At every election we hear a similar refrain from all political parties - education standards are too low and schools are failing children.
A frequently cited statistic is the proportion of children who leave primary school in England unable to read, write and add up properly.
It is used to justify a range of different policies, from right and left.
This includes the removal of school management, the need for a pupil premium, or creation of new schools.
I have heard politicians put the number of 11-year-olds failing by this measure as high as 40%. Others, perhaps more reasonably, say it is 20%.
But even setting aside this wide statistical variance, what does it actually mean?
Illiterate or average?
The key word here is "properly".
It is rather a weasel word, tacked on quietly at the end so as not to reduce the impact of the raw "facts"
Sometimes it is left off altogether, giving the entirely false impression that up to 40% of 11-year-olds are entirely illiterate and innumerate.
So what are the facts?
The first place to look is at the results of last year's Key Stage 2 national tests, or Sats as they are still popularly known.
These show that 80% of pupils achieved the expected level in English and 79% in Maths - Level 4.
As an aside, this is a big rise from 49% and 45% respectively in 1995 when the Sats began, although some argue this is down to the tests becoming easier and more teaching-to-the-test.
So, the 80% attaining Level 4 could be used to support the claim that one in five pupils cannot read, write and add-up "properly".
However, a tougher test would be the proportion of pupils who achieved the expected level in both English and maths.
In 2009 this figure in England was 72%.
So, one could argue that 28% of children leave primary school unable to read and write and add up properly.
But the reality is that this is a huge over-simplification of what the test results actually tell us.
A pupil who attains Level 3, but does not reach Level 4, is very far from being unable to read, write or add-up.
To gain Level 3 in maths, for example, a pupil must be able to do mental arithmetic with two digit numbers and solve written addition and subtraction problems using three digit numbers.
To reach Level 4 they must, amongst many other things, be able to: Use mental arithmetic to solve sums using addition, subtraction, multiplication and division; recognise fractions and percentages; use and make three-dimensional geometric patterns; measure the area of simple shapes, and understand data collection, including terms like average and range, and to be able to interpret frequency diagrams.
In other words, to get to Level 4 a child needs to be able to do a whole lot more than simply "add up properly".
The same is true in English. A child who reaches Level 3 but not Level 4 is still able to read a range of texts "independently" and "fluently and accurately".
To get to Level 4 they must, amongst many other things "show understanding of significant ideas, themes, events and characters, beginning to use inference and deduction".
So, as with maths, achieving a Level 4 in English involves much more than the basics.
Yet, politicians often cite these statistics as if pupils today are failing even to reach the Victorian schoolroom standards of knowing the alphabet, reciting times tables, and producing copper-plate handwriting.
So how did we get to the idea that failure to reach Level 4 amounts to a failure to read and add-up properly?
To understand this, you have to go back to the start of the National Curriculum 20 years ago.
It introduced the benchmark of "levels" in each subject. The idea was that the average child would be at Level 2 at the age of seven and would move up one level approximately every two years.
The key thing here is that the target levels were set at the attainment expected of the average child.
In other words, a normal distribution curve would put 50% of 11-year-olds at Level 4, with 25% above that level and 25% below.
Politicians who made a fuss about 25% of students performing below average would sound a bit daft.
However, even during the creation of the National Curriculum politicians pushed for the benchmark standards to be raised.
Duncan Graham, who ran the first National Curriculum Council, later related how the group that devised the maths curriculum deliberately set the standard "above existing practice" partly as a result of political pressure.
While it is commendable that politicians want the school system to aspire to ever-higher standards, surely the benchmark should stay constant?
In fact, the bar was quite deliberately raised by former Education Secretary David Blunkett, who said before taking office, that a Labour government would set "clear national targets" for the proportion of pupils reaching the benchmark levels.
David Blunkett made Level 4 the expectation rather than the average
As he put it: "The national tests offer a benchmark of what should be expected at that age, rather than an average".
From that point onwards Level 4 became the dividing line between success and failure at age 11.
Thus the Labour government likes to focus on the growing proportion of pupils who now achieve Level 4, while opposition parties prefer to draw attention to the large minority who fail to attain the benchmark.
But can the test results really bear such interpretation?
Quite apart from the confused issue of whether Level 4 is an average or a minimum level, there is also the question of consistency in the tests from year to year.
As Professor Peter Tymms of Durham University explained to a Commons Committee a couple of years ago, the Sats tests have to change each year and keeping the standard the same is a matter of judgement.
If the officials decide that the marks required to achieve a Level 4 should go up or down by even a single mark, the percentage of pupils achieving or missing the benchmark would change by 2% or 3%.
As he said then, that is enough to "make national headlines" but in reality the change may be entirely due to errors of measurement.
So, as the general election campaign continues, you might want to challenge politicians of all parties to explain exactly what they mean when they claim that 20%, 30% or even 40% of primary school leavers cannot read, write and add-up.
And while you are at it, ask them what they mean by the word "properly".