass change
Time (minutes) vs rate of diffusion

Answer:

Line graph - Sugar concentration is continuous numerical data with an order.
Bar graph - Vegetable type is categorical data (names, not numbers).
Line graph - Time is continuous numerical data and shows change over time.

How to plot data clearly and accurately

Presenting graphs properly requires attention to a few basic details, including:

Use graph paper and a ruler.
Put the independent variable on the horizontal (x) axis, and label both axes with the name of the variable and the unit.
Make sure the scale on the axes shows a good range based on your results.
Give the graph a descriptive, accurate graph title.
Take care with accurately plot the points, using crosses 'x'.

Step 1: Determine the axes

A graph has two axes drawn at right angles:

The horizontal axis (or x-axis) along the bottom for the independent variable.
The vertical axis (or y-axis) up the side for the dependent variable.

Step 2: Label each axis

Each axis has a label. The axis label shows the name of the variable and its unit. The usual convention is for the units to be enclosed in brackets.

Step 3: Add the tick marks

On each axis, there are tick marks (the little marks at regular intervals along each axis. There are also tick mark labels (the numbers next to the tick marks).

Step 4: Plot the data points

The data points are the values plotted on the graph. Each point is plotted using a pair of values for the variables (the x-coordinate and the y-coordinate).

Step 5: Draw the line of best fit

A line is drawn, which either connects all of the data points or is a line of best fit. A scatter graph may show just the data points or it may also have a line of best fit.

When scientists are trying to determine whether there is a relationship between two variables, it can be helpful to draw a graph and use a line of best fit, or trend line.

In many experiments, you cannot draw a line that passes through all the points. You have to draw a line that fits most of them. This is called the line of best fit.

Step 6: Write a graph title

Finally, a graph should have a graph title that describes what the graph is showing. If there is more than one line on the graph, there will also need to be a legend or key to show what each line represents.

How to spot patterns or trends in graphs

Understand the context of the graph

When interpreting a graph, the first thing always is to pay attention to what the graph is about, i.e. the variables involved, as indicated by the labels on the axes and any other information. It is also essential to be aware of the range for each axis, since this affects the visual appearance of the graph. This is particularly important when comparing two or more similar graphs.

Common trends in biology graphs

When thinking about the meaning of the line on a graph, a starting point is to identify whether the line goes up or down.

Positive correlation / gradients go up: 'As x increases, y increases'
Negative correlation / gradients go down: 'As x increases, y decreases'
Zero correlation / gradients are flat: 'As x increases, y has no clear pattern'

So in the osmosis context, this could be written as:

As sugar concentration increases, % change in mass becomes more negative.
This shows water leaving the cells by osmosis.

Anomalies

An anomaly is a data point that does not fit the trend. These must be identified and not ignored.

How to describe what a graph shows using scientific language

Describing a graph means saying what it shows, NOT why it happens. For this, you should include:

The direction of change
The overall pattern / trend
Correct variables and units

Do not include explanations because these are saved for the Discussion section of your report.

Sentence starters to use:

"The graph shows that..."
"There is a clear trend where..."
"As the independent variable increases..."
"The data indicates..."