Most people can understand the risk that share prices might fall after they have invested. Fewer people are likely to have considered the risk that total returns might also be reduced by failing to be invested on key days and other periods when share prices are rising.

Both risks are forms of bad timing and both are caused by the fact that share prices can fall - or rise - without warning. It is impossible for investors to be sure in advance whether prices will fall or rise after they buy shares, for example in investment trusts - or any other asset traded on the stock market.

However, if this investor missed just the 10 best days during this decade - and by this I mean hypothetically not being invested in shares for only one day each year for 10 years when share prices increased the most - then the total return during this decade would have plunged to £1,271. If the investor missed just the 20 best days, then they would have suffered a loss by the end of the decade - turning £1,000 into £872. Where they missed the 40 best days - still only equivalent to the best four days each year - they would have lost more than half their capital, ending the decade with just £484.

Remember that Bob only ever invested at the top of the market. So, just before the oil crisis of the 1970s pole-axed share prices, he invested the £3,000 he had saved in 1972. Bad Timing Bob then watched the FTSE All Share index - a broader measure of the London market than the FTSE 100, which didn't start until 1984 - collapse by 60% between 1973 and 1974.

Not an encouraging start, I think you will agree, but Bob's saving grace - quite literally - was that he never sold. He knew he wasn't wise enough to choose the right time to buy - indeed, he always chose the wrong time - but he remembered that his objective was to fund retirement and so he never tried to find the right time to sell.

As a result, even though he invested just before stock market setbacks - including 'Black Monday' in 1987; the bursting of the dot.com bubble in 2000 and the credit crunch in 2008 - he ended the 40-year savings period with a pension pot worth more than £632,000 (not including costs). Not a bad return on total investments of £112,000.

He never sold when share prices were depressed and thus benefited from the long-term upward trend in share prices and compound interest on dividend income. Bad Timing Bob's example shows us that short-term stock market setbacks need not matter much to people willing and able to take a long-term approach to investing in the stock market.