Guidance

Asymmetries: methodology notes

Updated 22 May 2024

Overview

This series outlines the asymmetry between UK trade in goods data and trade data reported by its partner countries. Each report covers the latest available three year’s trade in goods data.

Data source

The data in these reports comes from the United Nations International Trade Statistics Database. This database is also known as UN Comtrade. It is not possible to link tables directly. Instead, users must select their own filters to download data. The date of extraction can be found on each publication. Comtrade data is a live dataset. So, values are subject to frequent revisions.

All countries must collect their own trade data and submit data to UN Comtrade each month. UN Comtrade applies an annual average exchange rate to convert all data into United States Dollars.

For imports, UN Comtrade report the ‘country of origin’. For exports they report the ‘country of last known destination’ or ‘country of dispatch’.

Countries Included

If a country has not submitted imports data for each of the three years covered in a report, their imports data is excluded. The same applies for exports. This is so that asymmetry comparisons can be made between years. It also ensures that we are not including asymmetries calculated using incomplete data in our totals. 

However, this does not apply at a chapter level. So, as long as a country has submitted imports data for some chapters for each year covered in a publication, all their imports data is included. The same applies for exports. This allows the capture of misclassification of goods between countries. This is an important component of asymmetry.

Trade asymmetries

Trade asymmetries are the differences between the published trade data of a given country and that of its partner countries. Each country collects their own data on international trade in goods. In theory, each country’s exports to a partner country should match the partner country’s imports from them, and vice versa. For instance, UK-reported imports from France should match France-reported exports to the UK. Also, UK-reported exports to France should match France-reported imports from the UK. In reality, it is often the case that figures do not match exactly. We refer to these differences as trade asymmetries.

There are many reasons for asymmetries. These include differences in methods, valuation, and partner country assignment.

UK trade in goods statistics: Reasons for trade in goods asymmetries discusses these in detail.

Measuring trade asymmetries

There are many measurements of asymmetries. In this series we have chosen absolute asymmetry to be our leading asymmetry measure. Absolute asymmetry indicates the magnitude but not the direction of the asymmetry.

In the accompanying detailed data sets, we also provide the following for our expert users:

• non-absolute asymmetry figures
• relative asymmetry
• percentage share of total UK absolute asymmetry with the rest of the world

For non-absolute asymmetry, positive numbers indicate that UK figures are higher. Negative numbers indicate the opposite.

Different asymmetry measures and how they are calculated

Asymmetry

The asymmetry is defined as:

This is the simplest measure of asymmetry. The trade value recorded by the partner country is subtracted from that of the reporting country. This value can be positive or negative depending on which country is greater. In this series, the UK is the declarant or reporting country, ‘D’. Each other country is a partner country, ‘P’. Positive measures of asymmetry indicate that UK figures are higher.

Absolute asymmetry

The absolute asymmetry is defined as:

Absolute asymmetry is the same as asymmetry except negative numbers become positive. This means it does not show which country is greater. Instead, it focuses only on the size of the asymmetry.

In this series, we have chosen absolute asymmetry to be our leading asymmetry measure. This is because we are most interested in the size of the asymmetry.

Share of total absolute asymmetry

The share of the total absolute asymmetry is defined as:

The numerator is the absolute asymmetry for a particular category in a given flow and year. For example, the category could be a HS2 chapter. The denominator is the absolute total of the asymmetries for the flow and year in question. This value is given as a percentage. Being a part to whole proportion, it cannot take negative values or exceed 100%.  

Relative asymmetry

The relative asymmetry is defined as:

The numerator is the asymmetry for a particular category in a given flow and year. For example, the category could be a HS2 chapter. The denominator is the average of the UK and its partner country’s trade values for the same category. This value is given as a percentage. Since it is not a part to whole proportion, values can be negative and more than 100%. As for asymmetry, positive values show that UK figures are higher and vice versa.

How absolute asymmetry is calculated at different levels of aggregation

Asymmetry can be calculated for many different groups. For example, by partner country, flow, year, HS2 Chapter or any combination of the above. The same applies to absolute asymmetry. Although, in this case, the order of summing the groups and applying the absolute function matters.

At a country level, the absolute function is applied after grouping by partner country, flow, and year, but not by HS2 chapter. The focus is the size of the gap between countries’ total trade. Differences affecting HS2 chapter totals are irrelevant here.

At a HS2 chapter level, the absolute function is applied after grouping by HS2 chapter, flow, and year, but not by partner country. The focus is the size of the gap between what the UK reports for a certain chapter and what other countries report in total for said chapter. Differences affecting country totals do not matter here.

At a country and HS2 chapter level, the absolute function is applied after grouping by HS2 chapter, partner country, flow and year. Both differences by country and HS2 chapter matter here. These figures are only available in the detailed data set, for our expert users.

Totals at a world and regional level for a given flow and year are sums of absolute asymmetries calculated at a country level. The absolute function is applied at the country level only. This means that differences affecting HS2 chapters do not matter here. But, differences between the UK and individual partner countries do matter. Summing absolute asymmetries calculated at the HS2 chapter level will produce different totals.

Chapter 99 adjustments and omission from analysis

The Read Me First (Disclaimer) section of the UN Comtrade website says that a country’s total trade should match the sum of its chapter totals. Where this is not the case, it suggests that these differences are accounted for in each country’s Chapter 99 total. This was the approach we followed in this series where we found such differences.

Governance

The trade data presented in this series is sourced from UN Comtrade. 

HM Revenue & Customs released these statistics in accordance with the Code of Practice for Official Statistics, as well as, internationally agreed statistical guidance and standards.

Our statistical practice is regulated by the Office for Statistics Regulation (OSR).

OSR sets the standards of trustworthiness, quality, and value in the Code of Practice for Statistics that all producers of official statistics should adhere to.

You are welcome to contact us directly with any comments about how we meet these standards by emailing uktradeinfo@hmrc.gov.uk