Some exam boards' examinations are easier to pass than others, according to reports published by the Qualifications and Curriculum Authority.
A series of studies on standards over time in GCSE and A-level subjects by the authority show huge variations in syllabus content and the demands made on candidates.
Overall, while standards in most subjects have not changed, they have fallen in GCSE double science and teachers are marking coursework too generously.
GCSE English syllabuses offered by examining boards in Wales and Northern Ireland continue to be less demanding than those of boards in England.
The committee judged, as in a similar study in 1995-8, that the syllabus offered by the Welsh joint education committe board could be unbalanced because too high a proportion of marks was allocated to narrative and imaginative writing.
In GCSE English literature, the Northern Irish CCEA board was found to have become easier since 1995. One written paper of two-and-a-half hours had replaced two examinations of four hours in total, and three coursework assignments.
In GCSE geography, the Assessment and Qualifications Alliance (AQA) was criticised as potentially less demanding than that of other boards because it offered options with narrower content without ensuring there was increased depth.
Both Oxford, Cambridge and RSA (OCR) and CCEA, however, offered no option routes and all questions were compulsory.
But the AQA had the most difficult A-level German exam while the standard of reading required for this exam for Edexcel and CCEA has declined.
The WJEC board was found to have a less demanding German syllabus than other awarding bodies, in particular in the assessment of reading and listening.
The overall quality of students' coursework in GCSE double science was described as "disappointing", yet teachers often marked their projects too generously, masking poor work in written exams.
Reviewers carrying out comparisons on scripts said that the topics covered in the science papers had generally become more appropriate during the five years, (1995- 2000) partly because boards had trimmed syllabuses.
However, they found that the exams often failed to gauge the skills of both low and high-ability candidates and that pupils could gain too many marks by answering questions relating to general knowledge rather than science.
Occasional discrepancies were found between how pupils performed with one exam board compared to another, which were due to faults in individual papers.
In A-level geography, for example, syllabuses were at their most difficult in the mid-1990s. Twenty years ago more difficult skills were tested, but today's exam covers a broader range.
Barry McGaw, education director at the Organisation for Economic Co-operation and Development and head of the three-strong independent committee which the QCA has invited to review exam standards, said: "These reports are an important part of the QCA's monitoring work. We will examine them closely as part of our review of examination standards."
That report is expected to be published in the autumn.
Frank Wingate, of exam board Edexcel, said: "Of course there are slight differences. That's the point of having different boards. It encourages innovation and offers choice."
And Benet Steinberg, of the OCR, said: "Every year we design questions, reaching for a particular standard. We don't always get it right, so we tweak papers year on year. But the point is not whether the paper is harder or easier. The point is whether that is taken into account in the grading process, to deliver the same standard."
A-level geography (1980-2000)
The range of skills required expanded. In 2000 there was less emphasis on physical geography and more on aspects such as flood control and pollution which reviewers considered represented "a reduction in the level of demand".
But the range of skills required broadened to include fieldwork enquiry, problem-solving and decision-making as well as analysis of values and attitudes. So the loss of more challenging skills was compensated for largely by an increase in the breadth required.
Examinations moved from single sentence, opened-ended essay questions in 1980 to structured essay questions, which provided more opportunities for differentiation and allowed access to a wider range of candidates.
Overall the content of syllabuses expanded to reflect new dimensions of geography without any reduction in the depth of understanding required.
Exams were most difficult in the mid-1990s.
GCSE geography (1996-2001)
Syllabuses have been redesigned during this period to ensure a balance of physical, human and environmental geography.
The 1996 examination required 20-40 per cent recall, 20-40 per cent understanding and 20-40 practical skills. By 2001, new criteria meant that 30-40 per cent tested knowledge, 30-40 per cent understanding and application and 30-40 per cent skills and techniques.
In 1996 the emphasis was on human geography and minimal coverage of physical geography. By 2001 the balance improved, with a greater requirement to study physical processes. There was also a greater emphasis on environmental geography.
Overall the content, papers and coursework requirements were found to be almost identical.
GCSE English (1999-2002)
Reviewers found significant differences in the proportion of marks awarded for different aspects of the exam across boards. The study of poetry could account for 5 per cent or less of the overall marks for the CCEA and OCR, compared with 10 per cent or more in the AQA and Edexcel.
Edexcel increased the quantity of reading required from the anthology. In 1999 the required study was five poems and four non-fiction texts, in 2002 it was 14 poems and six non-fiction texts.
The QCA again reported concern about the poor technical accuracy of pupils' work at the grade F boundary. But overall it identified no differences in performance between 1999 and 2002
GCSE English literature (1980-2000)
Exam papers became accessible to a wider ability range in terms of the wording of questions. This did not alter the demands.
The number of texts studied increased from three to six and their range widened. However, while in 1980 candidates were expected to learn texts in detail, by memorising quotations, by 2000 they were allowed to take texts into the examination room and questions were based on interpretation and critical response.
In 1980 questions tended to be about plot, character and theme. By 1990 candidates were asked to give personal responses. Questions often included phrases such as "What do you think about..?"
Time pressures decreased, with the number of exam tasks varying from four or five in 1980 to two. The time allowed also decreased from two-and-a-half hours to two hours with many boards.
GCSE double science (1995-2000)
Reviewers found evidence that the standards of pupils' work had fallen. The overall quality of students' coursework was described as "disappointing" yet their projects often received overly generous marks, masking poor work in written exams.
Topics had generally become more appropriate, partly because boards had reduced the proportion of the syllabus which needed to be covered.
A-level German (1996 - 2001)
An increasing number of exams allowed candidates to use dictionaries, and fewer tested linguistic competence through translation. Some boards failed to set demanding enough tests for writing.
The standard of reading required by Edexcel and the CCEA declined while the AQA demanded the greatest linguistic breadth and sophistication. The report said that AQA's papers might sometimes be too difficult. Between 1996 and 2001 Edexcel and OCR reduced the amount of coursework required.
However, there was a general shift during that period towards testing in German as opposed to English, particularly in literature essays, leading to increased demands on candidates.
GCSE French (1996-2001)
The decision to let pupils use dictionaries in the exam has not made it any easier. Overall the exams was as demanding in 2001, when dictionaries were first allowed, as it was in 1996.
In some ways the exam seemed more difficult for less-able pupils, partly because the instructions and questions were now usually set in French rather than English.
The introduction of coursework did not make the exam easier, although the reviewers noted that pupils tended to gain higher marks than in the written papers. The standard of students' work remained roughly the same, although the quality of spoken French from the best candidates improved noticeably, with pupils "using a wider range of language more fluently".