Sir; Mystery shopping, like any other service or discipline, has to provide a worthwhile return on investment.

If it does not achieve that, it deserves to be binned and resources invested elsewhere.

Sue Grist's examples of shoppers being spotted and dubious results are real horror stories ('Money squandered on mystery shopping', The Grocer, 10 February, p19).

Proper mystery shopping conducted by accredited agencies is designed to

provide a positive return on investment by driving up customer service standards in store through training staff, measuring performance, raising the bar and achieving continuous improvement.

The staff game of 'spot the mystery shopper' has been played since mystery shopping was first invented. If allowed, it can make a programme worthless. Reputable agencies have strategies that they share with their clients to overcome such issues.

Despite the negativity expressed in the article, I was heartened by the comments from Asda and Sainsbury. Mystery shopping is clearly a valued tool among the leading operators.