This chart shows how the biggest argument for grammar schools is a total fantasy
The prime minister said she plans to trigger a new wave of grammar schools - secondary schools which select students by means of an exam that applicants will sit at the age of eleven.
Whether Britain should revisit a grammar school system has been a divisive issue ever since former Labour prime minister Tony Blair banned anymore being created in 1998.
May, and other Tory MPs who mainly reside in party's right-wing, argue it improves social mobility as it gives kids from less affluent backgrounds a better chance to fulfill their academic potential.
Critics say the old system enshrined class divisions because large numbers of children from wealthy families ended up in grammar schools while working class kids were more likely to go to "secondary moderns" - schools which kids who failed the grammar school entrance exam were sent to instead.
In 2013, Financial Times blogger Chris Cook collected a load of data and put together a chart which illustrates how the grammar school system actually impacted the achievements of children from poorer backgrounds.
Cook's findings were clear: poorer children achieved better grades at non-selective schools than at grammar schools, while at the other end of the scale, kids from the wealthiest backgrounds performed better at grammar schools.
The method
The point of Cook's analysis was to see whether attending grammar schools actually improved the achievements of poorer children. To do this, he first created a fictional region, "Selectivia", which represented the four areas in the UK with the most grammar schools in Britain - Kent, Lincolnshire, Medway, and Buckinghamshire.
He then established a simple points system based on GCSE results (labelled FT points on the chart). He gave pupils 8 points for an A* down to 1 point for a G, and then took a total for each pupil based on the grades they achieved in English, maths, and their three other best subjects.
The final step was to incorporate a means of measuring poverty known as the Income Deprivation Affecting Children Index (IDACI). This is a score based on the number of poor households in each region. By taking into account all of these factors, Cook was able to assess how pupils from performed in their GCSEs at selective and non-selective schools in relation to the socioeconomic status.
As the red arrow on the left shows, poorer children performed significantly worse at grammar schools than at selective schools. While, at the other end of the chart, kids from the country's wealthiest families achieved better grades at grammar schools than at non-selective schools.
Going off this research, the claim that grammar schools help kids from disadvantaged backgrounds to have better academic achievements isn't true.