Thank you for downloading this week's more or less podcast. Here's Tim Harford.

Hello, and welcome to a brand new series of more or less your weekly look at numbers in the news and in life.

This week a bus driver asks if we should get rid of traffic lights. A physicist tells us what would happen to New York if a terrorist set off a nuclear bomb in Central Park and a monkey tells us whether England or Australia will win the ashes.

Jack, the psychic cotton-top Tamarin, more on his astounding powers later. But first, let's talk about the hot political topic of the week, the increase in the cap on undergraduate tuition fees for English students, which was approved in the House of Commons last night.

Other government proposals fair and progressive, are they hanging a millstone around the neck of the nation's young people, dropping them into a deep hole of debt, or perhaps both.

Passions are high on both sides of the debate, but we couldn't help feeling the entire subject could do with a dose of numbers. So we decided to dig up Wesley Stevenson, dust him down and set him to work. What did you find, Wes?

Well, I decided it was time for a spreadsheet. We know quite a lot about how this new scheme will work, because in an act of kindness to number fiends like us, the Department for Business, Innovation and Skills have put all the data on their website.

Now, using this data, I've done some admittedly crude calculations based on a student borrowing £30,000 for their course. So this includes tuition fees and money for living expenses. And then looking at how much someone on various average salaries will end up paying back before the debt is written off in 30 years time.

What is striking is that someone who earns on average below £35,000 will pay very little or nothing at all. When you get to an average income of £50,000, you're still ending up paying back less than half of what you borrowed.

It's only when you get to an average of £70,000 annual income that you end up paying back £30,000, although even then, it's not the full amount of the loan once you include the interest.

Now, I don't want to get all economic see with you, but being asked to pay back money now is very different from being asked to pay back money in 30 years time. So how have you adjusted for that?

Well, the numbers I've given you are expressed in something called net present value, which adjusts for inflation plus the passage of time at a rate of 4.75% a year. This is the number of the government use in their calculations of the cost of tuition fees.

If you look at actual cash paid back, the numbers are substantially higher, but no economist would say this was the right way to think about debt payments.

Now, as you say, the figures are crude. Do we have any idea how many people won't have to pay anything back at all?

Because the salaries in my model rise by equal amounts each year, they're not really that realistic. So I decided I needed some expertise.

So I went to the institute for fiscal studies who have modeled this using much more realistic assumptions of how much people earn.

Lorraine Didden has been leading this work for the IFS. She's also a professor of economics at the Institute of Education. She gave me a better idea of who will pay.

Around 50% of graduates will, depending on the fee that they charge, will not pay back the loan, the higher the probability that they won't pay back the full value on the loan.

We estimate, on average, students will be paying around 25 to £30,000 in 2012 monetary terms for their university degree, which on average is more than they're paying.

At the moment, but there's a huge range in that the lowest earning graduates will be paying as little as £56,000, whereas the highest earning graduates will be paying on average, you know, well over £40,000 for their degree.

For those who aren't making enough money to pay back the loan quickly, it's basically functioning as a graduate tax.

Well, legally, it's not a tax. I mean, that would have different implications, for instance, for people who are emigrated, but it does look a lot like a tax.

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