In relation to abundance of organisms students should be able to: •…

The idea

When studying abundance, you often collect repeated measurements, such as quadrat counts. The mean is the arithmetic average: add all values and divide by the number of values. The mode is the most frequent value, useful for categorical data such as the most common species. The median is the middle value when data are ordered; for an even number of values, it is the mean of the two middle values. For example, counts 2, 3, 3, 5, 7 have mean 4, mode 3 and median 3. When plotting graphs, choose a sensible scale so the data fill the axes, label both axes with quantity and unit, and use a line graph for continuous data or a bar chart for categories. Select a scale that makes differences easy to see without distorting the pattern.

What earns the marks

• Define mean as the sum of all values divided by the number of values.
• Define mode as the value that occurs most often in a data set.
• Define median as the middle value when data are arranged in order, or the mean of the two middle values when there is an even number of values.
• Calculate an arithmetic mean correctly from a set of abundance data, including using a calculator where appropriate.
• Choose an appropriate graph type: line graph for continuous data, bar chart for discrete or categorical data.
• Select a scale for each axis that uses the available space and allows plotted points to be read accurately.
• Label axes with the variable and its unit, and give the graph a title where required.

Common mistakes and how to put them right

• Calculating the mean by dividing by the wrong number of values.

  Put it right: Count the number of values carefully and divide the total by that count.

• Finding the median without ordering the data first.

  Put it right: Arrange values in ascending order before identifying the middle.

• Choosing a scale that does not fit the data, such as starting at zero when values are all between 80 and 90, or using uneven intervals.

  Put it right: Use a consistent scale that covers the full range and uses most of the axis.

• Plotting a line graph for categorical data such as species names.

  Put it right: Use a bar chart for categories and a line graph for continuous variables.

• Forgetting to label axes or include units.

  Put it right: Write the quantity and unit, for example 'number of daisies per m²'.

Exam tips

• Show your working for the mean, especially if the numbers are not whole, so you can gain credit even if the final answer is wrong.
• When drawing a graph, check that the scale is uniform and that each axis is labelled with both the variable and its unit.
• If asked to plot data, use a sharp pencil and plot points accurately; for line graphs, join points with a ruler or draw a line of best fit as appropriate.
• Read the question carefully to decide whether a mean, mode or median is most appropriate for the data given.