Pass rates at Higher grade have not matched the improvement in Standard grade results over the past three years, according to a Scottish Examination Board analysis.
Each year since Standard grade was introduced in 1984, attainment has improved by 0.05 to 0.10 of a grade per subject as teachers become more familiar with the syllabus and format of the exam.
In a research paper that looks at predictions of Higher grade pass rates based on Standard grade results, the board reports that similar progress "has been more difficult to achieve at Higher grade. The overall pass rates at Higher have remained relatively constant."
The board notes, however, that many more pupils from the lower ability range are now taking the exam.
Pupils who achieve a grade 1 at Standard grade have a 90 per cent pass rate at Higher. Grade 4s either did not sit Highers in one year or performed poorly. The board has therefore placed the emphasis on grades 2 and 3 in its latest statistical report, aimed at helping teachers advise students on course options.
In 1993, 58 per cent of pupils attempting grade 2 in mathematics passed, against 48 per cent last year. Physics experienced a 9 per cent fall over the same period, as did history. Presentations at grade 3 followed a similar pattern.
Some variation may be due to school presentation policies and changes in examinations.