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Methodology - monthly sales indices in transport and communications


Methodology - monthly sales indices in transport and communications
 

I. Sales index in transport and communications
 
According to the CZ-NACE, units with predominating activities in transport, cargo handling and storage, post and courier services and telecommunications are classified in transport and communications. The sales index indicator serves for the evaluation, analyses of short-term development and possible predictions of future development in transport and communications. The development is observed by force of questionnaires SP 1-12 and SP rr, which are common for wholesale and retail trade, transport and communications and market services.
 
II. Formation of the selective sample
 
Reporting units are selected from the Business Register. Sample strata are defined by the size of the unit (number of employees) and principal activity (usually a 3‑digit level CZ‑NACE category), the sample is take for each strata separately. Included in this selection are all units with 50 or more employees. Proportional sampling is used in the other groups of enterprises. It is possible to apply the proportional sampling only in a situation when relevant information (in this case sales) exists for all units in the basic sample. The probability of including the unit in to the sample is directly proportional to the share of its sales on total sales of all units in strata. It is therefore ensured that units with above the average sales have a bigger probability to be included in the sample than units with lower sales and hence they are not exposed to unnecessary administrative burden. The importance of small enterprises however is not undervalued as the products of weights of individual enterprises and their sales are within the strata equal.
The sample strata are subjected to a so-called rotation i.e. partial replacement of part of the sample, with the aim to update the set of units to which the questionnaires are send and to retain the stability and hence the comparability with data produced in preceding years. That usually means that enterprises, having been selected at random, remain in the sample for two successive years; then they are excluded from the sample and replaced by new reporting units. The criterion for including all units into the sample is applied during the observed year; basic and selective samples are hence updated regularly with new units, which correspond with this criterion.
 
III. Description of indicators observed
 
Monthly questionnaire covers sales of own products, goods and services in the reference month. Sales are recorded both including VAT and not including VAT. Units report also sales of results of activities that are not subject to VAT (e.g. exports).
Initial questionnaire, which is send to units included in the selection for the first time contains information on whether the reporting unit is active, information about the principal activity of the unit, and sales in individual months of the preceding year.
 
IV. Description of calculating the sales index
 
The development of the branch is represented by the sales index without VAT, which is in compliance with demands of the European Commission and takes into consideration the fact that the price index used for the calculation of sales to constant prices does not include VAT.
Year-on-year sales indices at current prices are calculated for individual months for each   CZ‑NACE lowest aggregate (e.g. CZ‑NACE 60.1 ). The y‑o‑y price deflators for individual months of the current year are used for these CZ‑NACE categories; price deflators of individual months related to the year’s average are used for the base year. Absolute values of monthly sales at current prices for the lowest CZ‑NACE aggregates are converted to the price level of 2000 using the method of chaining deflators. By the summation of the lowest aggregates of absolute sales values at constant prices in individual months we get higher sale aggregates from which we calculate y‑o‑y sale indices at constant prices. The sum of monthly sales of aggregated CZ‑NACE categories at current prices for 2000 is equal to the sum of their sales at constant prices.
 
V. Retroactive corrections, revisions
 
Along with the processing of February data, revision of January data will be carried out. In case of requirements, data revisions are carried out along with the processing of the first month that follows the publication of quarterly data e.g. along with the processing of April data, data for January-March are revised. Final data are published following the processing of all of the months of the observed year, in June of the following year at the latest.
Along with the processing of April 2008 data, the base for computing base indices of seasonally adjusted data and trend was methodologically changed.
 
VI. International comparability
 
The area of short‑term statistics in the European Union is guided by EU Regulation              No 1165/98 and 1158/2005 on short‑term statistics. Under this Regulation, EU member states are obliged to provide information on turnover in transport and communication every quarter (current prices). The indicator published by CZSO is comparable.
VII. Seasonal adjustment
 
The size of sales during a year in some industries is strongly affected by seasons                 (e.g.  tourist season in services related to tourism and the like). In order to find out long-term trends and to be able to compare month – on – month or quarter – on - quarter development, the CZSO produces seasonally adjusted data, when the part of sales caused by a seasonal swing is separated from the original “rough” series, using a mathematical model. Seasonally adjusted data are comparable not only between years, but also between individual months (quarters).
Method TRAMO/SEAT recommended by Eurostat is used for seasonal adjustment. Program application DEMETRA is employed in the CZSO. Seasonal adjustment is made at the lowest level (usually 3-digits CZ-NACE); the so-called indirect method (sum of seasonally adjusted sales of 3-digit CZ-NACE items) is used for calculating seasonally adjusted aggregates     (CZ-NACE 2-digit level, services total, etc).
 
VIII. Comparability with other surveys in terms of methodology
 
The results of monthly surveys may differ from the results based on quarterly or annual surveys, which are designed to serve the estimate of the total volume of sales, not only index.
Discrepancies may also arise from different samples in some CZ-NACE activities where the volumes of sales are largely dispersed.
 
IX. Types of data released
 
The CZSO publishes the results of monthly survey in transport and communication on a regular basis in:
 
1. News Releases ( Transport and communication– quarterly, on 33rd working day after the end of the reference period).
 
2. Time series on the web pages of the CZSO - Time series
 
Results of the monthly survey are presented as sales indices. The sales indices are computed, using sales at current and constant prices (incl. VAT and not incl. VAT), both seasonally adjusted and not seasonally adjusted (gross). Among the basic types of indices are the following:
 
a) year-on-year index – the ratio of sales in the reference period and sales in the corresponding period of preceding year,
 
b) month-on-month or quarter-on-quarter index – the ratio of sales in the reference period and sales in the corresponding preceding period (it makes sense to publish this index only from seasonally adjusted data),
 
c) base index – ratio of sales in the reference period and average sales of the base year.       The year 2000 is now taken as the base year.