CIE Biology: How to Ace Paper 3

This post is specifically for the CIE Biology International A level qualification. What follows are my attempts to help guide students to do the best they can in the advanced practical skills exam, Paper 3. However, the information below is no substitute for proper revision and the dedicated practise of actually carrying out a variety of biology experiments. Additionally, I would strongly recommend you use the past papers section of CIE’s website to sit as many “mock” practical exams as possible.

Command words in a question


  • Say what is going on, e.g. the trend or pattern of results.
  • When describing data, always use units if appropriate (e.g. °C or cm3).


  • Say why a trend or pattern is occurring.
  • When explaining data, always link it to A level Biology.

Although describe and explain are the most common command words, you might find yourself answering questions with any of the following:

  • Calculate: use mathematics to find an answer. E.g. mean, percentage, percentage change, rate, ratio, etc.
  • Measure: use a suitable measuring instrument to take a reading, being sure to include units after the numerical answer.
  • Suggest: there is no one correct answer; you should look through the information you have been given for some clues.

Even more rarely you might find one of the below command words in a question:

  • State: give a brief answer – maybe one word or a phrase.
  • Define: give a definition – these should be concise.
  • Determine: explain how you could take measurements and calculate an answer from these measurements (e.g. in an experiment).

How many answers should I give?

  • If a question states “identify two” then only the first two responses will be marked.
  • If a question states “record observable differences” then all responses will be marked.

How great is the risk?

If a question asks you to “state the hazard with the greatest level of risk” for your practical, do not choose one you deem to be low risk. E.g. warm water in a thermostatically-controlled waterbath set at 30°C, is a hazard that is low risk and so would not gain a mark. A better answer would identify a risk that was either medium or high, e.g. hydrogen peroxide is harmful to the skin. The key is to identify a hazard with “the greatest level of risk”.

Deciding independent variable values

Give five values at roughly even intervals (e.g. every 5°C or 10 cm3) when deciding what values to use for your independent variable. Always use units if appropriate (e.g. °C or cm3).

Recording numbers and drawing tables

Do not go past one decimal place when recording the results from your experiment, usually whole numbers are fine. And always use units if appropriate (e.g. °C or cm3).

Table Headings:

  • Put the independent variable (IV) on the left and dependent (DV) on the right.
  • Draw a line between the top row and the body of the table, e.g. underline the IV and DV.
  • Use the full name of the IV and DV (e.g. temperature of hydrogen peroxide and time taken to rise).
  • Only use units  in the headings, not the body of the table (e.g. temperature of hydrogen peroxide / °C and time taken to rise / s).



  • Record results for at least five values of the IV.
  • Results should show the pattern or trend theoretically expected of the practical.
  • Use whole numbers.
  • Record results for two trials and calculate a mean average.
  • The mean average should be recorded to no more than one decimal place.

Identifying sources of error

Think carefully about your experiment, where might there have been an error? E.g. if you are looking at colour changes this is subjective and will be a source of error. Ensure you state the error and the reason it occurred.

Describe how an element of a practical can be investigated.

  • Use five values.
  • State how these five values will be made, e.g. if the IV is concentration of enzyme, “use simple or serial dilution” or if it is temperature “use a thermostatically-controlled waterbath at 20°C, 30°C, 40°C, 50°C and 60°C”.

Constructing graphs

  • You will normally use data that is given to you in the paper.
  • Put the IV on the x axis, DV on the y axis.
  • Use the full title from the table to label each axis, e.g. “initial rate of catalase activity / s-1“.
  • Always use units if appropriate (e.g. s or seconds but not sec).
  • Look at the values and use a logical scale. E.g. a scale of 0.06 or 0.04 for every large square of graph paper is not logical. However, a scale of 0.05 is logical.
  • Each plot will be checked to see whether it is accurate to within half a small square of the graph paper. It is recommended you use an x mark to do this.
  • There should be no labelling within the area of the graph.
  • Lines will be judged for their quality*.

Bar graphs

  • Ensure lines are not too thick, as the quality of each one will be judged*.
  • Bars can be touching. However, if you chose to leave gaps the gaps must be evenly spaced.
  • The horizontal lines at the top must be perfectly straight, parallel to the x axis.

Drawing diagrams

  • Quality of lines will be judged*.
  • No ruler straight lines for your diagram: nature is not straight!
  • No labels or writing within the drawing.
  • Label only what is asked of in the question.
  • Do not draw in anything that you do not see. E.g. smaller organelles.

Plan diagrams

  • The diagram should be at least 60 mm wide at its greatest width.
  • There should be no shading.
  • There should be no cells in  plan diagram. Do not be tempted to draw them in!
  • Use the correct section of the slide. This will be in the instructions of the questions, but make sure you actually draw what it wants you to!

Diagrams of cells

  • Cells should be at least 50 mm at their greatest width.
  • Draw exactly the number of cells stated in the question.
  • Do not include half cells. The questions always state “whole cells”.
  • Cells should not overlap but may be abutting (e.g. touching each other or sharing an outermost line).


  • Always show your working and the steps you take to come to your answer.
  • Always show units, but do not mix units. E.g. not 1 mm and 50 µm.
  • When converting units, show the conversion. E.g. 1 mm to µm: 1 x 1,000 = 1,000 µm.
  • Try not to go beyond one decimal place, or the same level of precision as is given in the question.
  • Give ratios to the lowest common denominator. E.g. 168:58 should be 84:29.

Comparing observable differences using a table

  • Organise the table as three columns; one for the differences and two for the samples. E.g


  • Underline the headings and divide the columns as per the table above.
  • If asked for differences, do not give similarities!
  • Differences would ideally be “X is thick” and “Y is thin”.

*Quality of lines

Each line you draw for a graph or drawing could be judged for its quality. E.g. whether it has been drawn by a sharp pencil as a thin and continuous line. This is really important as you don’t want to lose silly marks for not sharpening your pencil!


Image by Asim18, CC BY 2.5,


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