5.2 Calculating the price indices
This subsection concentrates on how the RPI is calculated. Generally the CPI is calculated in a similar way, though some of the details differ. To measure price changes in general, it is sufficient to select a limited number of representative items to indicate the price movements of a broad range of similar items. For each section of the RPI, a number of representative items are selected for pricing. The selection is made at the beginning of the year and remains the same throughout the year. It is designed in such a way that the price movements of the representative items, when combined using a weighted mean, provide a good estimate of price movements in the section as a whole.
For example, in 2012 the representative items in the ‘Bread’ section (which is contained in the ‘Food and catering’ group) were: large white sliced loaf, large white unsliced loaf, large wholemeal loaf, bread rolls, garlic bread. Changes in the prices of these types of bread are assumed to be representative of changes in bread prices as a whole. Note that although the price ratio for bread is based on this sample of five types of bread, the calculation of the appropriate weight for bread is based on all kinds of bread. This weight is calculated using data collected in the Living Costs and Food Survey.
Collecting the data
The bulk of the data on price changes required to calculate the RPI is collected by staff of a market research company and forwarded to the Office for National Statistics for processing. Collecting the prices is a major operation: well over 100 000 prices are collected each month for around 560 different items. The prices being charged at a large range of shops and other outlets throughout the UK are mostly recorded on a predetermined Tuesday near the middle of the month. Prices for the remaining items, about 140 of them, are obtained from central sources because, for example, the prices of some items do not vary from one place to another.
One aim of the RPI is to make it possible to compare prices in any two months, and this involves calculating a value of the price index itself for every month.
Changing the representative items
The Office for National Statistics (ONS) updates the basket of goods every year, reflecting advancing technology, changing tastes and consumers’ spending habits. The media often have fun writing about the way the list of representative items changes each year.
In the 1950s, the mangle, crisps and dance hall admissions were added to the basket, with soap flakes among the items taken out.
Two decades later, the cassette recorder and dried mashed potato made it in, with prunes being excluded.
Then after the turn of the century, mobile phone handsets and fruit smoothies were included. The old fashioned staples of an evening at home – gin and slippers – were removed from the basket.
So now, in 2012, it is the turn of tablet computers to be added to mark the growing popularity of this type of technology.
That received the most coverage when it was added to the basket of goods, with the ONS highlighting this digitalage addition in its media releases.
But those seafaring captains who once used the then unusual fruit as a symbol to show they were home and hosting might be astonished to find that centuries on, the pineapple has also been added to the inflation basket.
Technically, the pineapple has been added to give more varied coverage in the basket of fruit and vegetables, the prices of which can be volatile.
So, calculating the RPI involves two kinds of data:
the price data, collected every month
the weights, representing expenditure patterns, updated once a year.
Once the price data have been collected each month, various checks, such as looking for unbelievable prices, are applied and corrections made if necessary. Checking data for obvious errors is an important part of any data analysis.
Then an averaging process is used to obtain a price ratio for each item that fairly reflects how the price of the item has changed across the country. The exact details are quite complicated and are not described here. (If you want more details, they are given in the Consumer Price Indices Technical Manual, available from the ONS website. Consumer Price Indices: A brief guide is also available from the same website.)
For each item, a price ratio is calculated that compares its price with the previous January. For instance, for November 2011, the resulting price ratio for an item is an average value of
The next steps in the process combine these price ratios, using weighted means, to obtain 14 subgroup price ratios, and then the group price ratios for the five groups. Finally, the group price ratios are combined to give the allitem price ratio. This is the price ratio, relative to the previous January, for the ‘basket’ of goods and services as a whole that make up the RPI.
The allitem price ratio tells us how, on average, the RPI ‘basket’ compares in price with the previous January. The value of the RPI for a given month is found by the method described in Section 4, that is, by multiplying the value of the RPI for the previous January by the allitem price ratio for that month (relative to the previous January):
Thus, to calculate the RPI for November 2011, the final step is to multiply the value of the RPI in January 2011 by the allitem price ratio for November 2011.
Example 22 Calculating the RPI for November 2011
Here are the details of the last two stages of calculation of the RPI for November 2011, after the group price ratios have been calculated, relative to January 2011. The appropriate data are in Table 13.
Group  Price ratio:  Weight:  Ratio weight: 

Food and catering 
1.030 
165 
169.950 
Alcohol and tobacco 
1.050 
88 
92.400 
Housing and household expenditure 
1.037 
408 
423.096 
Personal expenditure 
1.128 
82 
92.496 
Travel and leisure 
1.026 
257 
263.682 
Sum 
1000 
1041.624 
You may have noticed that the weights here do not exactly match those in Table 12. That is because the weights here are the 2011 weights, and those in Table 12 are the 2012 weights, and as has been explained, the weights are revised each year.
The allitem price ratio is a weighted average of the group price ratios given in the table. If the price ratios are denoted by the letter r, and the weights by w, then the weighted mean of the price ratios is the sum of the five values of rw divided by the sum of the five values of w. The formula, from Subsection 2.3, is
The sums are given in Table 13. (The sum of the weights is 1000, because the RPI weights are chosen to add up to 1000.) Although Table 13 gives the individual values, there is no need for you to write down these individual products when finding a weighted mean (unless you are asked to do so). As mentioned previously, your calculator may enable you to calculate the weighted mean directly, or you may use its memory to store a running total of .
Now the allitem price ratio for November 2011 (relative to January 2011) can be calculated as
This tells us that, on average, the RPI basket of goods cost 1.041 624 times as much in November 2011 as in January 2011.
The published value of the RPI for January 2011 was 229.0. So, using the formula,
The final result has been rounded to one decimal place, because actual published RPI figures are rounded to one decimal place.
Example 22 is the subject of the following screencast. [Note that references to ‘the unit’ should be interpreted as ‘this course’. The original wording refers to the Open University course from which this material is adapted.]
Transcript: Screencast 5 Calculating an RPI
The same 2011 weights were used to calculate the RPI for every month from February 2011 to January 2012 inclusive. For each of these months, the price ratios were calculated relative to January 2011, and the RPI was finally calculated by multiplying the RPI for January 2011 by the allitem price ratio for the month in question. In February 2012, however, the process began again (as it does every February). A new set of weights, the 2012 weights, came into use. Price ratios were calculated relative to January 2012, and the RPI was found by multiplying the RPI value for January 2012 by the allitem price ratio. This procedure was used until January 2013, and so on.
The process of calculating the RPI can be summarised as follows.
Calculating the RPI
The data used are prices, collected monthly, and weights, based on the Living Costs and Food Survey, updated annually.
Each month, for each item, a price ratio is calculated, which gives the price of the item that month divided by its price the previous January.
Group price ratios are calculated from the price ratios using weighted means.
Weighted means are then used to calculate the allitem price ratio. Denoting the group price ratios by and the group weights by , the allitem price ratio is
The value of the RPI for that month is found by multiplying the value of the RPI for the previous January by the allitem price ratio:
The weights for a particular year are used in calculating the RPI for every month from February of that year to January of the following year.
Activity 21 Calculating the RPI for July 2011
Find the value of the RPI in July 2011 by completing the following table and the formulas below. The value of the RPI in January 2011 was 229.0. (The base date was January 1987.)
Group  Price ratio for July 2011 relative to January 2011:  2011 weights:  Price ratio weight: 

Food and catering 
1.024 
165 

Alcohol and tobacco 
1.042 
88 

Housing and household expenditure 
1.012 
408 

Personal expenditure 
1.053 
82 

Travel and leisure 
1.030 
257 

Sum 
Discussion
Group  Price ratio for July 2011 relative to January 2011:  2011 weights:  Price ratio weight: 

Food and catering 
1.024 
165 
168.960 
Alcohol and tobacco 
1.042 
88 
91.696 
Housing and household expenditure 
1.012 
408 
412.896 
Personal expenditure 
1.053 
82 
86.346 
Travel and leisure 
1.030 
257 
264.710 
Sum 
1000 
1024.608 
The published value for the RPI in July 2011 was 234.7, slightly different from the value you should have obtained in Activity 21 (that is, 234.6). The discrepancy arises because the government statisticians use more accuracy during their RPI calculations, and round only at the end before publishing the results.
The following activity is intended to help you draw together many of the ideas you have met in this section, both about what the RPI is and how it is calculated.
Activity 22 The effects of particular price changes on the RPI
Between February 2011 and February 2012, the price of leisure goods fell on average by 2.3%, while the price of canteen meals rose by 2.8%. Answer the following questions about the likely effects of these changes on the value of the RPI. (No calculations are required.)
(a) Looked at in isolation (that is, supposing that no other prices changed), would the change in the price of leisure goods lead to an increase or a decrease in the value of the RPI?
Would the change in the price of canteen meals (looked at in isolation) lead to an increase or a decrease in the value of the RPI?
Discussion
The RPI is calculated using the price ratio and weight of each item. Since the weights of items change very little from one year to the next, the price ratio alone will normally tell you whether a change in price is likely to lead to an increase or a decrease in the value of the RPI. If a price rises, then the price ratio is greater than one, so the RPI is likely to increase as a result. If a price falls, then the price ratio is less than one, so the RPI is likely to decrease. Therefore, since the price of leisure goods fell, this is likely to lead to a decrease in the value of the RPI. For a similar reason, the increase in the price of canteen meals is likely to lead to an increase in the value of the RPI.
(b) In each case, is the size of the increase or decrease likely to be large or small?
Discussion
Both changes are likely to be small for two reasons. First, the price changes are themselves fairly small. Second, leisure goods and canteen meals form only part of a household’s expenditure: no single group, subgroup or section will have a large effect on the RPI on its own, unless there is a very large change in its price.
(c) Using what you know about the structure of the RPI, decide which of ‘Leisure goods’ and ‘Canteen meals’ has the larger weight.
Discussion
The weight of ‘Leisure goods’ was 33 in 2012 (see Table 12). Since ‘Canteen meals’ is only one section in the subgroup ‘Catering’, which had weight 47 in 2012, the weight of ‘Canteen meals’ will be much smaller than 47. (In fact it was 3.) So the weight of ‘Leisure goods’ is much larger than the weight of ‘Canteen meals’.
(d) Which of the price changes mentioned in the question will have a larger effect on the value of the RPI? Briefly explain your answer.
Discussion
Since the weight of ‘Leisure goods’ is much larger than the weight of ‘Canteen meals’, and the percentage change in the prices are not too different in size, the change in the price of leisure goods is likely to have a much larger effect on the value of the RPI as a whole.