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Biology · Chapter 2: The Scientific Method (Solving a Biological Problem)
Data Collection, Graphs and Ratios
Recording data in tables, drawing line and bar graphs, and using ratio, proportion, percentage and mean in biology.
Key termRememberExampleWatch out
01Concept
The observations and measurements made during an experiment are called data. Data are useful only if they are recorded carefully and organised so that patterns can be seen. Biologists use tables, graphs and simple mathematics for this.
1. Recording data in a table
A results table should have a heading for each column, with the unit in the heading, not in every box. The independent variable goes in the first column.
| Time (days) | Height with fertiliser (cm) | Height without fertiliser (cm) |
|---|---|---|
| 0 | 0.0 | 0.0 |
| 7 | 5.5 | 4.2 |
| 14 | 11.3 | 8.3 |
| 21 | 15.2 | 11.2 |
2. Drawing graphs
| Type of graph | When to use it | Example |
|---|---|---|
| Line graph | When both variables are numbers and one changes continuously with the other | Height of a plant over 21 days |
| Bar graph | When the independent variable is in separate categories | Number of malaria cases in each province |
| Pie chart | To show parts of a whole, as percentages | Percentage of students with each blood group |
- The independent variable goes on the x-axis (horizontal).
- The dependent variable goes on the y-axis (vertical).
- Label both axes with the quantity and unit: Height (cm).
- Use an even scale (equal steps) and make the graph fill most of the grid.
- Give the graph a title.
3. Ratio, proportion and percentage
A ratio compares two quantities of the same kind, written as a : b. It is simplified by dividing both numbers by the same number.
Example: in a class of 40 students, 10 have blood group O and 30 do not. Ratio = 10 : 30 = 1 : 3.
A proportion says that two ratios are equal. It is used to scale up or down.
Example: a spray is mixed as 2 spoons of medicine in 1 litre of water. For 5 litres: 2 : 1 = x : 5, so x = 10 spoons.
A percentage is a ratio out of 100. If 18 of 24 seeds germinate, the percentage germination = 18 ÷ 24 × 100 = 75%.
4. Mean (average)
To make results reliable, experiments are repeated and the mean is taken. Heights 14, 15 and 16 cm give a mean of (14 + 15 + 16) ÷ 3 = 15 cm.
02Virtual Lab
Try this topic for yourself in the virtual lab. It opens on its own page with steps, tasks and questions.
03Key Formulas
Mean (average)
Repeat an experiment at least three times and use the mean.
Percentage
Example: 18 of 24 seeds germinate → 75%.
Ratio
- = a number that divides both a and b
Divide both parts by the same number to simplify.
04Real World Examples
Polio and dengue reports
The health department publishes bar graphs of polio and dengue cases by district and line graphs of cases week by week, so it can see where and when to send vaccination and spraying teams.
Your growth chart
The height and weight chart in a child's health card is a line graph. Doctors compare a child's points with the normal curve to check healthy growth.
Crop reports
Pakistan's yearly wheat and cotton production is reported in tables and graphs, and compared as percentages with the year before.
05Practice Questions
Try to answer in your own words first, then open the model answer.
Short questions
Q1.What are data? How are they organised?Show answer
Answer
Data are the observations and measurements recorded during an experiment. They are organised in tables and shown as graphs, and studied using simple mathematics such as ratios, percentages and means.
Q2.Which variable is plotted on the x-axis and which on the y-axis?Show answer
Answer
- x-axis (horizontal): the independent variable.
- y-axis (vertical): the dependent variable.
Q3.When would you use a bar graph instead of a line graph?Show answer
Answer
When the independent variable is in separate categories (for example, provinces or blood groups) rather than a continuous number.
Q4.In a garden there are 24 red roses and 16 white roses. Find the ratio of red to white roses in its simplest form.Show answer
Answer
- 24 : 16
- Divide both by 8: 3 : 2
Q5.Out of 50 seeds, 42 germinated. Find the percentage germination.Show answer
Answer
- Percentage = 42 ÷ 50 × 100
- = 84%
Q6.A student measured the pulse rate three times: 72, 76 and 74 beats per minute. Find the mean.Show answer
Answer
- Mean = (72 + 76 + 74) ÷ 3
- = 222 ÷ 3 = 74 beats per minute
Long questions
Q7.A student recorded the height of a bean plant: day 0: 0 cm, day 4: 3 cm, day 8: 7 cm, day 12: 12 cm, day 16: 16 cm. Explain how to draw a line graph of these results and state what it shows.Show answer
Answer
- Put time (days) on the x-axis and height (cm) on the y-axis, because time is the independent variable.
- Choose even scales: 0 to 16 days in steps of 4, and 0 to 16 cm in steps of 2.
- Label both axes with units and give the graph a title: Growth of a bean plant over 16 days.
- Plot each pair of readings as a point and join the points neatly.
- What it shows: the height increases steadily with time, by about 4 cm every 4 days, so the plant grows at a fairly constant rate of about 1 cm per day.