Drawing Graphs & Results Tables (Edexcel IGCSE Biology): Revision Note

Choosing a scale & plotting graphs

• When drawing the axes for your graph you should:

  • put the independent variable on the x-axis and the dependent variable on the y-axis

  • label both axes with the quantity and the unit

  • choose scales that allow your plotted points to cover more than half of the grid in both directions

  • use scales that are easy to read, e.g. 1, 2 or 5 units for each large square, or 10, 20 or 50

  • ensure that scales go up in equal steps all the way along the axes

• The axes do not have to start at zero, unless the question asks for this

• Edexcel "Plot a line graph" questions have usually been worth 5 marks, with one mark each for:

  • a linear scale that uses at least half of the grid

  • straight lines joining all the points

  • axes the correct way round, with the independent variable on the x-axis

  • units on both axes

  • points plotted accurately, to within half a small square

• If the table gives repeat readings and a mean, plot only the mean values

• A bar chart drawn instead of a line graph loses marks

• Plot each point with a small, neat cross (×) or a dot with a circle around it

  • Plot each point to an accuracy of half a small square

• Examiner reports show that students lose marks by:

  • choosing an unsuitable scale or one that uses too little of the grid

  • writing the data values along an axis as the scale, so the scale is not linear

  • leaving out units on the axes

    • For example, missing "per minute" from a rate

  • putting the axes the wrong way round

Joining points & lines of best fit

• In some questions you will be expected to join points on a graph

• In other questions you will be expected to draw a line of best fit

Joining points with straight lines

• Edexcel "Plot a line graph" questions usually tell you to join the points with straight lines, often adding "Use a ruler"

  • The mark scheme gives a mark for straight lines that pass through all of the points

• To join the points:

  • use a ruler and a sharp pencil

  • join each point to the next point with a straight line

  • do not draw a line of best fit or a curve

  • start the line at the first point and stop it at the last point; a line that is extended beyond the data loses the mark

• Joining the points is used when the values between your measurements cannot be assumed

  • For example, yearly figures that rise and then fall

Lines of best fit

• Draw a line or curve of best fit when the question asks for one, or when it does not tell you to join the points

• A line of best fit shows the trend in your results; it illustrates the general pattern or relationship between your variables

• When drawing a line, you should consider:

  • whether the line should be a straight line or a curve

    • If the points follow a constant linear pattern, plan to draw a straight line

    • If the rate of change varies or the trend levels off, plan to draw a smooth curve

  • where the line should start and end

    • Do not extend the line beyond your data; stop at the first and last plotted data points

    • Do not force the line through the origin (0, 0): only start at or pass through (0, 0) if this is shown in the data

  • how the points should be distributed around the line

    • Aim for a roughly even spread of points on each side of the line, along its whole length

    • If a data point is anomalous and clearly falls outside the pattern, treat it as an anomalous result and ignore it when positioning your line

• Your drawing technique should include the following:

  • Always draw using a sharp pencil so your line is thin, clear, and precise

  • Always use a ruler for straight lines

  • Draw curves in one smooth, continuous movement

Bar charts vs histograms

• Use a bar chart when the independent variable is in categories, e.g. species or food type

  • The bars of a bar chart should be of equal width and should not touch

• Use a histogram for continuous data that has been put into groups, e.g. leaf lengths of 0–4 cm, 5–9 cm

  • The bars of a histogram do touch

• Label the y-axis with the quantity and the unit, and label each bar or group on the x-axis

• Draw all lines with a ruler

Recording results in a table

• When designing a table you should put the independent variable in the first column and the dependent variable in the next

  • Add columns for any repeat readings, and a column for the mean if you need one

• Give each column a heading with the quantity and the unit

  • Put the unit in the heading only, not next to every number in the column

• Record all values in a column to the same number of decimal places

• Draw one table for all your results, not a separate table for each part of the experiment

  • Draw the table with a ruler

• Avoid:

  • putting units in the body of the table

    • For example, writing °C next to every temperature

  • giving headings that are not a quantity

    • For example, "bubbles" instead of "number of bubbles"