A monthly consumer price index traces changes in the monthly cost of a year’s
consumption using a sample of prices. But in some months the prices that can be
sampled will temporarily exclude some of the products that were bought in the
base year, Christmas trees providing a textbook example. Worse still, it becomes
permanently impossible to observe prices for sampled products that have been
completely superseded. There are methods for dealing with these two problems,
but they leave serious and irremediable defects in the index.