The shape of a distribution tells you the type of variation. A smooth, hump-shaped histogram means continuous variation, and separate bars mean discontinuous variation.
This lesson belongs to variation. It uses the two types from comparing continuous and discontinuous variation.
How do you read a distribution?
Use four steps every time. They give you a sentence for each mark.
- Read the axes and units.
- Identify the shape: smooth curve or separate bars.
- State the highest and lowest groups with figures.
- Explain the pattern using genes or the environment.
Worked example: one class, two features
A class of 50 students is measured for height and recorded for blood group.
| Height (cm) | 140-149 | 150-159 | 160-169 | 170-179 | 180-189 |
|---|---|---|---|---|---|
| Students | 3 | 12 | 20 | 11 | 4 |
| Blood group | A | B | AB | O |
|---|---|---|---|---|
| Students | 12 | 14 | 4 | 20 |
Check the totals: 3 + 12 + 20 + 11 + 4 = 50 and 12 + 14 + 4 + 20 = 50.
Height. The histogram rises to a peak at 160-169 cm with 20 students, then falls to 3 and 4 at the ends. This bell shape shows continuous variation, since there are intermediate values. Genes and nutrition together explain the range.
Blood group. The bar chart has four separate bars. Group O is the highest with 20 students and group AB the lowest with 4. This shows discontinuous variation, controlled by genes with no environmental effect.
A sentence frame for graph questions
Use this frame: “The graph shows [type]. The most common [group] is [value] with [figure], and the least common is [value] with [figure]. This is because [reason].”
It works for any data set and makes sure each part of the marking scheme is covered.
The mistake that loses marks
The common slip is to describe the shape but skip figures, or to give a figure with no reason.
| Weak | Strong |
|---|---|
| Most students are in the middle | 20 of 50 students are 160-169 cm |
| It is continuous | It is continuous because heights form a smooth curve with no gaps |
| Group O is high | Group O is highest at 20, so it is the most common group in this class |
Another slip is to join the bars of a blood group chart into a curve. Bars stay separate.
Check yourself
A second class of 40 students has 5 students at 140-149 cm, 15 at 150-159 cm, 14 at 160-169 cm and 6 at 170-179 cm. Check the total and state the modal group and the type of variation.
Answer
5 + 15 + 14 + 6 = 40, so the data are complete.
The modal group is 150-159 cm with 15 students. The smooth range of heights with no gaps shows continuous variation.
What to study next
Go on to connecting mutation and variation conceptually, then try the variation practice set. The biological data interpretation trainer offers more data sets.
If a teacher could give you fresh data and check your descriptions, see online one-to-one Biology tuition.