Two retail strategies are competing in the high street this Christmas. One is the Marks & Spencer belief that women want "investment" clothes, ie, safe and wearable rather than fashionable. The other is that women want something to lift their spirits beyond knee-length navy blue.

The trouble with the first strategy is that a solid piece of clothing can last about 20 years, and you will probably need to replace a coat only twice in an adult lifetime, three times if you bought an Afghan.

Even fashion is far more predictable than it tries to pretend. It seems to me that there are basically four styles: vampish, military, gypsy and Sarah Palin (which, interestingly, is described both as secretarial and boardroom, perhaps because men cannot tell the difference). The fifth style, ignored by fashion, is jeans or a variation on pyjamas, comfy and slovenly, which is how many women dress when they are not at work.

So it is not a surprising statistic that women wear only 30 per cent of the clothes that they own. We hardly ever need new clothes, in the way that one needs to replace lost glasses or gloves in winter. We acquire clothes through false logic, fantasy and hope. The sales probably account for 50 per cent of unworn clothes. The rest variously disappoint us.

Every successful dress works in the same way – Kate Winslet's simple, confident cocktail dress, anything worn by Cheryl Cole – but failed outfits are individual tragedies. My wardrobe contains skirts that have capriciously shrunk since I last wore them, dresses that are designed for tall blondes, suits that are perfectly OK, except I cannot warm to them.

The Clothes Show, which came up with the 30 per cent figure, gently chides women for refusing to let go of their pasts. Like Meryl Streep in Mamma Mia!, we recall past pleasures by hoarding glam rock silver outfits and six-inch heels.

Some clothes do have a haunting legacy. I have a beautiful, long, sleeveless black velvet dress with pearl buttons up the back and a white satin waistband. It was made for me by a friend called Magdalen Jebb for the opening of a film called The Killing Fields, in which my former husband played the journalist Jon Swain.

It survived the hopelessly youthful marriage and hung uselessly in my wardrobe for more than 20 years. The other week, my 14-year-old daughter, wearing the dress, stunningly appeared in my bedroom mirror. The Clothes Show must allow us a little sentimental value.

Fashion professionals regard the wasted 70 per cent in our wardrobes with concern. It is equivalent, to them, of a fridge full of mayonnaise and mustard past its sell-by. But I think the 70 per cent is the price we pay to settle on ourselves. The photographs of Kate Winslet in her perfect dress and fabulous shoes last week were accompanied in one paper by the old Kate in an unflattering black lace frock. It is a dress that anyone would have bought, as lace is sexy, a fashion perennial alongside military, gypsy, and Palin. It looked very expensive. I wonder if it has been thrown out.

Finding out what suits you, in clothes as in life, is trial and error. If the 30 per cent becomes 100 per cent, then the temperature drops. The reason that unfounded rumours about Anna Wintour's departure from American Vogue persist is that the risk is missing. Everyone has the right to fail, and the 70 per cent of lost wardrobes looks the right percentage to me.

Sarah Sands is editor in chief of British 'Reader's Digest'