Primary schools must not be judged on their SATs tests results alone, headteachers have warned.
Schools and parents know that the results, due to be published on Tuesday, must be taken with a "pinch of salt", according to the National Association of Head Teachers (NAHT).
More than half a million 11-year-olds across England took national curriculum tests, known as SATs, in May. The results are used in annual school league tables to assess a school's performance.
National figures on the proportions reaching the Government's expected standard for the age group will be released this morning.
Speaking ahead of the publication, NAHT general secretary Russell Hobby said: "Currently, the methods to hold schools to account aren't as fair or as reliable as they should be. SATs data only gives parents part of the picture when judging a pupil's success or a school's effectiveness. League tables are the least helpful way of knowing if a school is the right place for your child.
"At the moment, parents and schools know that these results have to be taken with a pinch of salt. This can't be right. Just looking at data misses the majority of the real work that schools do to help young people achieve their full potential.
"Schools do need to be held to account, but inspectors should look at more than just data. That way, when parents are reading Ofsted reports, they can have more confidence that the report properly reflects how good the school actually is."
There are signs that Ofsted is changing the way it uses data, Mr Hobby said, with chief inspector Amanda Spielman stating in a recent speech that inspections must examine what is behind school data, asking how they have achieved their results.
SATs tests have long-proved controversial, with teaching unions arguing that they put too much pressure on children, and the results are not a reliable measure of how schools are performing.
Last year, pupils sat tougher papers based on a new national curriculum, and 53% of pupils reached the new expected standard in reading, writing and maths.
The year before, under the old system, 80% achieved Level 4 - the old level expected of the age group - or above in these core subjects. Ministers were at pains to stress that the results were not comparable.
But union leaders branded last year's results a ''shambles''.
A Department for Education spokesman said: "Our new, knowledge-rich curriculum is designed to ensure pupils master the fundamentals of reading, writing and maths so that every child can reach their full potential.
"The results are an opportunity to celebrate the hard work that goes on in schools across the country, but they are just one of a number of measures that help us understand how primary schools are performing."