GCSE Computer Science

Transcription

1 GCSE Computer Science 4512/2 Computing Fundamentals Report on the Examination 4512 Summer 2015 Version: 1.0

2 Further copies of this Report are available from aqa.org.uk Copyright 2015 AQA and its licensors. All rights reserved. AQA retains the copyright on all its publications. However, registered schools/colleges for AQA are permitted to copy material from this booklet for their own internal use, with the following important exception: AQA cannot give permission to schools/colleges to photocopy any material that is acknowledged to a third party even for internal use within the centre.

3 General Comments Where possible these papers were marked online this June to ensure efficiency and accuracy of marking. Because of this it is possible to collect a wide range of statistics on each part of each question as well as on the exam paper as a whole. Many of these statistics are available to centres through the Enhanced Results Analysis which should be read in conjunction with this report. There was a broad spread of marks from candidates sitting this paper and, as with the last exam series, the extended answer question and the nine mark question on algorithm design showed a broad range of marks. Broadly speaking, candidates appeared well prepared for this exam although the question on network (dis)advantages caused some confusion with candidates assuming the question was on network topologies. Question 1 Over 70% of candidates were able to convert the binary number to denary in question 1(a). Many candidates used the space to show working which, although not creditworthy in itself, is encouraged as some candidates had made slight arithmetic mistakes when answering this onemark question. Only half of candidates were able to convert to hexadecimal being able to convert between denary, hexadecimal and binary is on the specification and candidates need to know how to do this. Just under half of candidates were able to convert 4C to denary in question 1(c). Some candidates multiplied 4 by 16 and then added 12 (C in hex) to this result whilst others converted the two hexadecimal digits to binary nibbles and then converted the binary to hexadecimal. Neither technique was necessarily more accurate than the other although many candidates did incorrectly translate C to 13 meaning that as long as their subsequent arithmetic was correct they would have received just one mark for their working. Almost three quarters of candidates received full marks in question 1(d) showing a good knowledge of bit units. Question 1(e) is a relatively simple question if candidates are aware of the way that ASCII encodings increment alphabetically (ie the code for g is one more than the code for f ). A good working knowledge of binary numbers would also mean that candidates would realise they only have to change the least significant bit from a 0 to a 1 to get the correct answer. Question 1(f)(i) was answered correctly by almost 80% of candidates, although the subsequent sub-questions of 1(f) proved more complex. 1(f)(ii) requires candidates to realise that there are four ways to arrange two bits (alternatively that 2 2 = 4). Part 1(iii) requires the reverse logic that 32 = 2 5 and so there are 5 bits needed to encode 32 separate colours; only 14% of candidates answered this question correctly (although some did give the answer 6 which, although incorrect, shows that their approach was probably correct). Finally, 1(f)(iv) was answered correctly by the majority of candidates although many candidates did have a variant on the number of bits used despite the instructions in the question. 3of 6

4 Question 2 Question 2(a) was answered correctly by less than half of candidates. All candidates should have the experience of coding with data structures and should be able to name the fundamental data structure(s) used in the language with which they are most familiar (eg a list in Python or an array in Java). The marks in part (b) were fairly evenly split with a similar number of candidates getting 1, 2, 3 and zero marks; the mark scheme gives a broad range of possible answers. Many candidates did not receive full marks here due to repetition of answers, for example answering, Requires writing less code as functions only have to be defined once, as well as, Saves programmers time when writing code. Question 3 For question 3(a) we use the BCS glossary (as we do in all cases of definitions) and define an algorithm as a series of instructions to perform a specific task although the mark scheme allows for other valid definitions. Question (b) required an analysis of two equivalent algorithms that many candidates found challenging. Less than a quarter of candidates were able to explain that the variable i has a role as a counter (any clear explanation would have been acceptable). Question 3(b)(ii), on data types, was answered successfully by more candidates. Part (b)(iii) was answered correctly by 26% - many candidates here just repeated the Boolean expression from one of the algorithms instead of interpreting the purpose of the algorithm as a whole. Approximately the same number of candidates were able to answer part (b)(iv) the comparative size of the two trace tables was a clue to answering this question correctly. Question 4 In question 4(a) a number of candidates put the analysis stage at the end of the simplified waterfall model rather than at the start. Most candidates were able to give another development model in part (b)(i). The extended answer question had a broad spread of marks. In some cases candidates did not describe prototyping which limited their marks. Additionally, some candidates provided weak (dis)advantages of prototyping such as increased cost or time (prototyping is frequently used in agile development to reduce both of these) and programmer attachment to a prototype. It should also be noted that it is entirely possible to receive all six marks in this question by two carefully written, succinct paragraphs that use technical vocabulary where appropriate. Question 5 Over 80% of candidates were able to identify the primary key and only slightly less were able to give the number of records for the second question 5(a)(ii). Over 60% of candidates were also able to answer question 5(a)(iii) and 68% chose the correct SQL statement in question 5(b), showing a good overall understanding of relational database tables. Question 5(c) rewarded candidates for choosing the correct records, displaying the correct fields and finally ordering them in ascending order; broadly speaking this is the order of complexity too with only 22% of candidates doing all three of these correctly. 4of 6

5 Question 6 We would encourage all candidates to know that a computer system is defined in the specification as a combination of hardware and software. Some candidates wrote that it is an input-processoutput system, but that does not go into sufficient depth to be awarded the mark for this question. In 6(b) candidates were required to describe, for up to four marks, how instructions are held in memory, requested and then decoded and executed by a processor, possibly processing data involving further reading and writing from/to memory and how this is a continuous cycle. Many candidates spoke at length about RAM and ROM and memory volatility as well as secondary storage and caching which were not necessary or directly relevant. Over half of candidates could provide a reason why a dual core processor might perform faster than an equivalent single core processor in question 6(c). In question 6(d), on memory terminology, over 74% of candidates received at least two marks. The final question on a possible situation when virtual memory might be needed was less well answered there are many different ways to word this answer, however, most correct answers were variations on the, when there is insufficient main memory concept. Question 7 Just under half of candidates realised that the RETURN keyword indicated that CarPrice was a function in question 7(a) with many candidates erroneously giving the keyword CASE. Almost exactly half of candidates correctly deduced that the datatype of cost must be a real in 7(b) (it was the only numeric datatype in the list of possible answers). Question 7(c) was completed correctly by a third of candidates with a half of all candidates gaining at least three of the four possible marks. Being able to follow an algorithm is a core skill for computer science and we would encourage all candidates to have significant practice with this (combined with understanding function calls and parameters for this particular instance). Candidates were fairly evenly split in the marks awarded for 7(d). As in a similar style of question earlier in the paper, many candidates gave the same answer twice instead of three distinct answers. Question 8 Few candidates achieved full marks in this extended answer question (5%) but over half received three or more marks. The question stem asked for a discussion on the advantages and disadvantages and we refer to the mark scheme to see how answers fall into the three mark ranges. Many candidates were obviously well-prepared on network topologies and gave lengthy answers on the comparative merits of ring, bus and star topologies but this was not what the question was asking. This points to a need for candidates to practice exam technique more deeply. Question 9 36% of candidates received seven or more marks in Question 9, answering the question fully in either pseudocode or a flowchart (there was no noticeable difference in achievement in the two different methods). We would encourage candidates to be explicit in their answers, for instance instead of writing, increase time by 40, a more complete solution would involve instantiating a variable called, for example, time to zero at the start of the algorithm and then writing time time + 40 at the point where it should increase. The mark scheme gives a point-by-point explanation of how marks are awarded with pseudocode and flowchart examples. Some candidates chose to 5of 6

6 declare functions in their answers and then call them unless the question specifically asks for a function then we would discourage this as the candidate puts themselves at risk of introducing errors due to overcomplicating their answer. Question 10 Over two thirds of candidates correctly answered questions 10(a)(i) and 10(a)(ii) although many candidates incorrectly gave both characters at position 1 and 3 in the second question instead of the single character at position 4. Question 10(b) was a relatively complex algorithm to deduce if a string is palindromic; the marks awarded reflected this with only 47% gaining three or more marks. As stated earlier, we would encourage candidates to have significant practice in analysing and tracing algorithms to assist their understanding of code and also their ability to develop their own. Question 10(c)(i) and (ii) were both answered correctly by over two thirds of candidates showing a good general knowledge of error types. Mark Ranges and Award of Grades Grade boundaries and cumulative percentage grades are available on the Results Statistics page of the AQA Website. Converting Marks into UMS marks Convert raw marks into Uniform Mark Scale (UMS) marks by using the link below. UMS conversion calculator 6of 6