Exam Tips (Cambridge International A-Level Computer Science)

How the 9618 exams are structured

What papers do you sit?

• Cambridge International AS & A Level Computer Science (9618) is assessed by up to four papers, each worth 75 marks
• AS Level students sit Papers 1 and 2
• A Level students sit all four papers, either all at once or staged over two years (Papers 1 and 2 first, then Papers 3 and 4)
• Calculators are not allowed in any paper, so binary, hexadecimal and file size calculations must be done by hand

How long should you spend per mark?

• Every paper is out of 75, so the time per mark depends on the paper's length

  • Papers 1 and 3: about 1 minute 10 seconds per mark
  • Paper 2: about 1 minute 35 seconds per mark
  • Paper 4: about 2 minutes per mark

• A 6-mark algorithm question in Paper 2 is therefore worth about 10 minutes, so if you have spent 20 minutes on it, move on and come back later

What examiners are looking for

What are the assessment objectives?

• Every mark in 9618 is awarded against one of three assessment objectives (AOs):

  • AO1: demonstrate knowledge and understanding of the principles and concepts of computer science
  • AO2: apply that knowledge and understanding, including to analyse problems in computational terms
  • AO3: design, program and evaluate computer systems to solve problems, making reasoned judgements about these

• The balance changes between papers:

  • Papers 1 and 3 are 60% AO1 and 40% AO2, so nearly half of the marks need you to apply knowledge, not just recall it
  • Paper 2 is 40% AO2 and 60% AO3, so most marks come from designing and writing pseudocode
  • Paper 4 is 100% AO3, so every mark comes from working code and testing evidence

• Across the full A Level, the split is AO1 30%, AO2 30% and AO3 40%: more marks come from programming and problem-solving than from recall

Top exam tips for 9618

Answer in the context of the scenario

• Many questions are set around a scenario, such as a company, a school system or a device
• Examiner reports note in almost every series that generic answers lose marks when the question gives a scenario
• For example, if a question asks why lossy compression would not be used for a particular file, a description of lossy compression on its own scores nothing; the answer must say why it is unsuitable for that file

Use technical terms precisely

• Computer science terms have exact meanings, and using the wrong one can change the meaning of the whole answer
• Examiners give this example: compression does not make "the transmission speed faster"; it makes the time taken to transmit the file shorter, because transmission speed depends on the connection, not the file
• Other distinctions examiners check include data vs information, memory vs storage, record vs file, identifier vs literal value, and OUTPUT vs RETURN
• Avoid brand names: write "a word processor" rather than naming a product, as brand and company names cannot be credited

Match your answer to the command word

• The command word tells you how much detail is needed

  • Identify or State needs a short, precise answer
  • Describe needs the features or steps
  • Explain needs the reason why or how, usually with a "because" or "so that"

• Examiners point out that an "explain" question needs more detail than an "identify" question, so match the length and depth of your answer to the word

• See the 9618 command words page for every word used in 9618 and how to answer each one

Do not repeat the question

• Paper 2 examiner reports give this warning in most series: answering by repeating phrases from the question gains no marks
• If the question says "a program stores scores in an array", writing "the program uses an array to store scores" adds nothing; each mark needs a new point

Read the whole question before you start

• Some questions give an example in the stem and then exclude it from the answer
• For example, if a question states that a range check is a validation check and asks for two other validation checks, a range check scores nothing
• Look for words such as "other", "different", "apart from" and "using the given data"

Use Cambridge pseudocode, not your programming language

• In Papers 2 and 3, pseudocode must follow the Cambridge Pseudocode Guide for Teachers
• Python syntax such as len(), += or elif is not acceptable when a question asks for pseudocode
• In Paper 2, only use the functions listed in the Insert; examiners will not credit language-specific functions that are not in it
• Do not use reserved words as identifiers: DECLARE Input : STRING and DECLARE Integer : INTEGER both lost marks in recent series

Show your working and make the final answer clear

• For number base conversions, floating-point questions and low-level program traces, write out your working
• Some questions say "show your working" and award marks for it, so a correct answer alone may not get full credit
• Underline or box your final answer so the examiner can find it

Attempt every question

• Examiners report that even the hardest Paper 2 algorithm questions contain accessible marks, such as declaring variables, setting up a loop or returning a value
• In Paper 4, students who skipped an earlier part could still gain marks on later parts, for example by calling a subroutine they had not managed to write
• A blank answer is guaranteed to score zero

Worked Example A photographer emails high-resolution photographs to a client. The client prints the photographs as large posters.

Explain why the photographer should not use lossy compression for these files.

[2]

Answer:

Weak answer (0 marks): "Lossy compression removes data from the file to make it smaller. It is used for images and sound."

This only describes lossy compression and never mentions the photographs or the posters.

Strong answer (2 marks):

• Lossy compression permanently removes data from the file, so the original image cannot be restored [1 mark]
• The posters are printed at a large size, so the lost detail would make them visibly lower in quality [1 mark]

The second mark is only available because the answer links the lost data to the client printing large posters.

Exam Hint

• Before you write, underline the scenario details in the question; aim to use at least one of them in every answer that is worth more than 1 mark
• When you finish a question, check that every point you have written is a new point, not the question reworded, because each mark needs a separate, creditworthy idea

Common mistakes that lose marks

• These mistakes all appear in Cambridge examiner reports for 9618, most of them in more than one series

Paper 4: how do you lose marks on evidence?

• In Paper 4, marks can only be given for what is in the evidence document

• Examiner reports highlight these problems:

  • Code pasted as a screenshot instead of copied as text, which can be unreadable (especially on a dark integrated development environment (IDE) background)
  • Code cut off at the edge of the page, or the start of a subroutine missing
  • Test screenshots that do not show the input data, so the examiner cannot check that the output matches it

• Paste code as text, check that every line is visible, and make sure each test screenshot shows both the input and the output

How to revise for each paper

• Each paper needs a different kind of practice:

  • Paper 1: learn definitions precisely, then practise applying them to unfamiliar scenarios
  • Paper 2: write pseudocode by hand under timed conditions, using the Insert
  • Paper 3: practise the multi-step methods (floating-point normalisation, Boolean algebra, Karnaugh maps, Dijkstra's algorithm) until each step is automatic
  • Paper 4: write and test full programs on a computer in your chosen language, and practise filling in an evidence document

• See the guide for each paper for its question types, plus the command words and pseudocode pages:

  • How to answer Paper 1: Theory Fundamentals
  • How to answer Paper 2: Problem-solving and Programming
  • How to answer Paper 3: Advanced Theory
  • How to approach Paper 4: Practical
  • 9618 command words
  • Writing Cambridge pseudocode in the exam