Whatever you write, whatever you design, however much you spend on preparation, do not go flying in with a mailing to all 5000 secondary schools - or worse all 25000 primary schools.

That's always my message, and always people say, "but I have to get this out now - this is our selling season."

I write direct mail for a living, and about 80% of what I write works in the sense of doing what the client wants (improving response rates or holding them). Which means 20% of what I write doesn't work. Even after 20 years experience.

I never try to hide this - rather I always tell my clients about it, because what I want them to do is trial every change we make. How many you trial depends on the response rate you are looking for - but there should always be a trial.

This is how it works.

Firstly work out the percentage response rate you need to make enough profit to make the mailing worthwhile.  So, for example, if you make £20 profit per sale, and we take it that your total cost of mailing is going to be £40 per hundred, it is clear that 2 sales per hundred (2% obviously) breaks even. 3% is profit.  So if your trial gets 3% you are going to go ahead and mail the UK.

Now you know that you want 3%, do this sum: 600 divided by the percentage response rate. In this case that is obviously 600 divided by 3 - the answer is 200, and so the absolute minimum number of schools in your test run should be 200.

But let us imagine that, when you do your analysis of the percentage response rate you require, the answer is much lower - say 0.5%.  In other words you make enough money on each sale to only need to sell 1 for every 200 you mail.  Just mailing 200 schools here might well not give you an adequate answer - you might get the 1 sale, but you might not - and the statistics are simply not solid enough to give you any real information here.

The sum of 600 divided by 0.5 gives the answer 1200.  Mail 1200 and you should get 6 sales - and that is enough to give a fairly clear indication of just how near to your target you are getting.

Of course you might not want to go for 1200 straight off - you could mail 500 reasoning that if you don't get a single reply from the 500 you are clearly a long way from your target.  But if you do get a couple of replies, then it is worth pushing on with another 700 to see if overall the response rate rises a little.