Retail Price Index sample methodology

FOI request: FOI/2022/3985

You asked

Request 1: Foreign holidays - Late Bookings

am writing to request the ONS provides additional information on sampling and price collection for the following RPI item: *foreign holidays - late bookings (640304).

I wanted to request the following information be made publicly available in regards to said item.

1. Price collection and check-in dates: **Could you please make publicly available the price collection methodology? Specifically, could you please advise how the price collection date relates to the RPI index day, and how this subsequently relates to the check-in and check-out date for this item?

2. Length of stay: Could you please make publicly available how many nights are included in each month's booking? Also, are the first and last day of stay set to coincide with particular weekdays and/or days of the month? Does length of stay vary by destination?

3. Sample size: Could you please make publicly available how the ONS identifies the relevant destinations in its sample of quotes for this item? Is the sample constructed based on any particular survey? Does the sample change from one month to the next or does it remain constant? How do you assign weights to each destination?

4. Trip definition: Could you please make publicly available how the ONS defines the relevant trips? Would quotes be for double-rooms at a set location and set star rating (eg, double-room at 3-star hotel at relevant holiday destination)? Would you include meals in the booking (eg, breakfast only, full boarding, etc)?

5. Selection: Even after narrowing the likely permutations for the previous parameters (1,2,3,4) there are likely to be a large number of eligible holidays. Does the ONS typically try to choose the cheapest of those eligible holidays?

6. Sub-indices: Are there sub-indices for this item and, if so, could you make available the differentiating parameters of those sub-indices.

Request 2: Foreign holidays - City breaks

I am writing to request the ONS provides additional information on sampling and price collection for the following RPI item: *foreign holidays - city breaks (640308).

I wanted to request the following information be made publicly available in regards to said item.

1. Price collection and check-in dates: Could you please make publicly available the price collection methodology? Specifically, could you please advise how the price collection date relates to the index day, and how this subsequently relates to the check-in and check-out date for this item?

2. Length of stay: Could you please make publicly available how many nights are included in each month's booking? Also, are the first and last day of stay set to coincide with particular weekdays and/or days of the month? Does length of stay vary by destination?

3. Sample size: Could you please make publicly available how the ONS identifies the relevant destination in its sample of quotes for this item? Is the sample constructed based on any particular survey? Does the sample change from one month to the next or does it remain constant? How do you assign weights to each destination?

4. Trip definition: Could you please make publicly available how the ONS defines the relevant trips? Would quotes be for double-rooms at a set location and set star rating (eg, double-room at 3-star hotel at relevant city centre)? Would you include meals in the booking (eg, breakfast only, full boarding, etc)?

5. Sub-indices: Are there sub-indices for this item and, if so, could you make available the differentiating parameters of those sub-indices.

Request 3: Domestic holidays - Hotel 1 night price

I am writing to request the ONS provides additional information on sampling and price collection for the following RPI item: *domestic holidays - hotel 1 night price (640406).

I wanted to request the following information be made publicly available in regards to said item.

1. Price collection and check-in dates: Could you please make publicly available the price collection methodology? Specifically, could you please advise how the price collection date relates to the index day, and how this subsequently relates to the check-in and check-out date for this item?

2. Length of stay: The item title suggests the duration is one night. Could you confirm this is the case?

3. Check-in and check-out parameters: Is there a particular day of the week chosen to check-in and day of the week chosen to check out?

4. Sample size: Could you please make publicly available how the ONS identifies the relevant destination in its sample of quotes for this item? Is the sample constructed based on any particular survey? Does the sample change from one month to the next or does it remain constant? How do you assign weights to each destination?

5. Room parameters: Could you please advise on the following parameters:

• Bed-size (e.g. single, double, king, all of those options)
• Hotel rating (e.g. do you select hotels with a 3-star rating for example as that is probably the mostly commonly booked?)
• Meals (would you include meals in the booking, e.g. breakfast)

6. Sub-indices: Are there sub-indices for this item and, if so, could you make available the differentiating parameters of those sub-indices.

Request 4: Foreign holidays - Self catering

I am writing to request the ONS provides additional information on sampling and price collection for the following RPI item: *foreign holidays - self catering (640305).

I wanted to request the following information be made publicly available in regards to said item.

• Sub-indices: Are there sub-indices for this item? And, if so, could you please a list of all the sub-indices.
• Trip types: What exactly constitutes "self catering" and within each country is there a sub index per different type of self catering category?
• Length of stay: Could you please make publicly available the length of stay for each holiday trip? Is it always one-week? And would that run from, say, a set day of the week until the following day of the week?
• Weights: At what level of aggregation are weights applied and where are those weights obtained?
• Price collection: The inflation manual suggest that the ONS uses brochures and travel agency websites to collect holiday prices. Could you confirm the frequency of this price collection?

Request 5: Mobile phone applications

I am writing to request the ONS provides additional information on sampling and index construction for the following item RPI > Housing and household expenditure > Household Services > Telephone > MOBILE PHONE APPLICATIONS (Item Code 440408).

I would request the following information be made publicly available:

1. Price collection: Could you please make public the price sampling methodology? Specifically, which sampling method is used to select the applications from each digital storefront?

• Is a chart typically used? If so

• Are you basing the sampling using one single industry chart, or using the chart per shop/website sampled?

• Are you collecting prices for the top 10, 20, or other number of items on the chart?

• Do you collect prices from more than one shop/website?

2. Price dates: are the application prices sampled once per month on the 'index date', as with most other items, or does this item have a different sampling rate/schedule?

3. Index Construction: Could you please make public information on the index construction methodology for this item? Specifically, how are the sampled prices aggregated to produce a comparable item index value? For example, are there sub-indices. Furthermore, are all of the sampled applications given an equal weight in the aggregation?

Request 6: Eurotunnel Fares - RPI

I am writing to request the ONS provides additional information on sampling and price collection for the RPI item Euro Tunnel Fares (620106).

I wanted to request the following information be made publicly available in regards to said item.

1. Price collection and departure dates: Could you please make publicly available the price collection methodology? Specifically,

• could you please advise how the price collection date relates to the index day
• how are departure and return dates are determined and please clarify if you have sub-indices for different permutations (e.g. one week ahead index, perhaps weekend return leaving on a Friday and returning on a Sunday, etc.)

2. Types of vehicles: Do you collect fares for both cars and motorbikes?

3. Determine which types of vehicles to collect fares for:

• Does the ONS use a reference of popular vehicles (both cars and motorbikes) and input under the Make and Model section to determine pricing?
• How many car types are prices collected for?
• How many motorbikes are prices collected for?

4. Add-on parameters: do you collect prices including "towing a caravan or trailer" and/or "will your vehicle have items on the rear or roof"?

Request 7: New cars and electric/hybrid cars

NEW CARS (610113)

1. Regarding NEW CARS, the CPI technical handbook states that a sample of 'approximately 35' cars covering a range of manufacturers are sampled each month.

2. Has this number (approximately 35) been altered substantially since the last update of the technical manual?

3. What is the approximate number of manufacturers which are sampled within this sample of 35 cars?

4. How is the sample size split between different types of cars? (e.g. hatchback, SUV etc.)

5. What methodology is used to select the manufacturers and models sampled, e.g. is market share data used to determine the cars to sample?

6. Within the manufacturers and models sampled, are different weights assigned to the different cars in the sample? What method is used to determine the weights to each car in the sample?

7. When sampling the prices for the new cars models, does price collection tend to stick to a typical engine size where available, when the model offers multiple choices of engine size? If so, what is that typical engine size, or if not, how does ONS decide which engine size(s) to sample from the available options for each car?

8. When constructing the index for NEW CARS, are additional features/add-on options considered, or only the basic, default models of each car in the sample?

9. Within the new cars item index, does ONS produce any subindices for different types of new cars (e.g. hatchback, SUV etc.) or any other subindices, similarly to how the AIR FARES item is segmented into different subindices, or do all cars sampled go into a single index?

10. Regarding 4), if any subindices are indeed produced, we would request that you kindly publish those subindices from the first month that NEW CARS was included in the RPI basket, up to the most recent publication month.

11. The majority of items in the RPI basket are tracked using an index relative of the current month's price over the price in the base period. Is a comparable base price used in the new cars index, in order to calculate an index relative? How does ONS handle a switch in the basket of cars between base periods when a car is removed and a new model enters the sample? Is the base price switched, adjusted etc.?

ELECTRIC/HYBRID CARS (610114)

1. Regarding ELECTRIC/HYBRID CARS,

1. How many ELECTRIC/HYBRID cars are included in the sample each month? 2. What is the approximate number of manufacturers which are sampled within this sample? 3. What methodology is used to select the manufacturers and models which go into the sample? 4. If different weights are assigned to different electric/hybrid cars in the sample, how are these determined? 5. What is the sample size and weight split of a) electric cars and b) hybrid cars respectively in the electric/hybrid cars item?

2. Does the ELECTRIC/HYBRID CARS item consist of new electric/hybrid cars, used electric/hybrid cars, or a mixture of both new and used electric/hybrid cars?

3. If the item consists of a mixture of new electric/hybrid cars and used electric/hybrid cars, we would request that you kindly publish separate NEW and USED electric/hybrid cars subindices (similarly to AIR FARES subindices) from the first month this item was included in the RPI basket, up to the most recent month of publication. 4. If the item consists of a mixture of new electric/hybrid cars and used electric/hybrid cars, how is the weight split between these subcomponents? 5. When constructing the index for ELECTRIC/HYBRID CARS, are additional features/add-on options considered, or only the basic, default models of each car in the sample?

We said

​Thank you for your requests. You submitted seven requests to us in total within the space of a 20-working day period. Our response will cover all aspects of those requests.

Under the Freedom of Information Act 2000 (FOIA), members of the public have the right to ask for the release of information held by public authorities. This right only covers recorded information. ICO guidance states that a request can be in the form of a question, but the authority does not have to answer questions if this would mean creating new information or giving an opinion or judgement that has not already been recorded.

Having looked at the questions asked, and discussions with our Prices Team, we have concluded that most of these are not requests for information held.

We are able to provide some information in relation to the following:

International rail fares (620106)

Information on how the price collection relates to index day, and departure and return dates.

Petrol and diesel cars (610113), and electric or hybrid cars (610114)

Information on the sample sizes for each item, the specifications which are recorded and some detail on the sample selection methodology.

Please note that some of the information under the 'Description of prices used in the index' heading of this document is out of date.

In 2022 the sample sizes are split as follows:

• Petrol and diesel: 22
• Electric and hybrid: 19

Also, as a point of clarification, where the 'N' indicator code is used, as discussed under point 3 of the 'Obtaining the data' section,  new base prices are imputed using class mean imputation rather than the quality adjustment method discussed earlier in the document.

Foreign holidays - self-catering (640305)

Length of stay and the timing of holidays

The level at which weights are applied

Foreign holidays - city breaks (640308)

Length of stay and the timing of holidays

Assignment of weights to location

Hotel one night price (640406)

Price collection date and its relation to check-in and check-out date

Length of stay

Check-in and check-out parameters

Room parameter and meals

Mobile phone applications (440408)

Information on the use of charts and numbers taken from each chart

Index construction and weighting

Some of the requested information is held in our internal guidance documents. The relevant documents can be found in the associated downloads.