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Official figures show children who completed their primary school education in England this summer did better in maths and English tests than last year.

But around a quarter of primary schools did not administer the tests - known as Sats - in May because members of two large teaching unions staged a boycott of them.

What did this action involve and what was its impact?

National curriculum tests - popularly called Sats - are taken by 11 year olds in England in their last year of primary school, at the end of Key Stage 2. Pupils sit written tests in English and maths. The results are published by the government and are used by the media to compile primary school league tables.

Pupils are also tested at the end of Key Stage 1, aged seven, in reading, writing and maths. However, these tests are low-key and pupils may not be aware they are being tested. The results are not used for league tables.

Sats for 14 year olds, which were taken at the end of Key Stage 3, were scrapped by former Education Secretary Ed Balls in October 2008.

The National Association of Head Teachers (NAHT) and the National Union of Teachers (NUT) staged the boycott of the tests this year. In ballots of NAHT members and some members of the NUT in April, 64% voted in favour of the action. This meant many union members who were head teachers or deputy heads refused to administer the tests in their schools. However, the action was not mandatory, which meant members were within their rights not to take part in the boycott.

The NAHT and the NUT believe the tests are bad for children's education. They say teachers feel pressurised to spend so much time preparing pupils for the tests that it squeezes out other lessons. They are also deeply opposed to the way the results of the tests are used to compile primary school league tables. Other teachers' unions also have concerns about the tests, but were not involved in the boycott. Unions also say test and exam results used for league tables are too crude a way of measuring all a school offers. Sats were introduced in the mid-1990s to enable comparisons of primary schools and pupils.

The Department for Education said 4,005 schools of the 15,515 schools did not administer the tests. About 600,000 10 and 11-year-olds were due to take the national tests in English and maths in May, but only 420,000 pupils sat them. Some sat old papers which were marked by neighbouring schools so that the scores could be passed on to secondary schools, but the data would not be used for national comparisons.

Media organisations draw up the tables from data released by the government - the government does not draw up the tables itself. Around one in four schools will not have figures listed for their Sats results this year. This will make local comparisons between schools less meaningful.

The Sats data released by the Department for Education in August could not give a reliable figure for attainment in 20 local authorities, because so many schools in those areas had taken part in the boycott.

Critics of Sats say the results are not an accurate measure of pupils' attainment. In July, a report from the exams regulator, Ofqual, suggested 17.4% of grades awarded in English reading tests could be wrong because of inconsistencies in marking and flaws in the test design. The study found 1,387 pupils, who sat a sample reading test in 2007, had only an 82.6% chance of being graded correctly.

What about teacher assessment?

Teachers' unions have long argued that teacher assessment is the best way to assess pupils' progress and is less intimidating and stressful for children. This year, as well as publishing the Sats results, the Department for Education has also published the assessments of teachers. This data shows teachers gave the same figure as the test results for English and one percentage point higher for maths.

Some schools like the tests, because they give them a chance to show how much they have brought pupils on during their time there. The test data used to compile league tables gives a "contextual value added" or CVA score, which takes in a range of factors outside a school's control, such as pupils' gender, ethnicity and level of poverty. Some top schools like the tests because their pupils do well and that success is showcased.

While many parents think the tests are too stressful for their children, others like the tests because it shows them how well - or otherwise - their child is progressing. Crucially, parents like to look up schools they are considering for their children on league tables. No Sats means no league tables (in their current form, at least).

The government says the publication of test results keeps teachers on their toes and is an effective way of ensuring schools in the maintained sector are accountable.

Education Secretary Michael Gove has said the tests will go ahead next year. However, he has said there were "flaws" in the testing system and has promised a review.

Prior to the election, the Conservatives said the tests need to be "reformed but not scrapped" and the Liberal Democrats said they would keep Sats, but refine them with more weight put on internal teacher assessment and greater external checks to guarantee quality and consistency.

This being the case, it is unlikely the coalition government will scrap the tests totally, but schools, parents and pupils can expect to see some change to the way they are administered currently.

The tests have been abolished in Wales and Northern Ireland and have never been taken in Scotland.