# What date in a tax year to invest in an ISA?

There is a well known result that if you have to choose the best option from a set of randomly distributed options then you should sample the first 37% of them, then from the remaining 63% select the first option better than the best of the first 37%. This algorithm doesn't guarantee you have selected the best option, but it gives the best results when averaged over many runs.

With the start of the UK financial year imminent I have to choose when in the next financial year to invest in a stocks and shares ISA. Normally I just invest the maximum as soon as the financial year starts on the grounds that it's hard (impossible?) to game the system, and I'll probably do the same this year. However the stock market has reached a high just as the next financial year starts, and this made me wonder if it was worth waiting to see if the market goes down again.

I realise there is no way to guarantee you are investing at the optimal time, and I am not asking for advice as to when to invest. My question is whether there is any strategy akin to the 37% rule that gives you the best chance of investing at a good time in the next twelve months? Is this a problem that anyone has approached mathematically or statistically? It does not seem likely the 37% rule would apply since the 37% rule assumes that on average the value of the options remains constant, but on average the value of the stock market increases.

Somewhat belatedly I see there is a similar question: Investing in stocks & shares ISA- when to make deposits? To distinguish my question from the previous one let me clarify that I am not asking when I should invest. I am asking whether there are any studies of the optimal strategy in these circumstances?

• So the idea is to invest the first time it goes below the lowest point in your 37% (or else at the end of the year)? Commented Apr 5 at 0:10
• The secretary problem is not a good way of looking at this as it gives a 63% probability you will not invest at all. Since the purpose of the ISA wrapper is to avoid income tax and capital gains tax on a given investment made in a tax year, the cost of not investing if you have the money available would be bigger than trying to time the market. Commented Apr 5 at 10:07