Command Words (Cambridge International A-Level Computer Science)

What are command words?

Why do command words matter in 9618?

• A command word is the instruction at the start of an exam question, such as State, Describe or Explain
• It tells you what kind of answer the examiner wants and how much detail to give
• Cambridge examiner reports for 9618 regularly advise students to learn the meanings of the command words and to "form their answers accordingly"
• A correct fact written in the wrong form can still lose marks; for example, a one-word answer to an Explain question will rarely score

The most common command words in 9618

• The 9618 syllabus lists 27 command words, but a small number make up most of the questions

• In recent papers, the most frequent are Write, Describe, Complete, Explain, State and Identify

  • Paper 1 uses Describe and Explain the most
  • Papers 2 and 3 use Write the most: in Paper 2 it usually asks for pseudocode, and in Paper 3 it also asks for Boolean expressions, Reverse Polish Notation (RPN), declarative clauses and floating-point values

Command words that students confuse

State, Describe or Explain?

• These three words ask about the same topic at three different depths

  • State asks what: one clear fact
  • Describe asks what happens: the features or the steps, in order
  • Explain asks why or how: each point needs a reason

• Examiners point out that an Explain question needs more detail than an Identify question

• If an Explain question is worth 2 marks, aim for one point and its reason, or two linked points

Identify or Outline?

• Identify only needs the name of the thing; extra detail will not gain more marks

• Outline needs the main points of each item, briefly

  • "Outline two methods of overcoming this issue" needs a short description of how each one works
  • If the same question used Identify instead, the names of two methods would be enough

Evaluate means "work out the value"

• In most subjects, Evaluate means making a judgement

• In 9618 papers, Evaluate is used to ask for the value of an expression, matching the second half of the Cambridge definition: "judge or calculate the quality, importance, amount, or value of something"

  • "Evaluate this RPN expression" means work out the numeric result
  • "Evaluate each expression in the table by using the data values shown" means work out what each expression returns (for example, TRUE, FALSE or a number)

• Write the final value clearly and, where asked, show the stack contents or working that produced it

Compare and contrast

• Compare asks for similarities and/or differences; Contrast asks for differences only

• Every point must mention both items

  • "A linear search checks every item in turn, whereas a binary search halves the search area each time" scores
  • "A binary search halves the search area each time" on its own does not, because it says nothing about a linear search

Draw in 9618

• The syllabus defines Draw as "draw a line to match a term with a description", and 9618 papers do use it this way

• 9618 papers also use Draw for diagrams, such as:

  • logic circuits
  • structure charts
  • syntax diagrams
  • loops around groups in a Karnaugh map

• Use a ruler, follow the symbol conventions used in the question and label everything the question asks for

Worked Example A student sends a long document from a laptop to a printer.

(a) State what is meant by a buffer.

[1]

(b) Describe how a buffer is used when the document is printed.

[3]

(c) Explain why a buffer is needed when the document is printed.

[2]

Answer:

(a) A temporary area of memory that holds data while it is being transferred between devices [1 mark]

(b) Any three from:

• The laptop sends data for the document to the buffer [1 mark]
• The printer takes data from the buffer at its own speed [1 mark]
• When the buffer is empty, an interrupt is sent to the laptop [1 mark]
• ... requesting more data to be sent [1 mark]

(c) The laptop sends data much faster than the printer can print it [1 mark]

• ... so the buffer holds the data and the processor can carry on with other tasks instead of waiting for the printer [1 mark]

Part (a) needs one fact, part (b) needs the steps in order, and part (c) needs a reason, so the same topic is worth 1, 3 or 2 marks depending on the command word.

Exam Hint

• Before you answer, circle the command word and check the marks: an Explain question worth 2 marks needs a point and a reason, not two separate facts
• If a question uses Evaluate with an expression, it wants a value, so do not write a paragraph judging the expression

Command words in programming and working-out questions

Write

• Write is the most common command word in Papers 2 and 3

• In 9618 it usually asks for an answer in a specific form:

  • "Write pseudocode for the function..."
  • "Write a Structured Query Language (SQL) script to..."
  • "Write the Boolean expression that corresponds to..."

• When Write asks for pseudocode, it must follow the Cambridge pseudocode conventions; marks are lost for using programming language syntax instead

• See Writing Pseudocode in the Exam for the conventions examiners check

Complete

• Complete asks you to fill in something that is partly given, such as a trace table, a truth table, an assembly language trace or a Backus-Naur Form (BNF) definition
• Use the format already in the table; for example, if a column shows values in binary, do not switch to denary
• In a trace table, write a value only when it changes; in a truth table, fill in every cell

Show

• Show is not in the syllabus list, but it appears often in Papers 1 and 3

  • "Show your working" means the method earns marks, not just the answer
  • "Show the changing contents of the stack" means write out every state, not only the final one
  • "Show the result of a 2-place left logical shift" means write the resulting binary number in full

Other command words in the 9618 syllabus

• These words are in the syllabus list but appear rarely in 9618 papers

• Justify is the exception: it is common in 9618, usually as "Justify your choice" or "justify your answer"; give a reason linked to the scenario, saying why your choice suits it better than the alternative, not just how each works