The Turkish Statistical Institute (TÜİK) publishes indices every month that show how retail trade is developing. This data is one of the most valuable open sources for a retail company to compare its own performance with the market. But reading the indices correctly requires knowing a few concepts.
The difference between the volume index and the turnover index
The sales volume indexis adjusted for price changes, meaning it shows how much was actually sold in real quantity terms. The turnover index , on the other hand, measures sales at current prices, meaning their monetary value. In periods of high inflation, turnover can rise quickly while volume rises much more slowly or even falls. For example, according to data published by TÜİK on 13 April 2026, total turnover rose by about 34% year on year in February 2026, while retail sales volume rose by 15.6% year on year. The gap is largely due to price increases.
Calendar and seasonal effects
- Calendar-adjusted: A series from which the effect of differences in working days and holidays between months has been removed. It is used for year-on-year comparisons (this February versus last February).
- Seasonally and calendar-adjusted: Regular seasonal fluctuations such as religious holidays, school terms and New Year are also removed. It is used for month-on-month comparisons (this month versus last month).
For example, in the same bulletin retail sales volume rose 15.6% year on year, but fell 0.2% compared with the previous month according to the seasonally and calendar-adjusted series. The two are not contradictory; they answer different questions.
Sub-categories
Retail sales indices are usually published with sub-groups such as food, beverages and tobacco; non-food products (excluding automotive fuel); and automotive fuel. For a grocery chain, the most meaningful comparison is naturally the food group.
How do you compare them with your own data?
- Aggregate your own sales over the same periods (monthly).
- Use the TÜİK turnover series if you are comparing revenue, and the volume series if you are comparing units or baskets.
- Compare year-on-year changes with the calendar-adjusted series.
- Track the two growth rates on the same chart to see whether you are growing above or below the market.
- Note calendar effects in your own data too, such as shifting holiday dates.
Take a look at our TÜİK retail index report project, in which we automated this comparison in Power BI. For the latest data and methodology notes, we recommend referring to TÜİK’s own bulletins.