Health issues: Students should be able to translate disease incidence information between graphical and numerical forms, construct and interpret frequency tables and diagrams, bar charts and histograms, and use a scatter diagram to identify a correlation between two variables.

The idea

Disease incidence is the number of new cases of a disease in a population over a stated time. Scientists present these data in frequency tables, bar charts, histograms and scatter diagrams, and must move between numerical and graphical forms. A frequency table records categories or intervals with their counts. Bar charts compare discrete categories, leaving gaps between bars. Histograms show continuous numerical data grouped into intervals, so bars touch and the area of each bar represents frequency. A scatter diagram plots one variable against another, such as vaccination rate against disease incidence, and a correlation is identified from the overall pattern of points, not from a single point.

What earns the marks

• Defines disease incidence as the number of new cases in a population over a given time period.
• Constructs a frequency table with correctly labelled categories or intervals and matching frequencies.
• Chooses a bar chart for discrete categories and a histogram for continuous grouped data, with bars touching in a histogram.
• Reads values accurately from a graph or table and converts between graphical and numerical forms.
• Plots a scatter diagram with the explanatory variable on the x-axis and the response variable on the y-axis.
• Identifies positive, negative or no correlation from the overall pattern of points on a scatter diagram.
• Interprets a correlation as an association, not proof that one variable causes the other.

Common mistakes and how to put them right

• Leaving gaps between bars in a histogram.

  Put it right: Histogram bars touch because the data are continuous and grouped into intervals.

• Using a bar chart for continuous data such as age ranges.

  Put it right: Use a histogram when data are grouped into numerical intervals.

• Claiming that a correlation proves causation.

  Put it right: A correlation shows an association, and other factors may explain it.

Exam tips

• Label both axes with the quantity and its unit, and give the graph a title that names the disease and time period.
• When reading a histogram, check the scale on the frequency density axis before estimating a frequency.
• Describe a correlation using both its direction and strength, for example 'a strong negative correlation', and avoid saying 'the points are random' without checking the overall pattern.