Describing Graphs & Trends (Cambridge (CIE) IGCSE Biology): Revision Note

How to describe the trend in a graph

• When a question asks you to describe a graph or a set of results, say what the data show

  • Avoid explaining why the data look like this

• Read as much of the data as you have been asked to consider; not more and not less

  • Some questions may ask you to describe an entire data set

    • You will lose marks if you miss out part of the pattern

  • Other questions may only require you to look at data points between, e.g. certain times or temperatures

    • You will waste time if you extend your description beyond these points

• A complete description usually has three parts:

  • The overall trend, e.g. as the temperature increases, the rate of enzyme activity increases

  • Any change in the pattern, e.g. the line levels off, reaches a peak or starts to curve

  • Figures from the graph or table, with their units, to support what you say

• Useful words for describing a pattern include:

  • "increases at a constant rate", for a straight line

  • "increases more quickly" or "increases more slowly", for a curve

  • "levels off" or "stays constant at…", for a line that becomes flat

  • "reaches a maximum of …, then decreases", for a peak

• Examiner reports show that students lose marks by:

  • stating that two quantities are directly proportional just because one increases when the other increases

    • A directly proportional relationship is shown by a straight line through the origin

  • describing only part of the graph

Reading values from a graph

• When reading data points from graphs, be sure to:

  • check the scale on each axis

    • convert any values if necessary, e.g. minutes to seconds

  • determine what one small square is worth, as it is not always 1

  • read values to an accuracy of half a small square

  • use a ruler to draw lines from the axis to the graph line and across to the other axis, so you read the value accurately

    • Leave these lines on the graph so the examiner can see what you did

• Examiner reports show that students lose marks by:

  • reading the wrong value from the graph

  • not checking the scale on each axis

  • forgetting to convert a value read from the graph into the unit needed, e.g. minutes into seconds

Quoting data from graphs & tables

• Marks are often available for quoting figures from a graph or table

• A quoted figure must include:

  • the value, read as accurately as the scale allows

  • the unit, taken from the Y axis label or the table heading for the dependent variable

  • the independent variable at which it applies, e.g. "at 30 °C" or "after 20 s"

• When you describe an increase or a decrease, say how much a quantity changes, not just that it changes

  • E.g. "it increases from 12 mm to 48 mm", not "it increases a lot"

• When you compare two sets of data, quote a pair of values from each set