The toothpaste survey

The book's sample project surveys the use of toothpaste. Its questionnaire asks about the family (size, income, occupation), the brand used, whether the family is satisfied with it, the qualities it looks for in a toothpaste, and which media (television, newspapers…) influence its choice.

The survey covers 100 households. About two-thirds (67%) are urban. Most families have 3 to 6 members, and most monthly incomes lie between ₹10,000 and ₹30,000.

The data are then analysed with the tools of the earlier chapters:

• Income: using the step deviation method (with A = 20,000 and c = 5,000), the mean income is ₹18,000, and the standard deviation (a measure of spread) is ₹9,000.
• Spending on toothpaste: the mean is ₹104. The book prints the standard deviation as ₹35.60, but working it out from the book's own table gives ₹37.74, so the printed figure is a slip.

The conclusions: Pepsodent, Colgate and Close-up are the leading brands; most people prefer gel and antiseptic toothpastes; and television is the main medium that influences buyers.

The lesson for your own project: check every table and every calculation before you trust a conclusion. Even a printed textbook can contain an error.