The BOT distributes the BSI questionnaires during the first week of the survey month. The BSI is compiled by the first working day of the following month. The surveyed businesses are in manufacturing sector, trading sector, and service sector. The samples include small, medium, and large businesses (registered capital less than 50 million baht, 50-200 million baht, more than 200 million baht respectively). The overall response rate is around 60 per cent of total sent-out questionnaires or 500 observations per month on average.

The BSI is constructed as diffusion index in the same way of Purchasing Managers’ Index (PMI) in other countries. The BSI is made up of 6 components: production, total order books, investment, production cost, and employment. Each component is applied with equal weight to calculate the monthly composite index.

Each component index is calculated according to the following formula:

Index = (100 × percentage of respondents indicating “improvement”) + (50 × percentage of respondents indicating “stable”) + (0 × percentage of respondents indicating “deterioration”)

Interpretation of the index is as follows:

Index = 50 indicates that the respondents’ business sentiment remains stable from the previous month.

Index > 50 indicates that the respondents’ business sentiment has improved from the previous month.

Index < 50 indicates that the respondents’ business sentiment has deteriorated from the previous month.

For more information, see construction.