Notes on the arithmetic

Comparing prices across time is fraught with inaccuracies. Quite apart from the problems of selecting comparable products, the comparison is affected by the fact that any list selected from an old price list will reflect the popular products of the day, with no account taken of subsequent changes in tastes, or of the availability of new lines. To some extent this problem is reinforced by the use of an inflation factor derived from the retail price index for food, which is based on a basket of goods which is carefully selected to reflect contemporary tastes.

It is also difficult to compare quantities. Where possible, the prices given for the 2001 price are for imperial weights. For products that are today only available in metric pack sizes, for example butter, cocoa, tea, this is the price that has been given, on the basis that this is what shoppers actually buy. In each of these cases, the metric size is larger than its imperial equivalent. In 1911, many products were simply sold in 'large' or 'small' sizes, which makes accurate comparison impossible.

The process of converting prices from pounds, shillings and pence into decimal currency introduces considerable inaccuracies. In 1971, when Britain's currency was converted, the conversion rate was one new penny to 2.4 old pence (1p = 2.4p). This meant that very few prices converted exactly and had to be rounded either up or down. The pattern spreadsheet given here compounds this problem, because it rounds 0.5. Most of the prices given here have been rounded.